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href="https://nodejs.org/docs/latest-v4.x/api/events.html">4.x</a></li> 134<li><a href="https://nodejs.org/docs/latest-v0.12.x/api/events.html">0.12.x</a></li> 135<li><a href="https://nodejs.org/docs/latest-v0.10.x/api/events.html">0.10.x</a></li></ol> 136 </li> 137 138 <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/master/doc/api/events.md">Edit on GitHub</a></li> 139 </ul> 140 </div> 141 <hr> 142 </header> 143 144 <details id="toc" open><summary>Table of contents</summary><ul> 145<li><span class="stability_2"><a href="#events_events">Events</a></span> 146<ul> 147<li><a href="#events_passing_arguments_and_this_to_listeners">Passing arguments and <code>this</code> to listeners</a></li> 148<li><a href="#events_asynchronous_vs_synchronous">Asynchronous vs. synchronous</a></li> 149<li><a href="#events_handling_events_only_once">Handling events only once</a></li> 150<li><a href="#events_error_events">Error events</a></li> 151<li><span class="stability_1"><a href="#events_capture_rejections_of_promises">Capture rejections of promises</a></span></li> 152<li><a href="#events_class_eventemitter">Class: <code>EventEmitter</code></a> 153<ul> 154<li><a href="#events_event_newlistener">Event: <code>'newListener'</code></a></li> 155<li><a href="#events_event_removelistener">Event: <code>'removeListener'</code></a></li> 156<li><a href="#events_emitter_addlistener_eventname_listener"><code>emitter.addListener(eventName, listener)</code></a></li> 157<li><a href="#events_emitter_emit_eventname_args"><code>emitter.emit(eventName[, ...args])</code></a></li> 158<li><a href="#events_emitter_eventnames"><code>emitter.eventNames()</code></a></li> 159<li><a href="#events_emitter_getmaxlisteners"><code>emitter.getMaxListeners()</code></a></li> 160<li><a href="#events_emitter_listenercount_eventname"><code>emitter.listenerCount(eventName)</code></a></li> 161<li><a href="#events_emitter_listeners_eventname"><code>emitter.listeners(eventName)</code></a></li> 162<li><a href="#events_emitter_off_eventname_listener"><code>emitter.off(eventName, listener)</code></a></li> 163<li><a href="#events_emitter_on_eventname_listener"><code>emitter.on(eventName, listener)</code></a></li> 164<li><a href="#events_emitter_once_eventname_listener"><code>emitter.once(eventName, listener)</code></a></li> 165<li><a href="#events_emitter_prependlistener_eventname_listener"><code>emitter.prependListener(eventName, listener)</code></a></li> 166<li><a href="#events_emitter_prependoncelistener_eventname_listener"><code>emitter.prependOnceListener(eventName, listener)</code></a></li> 167<li><a href="#events_emitter_removealllisteners_eventname"><code>emitter.removeAllListeners([eventName])</code></a></li> 168<li><a href="#events_emitter_removelistener_eventname_listener"><code>emitter.removeListener(eventName, listener)</code></a></li> 169<li><a href="#events_emitter_setmaxlisteners_n"><code>emitter.setMaxListeners(n)</code></a></li> 170<li><a href="#events_emitter_rawlisteners_eventname"><code>emitter.rawListeners(eventName)</code></a></li> 171<li><span class="stability_1"><a href="#events_emitter_symbol_for_nodejs_rejection_err_eventname_args"><code>emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])</code></a></span></li> 172</ul> 173</li> 174<li><a href="#events_events_defaultmaxlisteners"><code>events.defaultMaxListeners</code></a></li> 175<li><a href="#events_events_errormonitor"><code>events.errorMonitor</code></a></li> 176<li><a href="#events_events_geteventlisteners_emitterortarget_eventname"><code>events.getEventListeners(emitterOrTarget, eventName)</code></a></li> 177<li><a href="#events_events_once_emitter_name_options"><code>events.once(emitter, name[, options])</code></a> 178<ul> 179<li><a href="#events_awaiting_multiple_events_emitted_on_process_nexttick">Awaiting multiple events emitted on <code>process.nextTick()</code></a></li> 180</ul> 181</li> 182<li><span class="stability_1"><a href="#events_events_capturerejections"><code>events.captureRejections</code></a></span></li> 183<li><span class="stability_1"><a href="#events_events_capturerejectionsymbol"><code>events.captureRejectionSymbol</code></a></span></li> 184<li><span class="stability_0"><a href="#events_events_listenercount_emitter_eventname"><code>events.listenerCount(emitter, eventName)</code></a></span></li> 185<li><a href="#events_events_on_emitter_eventname_options"><code>events.on(emitter, eventName[, options])</code></a></li> 186<li><a href="#events_events_setmaxlisteners_n_eventtargets"><code>events.setMaxListeners(n[, ...eventTargets])</code></a></li> 187<li><span class="stability_1"><a href="#events_eventtarget_and_event_api"><code>EventTarget</code> and <code>Event</code> API</a></span> 188<ul> 189<li><a href="#events_node_js_eventtarget_vs_dom_eventtarget">Node.js <code>EventTarget</code> vs. DOM <code>EventTarget</code></a></li> 190<li><a href="#events_nodeeventtarget_vs_eventemitter"><code>NodeEventTarget</code> vs. <code>EventEmitter</code></a></li> 191<li><a href="#events_event_listener">Event listener</a></li> 192<li><a href="#events_eventtarget_error_handling"><code>EventTarget</code> error handling</a></li> 193<li><a href="#events_class_event">Class: <code>Event</code></a> 194<ul> 195<li><a href="#events_event_bubbles"><code>event.bubbles</code></a></li> 196<li><a href="#events_event_cancelbubble"><code>event.cancelBubble()</code></a></li> 197<li><a href="#events_event_cancelable"><code>event.cancelable</code></a></li> 198<li><a href="#events_event_composed"><code>event.composed</code></a></li> 199<li><a href="#events_event_composedpath"><code>event.composedPath()</code></a></li> 200<li><a href="#events_event_currenttarget"><code>event.currentTarget</code></a></li> 201<li><a href="#events_event_defaultprevented"><code>event.defaultPrevented</code></a></li> 202<li><a href="#events_event_eventphase"><code>event.eventPhase</code></a></li> 203<li><a href="#events_event_istrusted"><code>event.isTrusted</code></a></li> 204<li><a href="#events_event_preventdefault"><code>event.preventDefault()</code></a></li> 205<li><a href="#events_event_returnvalue"><code>event.returnValue</code></a></li> 206<li><a href="#events_event_srcelement"><code>event.srcElement</code></a></li> 207<li><a href="#events_event_stopimmediatepropagation"><code>event.stopImmediatePropagation()</code></a></li> 208<li><a href="#events_event_stoppropagation"><code>event.stopPropagation()</code></a></li> 209<li><a href="#events_event_target"><code>event.target</code></a></li> 210<li><a href="#events_event_timestamp"><code>event.timeStamp</code></a></li> 211<li><a href="#events_event_type"><code>event.type</code></a></li> 212</ul> 213</li> 214<li><a href="#events_class_eventtarget">Class: <code>EventTarget</code></a> 215<ul> 216<li><a href="#events_eventtarget_addeventlistener_type_listener_options"><code>eventTarget.addEventListener(type, listener[, options])</code></a></li> 217<li><a href="#events_eventtarget_dispatchevent_event"><code>eventTarget.dispatchEvent(event)</code></a></li> 218<li><a href="#events_eventtarget_removeeventlistener_type_listener"><code>eventTarget.removeEventListener(type, listener)</code></a></li> 219</ul> 220</li> 221<li><a href="#events_class_nodeeventtarget">Class: <code>NodeEventTarget</code></a> 222<ul> 223<li><a href="#events_nodeeventtarget_addlistener_type_listener_options"><code>nodeEventTarget.addListener(type, listener[, options])</code></a></li> 224<li><a href="#events_nodeeventtarget_eventnames"><code>nodeEventTarget.eventNames()</code></a></li> 225<li><a href="#events_nodeeventtarget_listenercount_type"><code>nodeEventTarget.listenerCount(type)</code></a></li> 226<li><a href="#events_nodeeventtarget_off_type_listener"><code>nodeEventTarget.off(type, listener)</code></a></li> 227<li><a href="#events_nodeeventtarget_on_type_listener_options"><code>nodeEventTarget.on(type, listener[, options])</code></a></li> 228<li><a href="#events_nodeeventtarget_once_type_listener_options"><code>nodeEventTarget.once(type, listener[, options])</code></a></li> 229<li><a href="#events_nodeeventtarget_removealllisteners_type"><code>nodeEventTarget.removeAllListeners([type])</code></a></li> 230<li><a href="#events_nodeeventtarget_removelistener_type_listener"><code>nodeEventTarget.removeListener(type, listener)</code></a></li> 231</ul> 232</li> 233</ul> 234</li> 235</ul> 236</li> 237</ul></details> 238 239 <div id="apicontent"> 240 <h2>Events<span><a class="mark" href="#events_events" id="events_events">#</a></span></h2> 241 242<p></p><div class="api_stability api_stability_2"><a href="documentation.html#documentation_stability_index">Stability: 2</a> - Stable</div><p></p> 243 244<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v14.20.1/lib/events.js">lib/events.js</a></p> 245<p>Much of the Node.js core API is built around an idiomatic asynchronous 246event-driven architecture in which certain kinds of objects (called "emitters") 247emit named events that cause <code>Function</code> objects ("listeners") to be called.</p> 248<p>For instance: a <a href="net.html#net_class_net_server"><code>net.Server</code></a> object emits an event each time a peer 249connects to it; a <a href="fs.html#fs_class_fs_readstream"><code>fs.ReadStream</code></a> emits an event when the file is opened; 250a <a href="stream.html">stream</a> emits an event whenever data is available to be read.</p> 251<p>All objects that emit events are instances of the <code>EventEmitter</code> class. These 252objects expose an <code>eventEmitter.on()</code> function that allows one or more 253functions to be attached to named events emitted by the object. Typically, 254event names are camel-cased strings but any valid JavaScript property key 255can be used.</p> 256<p>When the <code>EventEmitter</code> object emits an event, all of the functions attached 257to that specific event are called <em>synchronously</em>. Any values returned by the 258called listeners are <em>ignored</em> and discarded.</p> 259<p>The following example shows a simple <code>EventEmitter</code> instance with a single 260listener. The <code>eventEmitter.on()</code> method is used to register listeners, while 261the <code>eventEmitter.emit()</code> method is used to trigger the event.</p> 262<pre><code class="language-js"><span class="hljs-keyword">const</span> <span class="hljs-title class_">EventEmitter</span> = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 263 264<span class="hljs-keyword">class</span> <span class="hljs-title class_">MyEmitter</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">EventEmitter</span> {} 265 266<span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MyEmitter</span>(); 267myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-function">() =></span> { 268 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'an event occurred!'</span>); 269}); 270myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>);</code></pre> 271<section><h3>Passing arguments and <code>this</code> to listeners<span><a class="mark" href="#events_passing_arguments_and_this_to_listeners" id="events_passing_arguments_and_this_to_listeners">#</a></span></h3> 272<p>The <code>eventEmitter.emit()</code> method allows an arbitrary set of arguments to be 273passed to the listener functions. Keep in mind that when 274an ordinary listener function is called, the standard <code>this</code> keyword 275is intentionally set to reference the <code>EventEmitter</code> instance to which the 276listener is attached.</p> 277<pre><code class="language-js"><span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MyEmitter</span>(); 278myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-keyword">function</span>(<span class="hljs-params">a, b</span>) { 279 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(a, b, <span class="hljs-variable language_">this</span>, <span class="hljs-variable language_">this</span> === myEmitter); 280 <span class="hljs-comment">// Prints:</span> 281 <span class="hljs-comment">// a b MyEmitter {</span> 282 <span class="hljs-comment">// domain: null,</span> 283 <span class="hljs-comment">// _events: { event: [Function] },</span> 284 <span class="hljs-comment">// _eventsCount: 1,</span> 285 <span class="hljs-comment">// _maxListeners: undefined } true</span> 286}); 287myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>, <span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>);</code></pre> 288<p>It is possible to use ES6 Arrow Functions as listeners, however, when doing so, 289the <code>this</code> keyword will no longer reference the <code>EventEmitter</code> instance:</p> 290<pre><code class="language-js"><span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MyEmitter</span>(); 291myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-function">(<span class="hljs-params">a, b</span>) =></span> { 292 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(a, b, <span class="hljs-variable language_">this</span>); 293 <span class="hljs-comment">// Prints: a b {}</span> 294}); 295myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>, <span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>);</code></pre> 296</section><section><h3>Asynchronous vs. synchronous<span><a class="mark" href="#events_asynchronous_vs_synchronous" id="events_asynchronous_vs_synchronous">#</a></span></h3> 297<p>The <code>EventEmitter</code> calls all listeners synchronously in the order in which 298they were registered. This ensures the proper sequencing of 299events and helps avoid race conditions and logic errors. When appropriate, 300listener functions can switch to an asynchronous mode of operation using 301the <code>setImmediate()</code> or <code>process.nextTick()</code> methods:</p> 302<pre><code class="language-js"><span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MyEmitter</span>(); 303myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-function">(<span class="hljs-params">a, b</span>) =></span> { 304 <span class="hljs-title function_">setImmediate</span>(<span class="hljs-function">() =></span> { 305 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'this happens asynchronously'</span>); 306 }); 307}); 308myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>, <span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>);</code></pre> 309</section><section><h3>Handling events only once<span><a class="mark" href="#events_handling_events_only_once" id="events_handling_events_only_once">#</a></span></h3> 310<p>When a listener is registered using the <code>eventEmitter.on()</code> method, that 311listener is invoked <em>every time</em> the named event is emitted.</p> 312<pre><code class="language-js"><span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MyEmitter</span>(); 313<span class="hljs-keyword">let</span> m = <span class="hljs-number">0</span>; 314myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-function">() =></span> { 315 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(++m); 316}); 317myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>); 318<span class="hljs-comment">// Prints: 1</span> 319myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>); 320<span class="hljs-comment">// Prints: 2</span></code></pre> 321<p>Using the <code>eventEmitter.once()</code> method, it is possible to register a listener 322that is called at most once for a particular event. Once the event is emitted, 323the listener is unregistered and <em>then</em> called.</p> 324<pre><code class="language-js"><span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MyEmitter</span>(); 325<span class="hljs-keyword">let</span> m = <span class="hljs-number">0</span>; 326myEmitter.<span class="hljs-title function_">once</span>(<span class="hljs-string">'event'</span>, <span class="hljs-function">() =></span> { 327 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(++m); 328}); 329myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>); 330<span class="hljs-comment">// Prints: 1</span> 331myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>); 332<span class="hljs-comment">// Ignored</span></code></pre> 333</section><section><h3>Error events<span><a class="mark" href="#events_error_events" id="events_error_events">#</a></span></h3> 334<p>When an error occurs within an <code>EventEmitter</code> instance, the typical action is 335for an <code>'error'</code> event to be emitted. These are treated as special cases 336within Node.js.</p> 337<p>If an <code>EventEmitter</code> does <em>not</em> have at least one listener registered for the 338<code>'error'</code> event, and an <code>'error'</code> event is emitted, the error is thrown, a 339stack trace is printed, and the Node.js process exits.</p> 340<pre><code class="language-js"><span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MyEmitter</span>(); 341myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(<span class="hljs-string">'whoops!'</span>)); 342<span class="hljs-comment">// Throws and crashes Node.js</span></code></pre> 343<p>To guard against crashing the Node.js process the <a href="domain.html"><code>domain</code></a> module can be 344used. (Note, however, that the <code>domain</code> module is deprecated.)</p> 345<p>As a best practice, listeners should always be added for the <code>'error'</code> events.</p> 346<pre><code class="language-js"><span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MyEmitter</span>(); 347myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">err</span>) =></span> { 348 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'whoops! there was an error'</span>); 349}); 350myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(<span class="hljs-string">'whoops!'</span>)); 351<span class="hljs-comment">// Prints: whoops! there was an error</span></code></pre> 352<p>It is possible to monitor <code>'error'</code> events without consuming the emitted error 353by installing a listener using the symbol <code>events.errorMonitor</code>.</p> 354<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">EventEmitter</span>, errorMonitor } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 355 356<span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 357myEmitter.<span class="hljs-title function_">on</span>(errorMonitor, <span class="hljs-function">(<span class="hljs-params">err</span>) =></span> { 358 <span class="hljs-title class_">MyMonitoringTool</span>.<span class="hljs-title function_">log</span>(err); 359}); 360myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(<span class="hljs-string">'whoops!'</span>)); 361<span class="hljs-comment">// Still throws and crashes Node.js</span></code></pre> 362</section><section><h3>Capture rejections of promises<span><a class="mark" href="#events_capture_rejections_of_promises" id="events_capture_rejections_of_promises">#</a></span></h3> 363<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - captureRejections is experimental.</div><p></p> 364<p>Using <code>async</code> functions with event handlers is problematic, because it 365can lead to an unhandled rejection in case of a thrown exception:</p> 366<pre><code class="language-js"><span class="hljs-keyword">const</span> ee = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 367ee.<span class="hljs-title function_">on</span>(<span class="hljs-string">'something'</span>, <span class="hljs-keyword">async</span> (value) => { 368 <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(<span class="hljs-string">'kaboom'</span>); 369});</code></pre> 370<p>The <code>captureRejections</code> option in the <code>EventEmitter</code> constructor or the global 371setting change this behavior, installing a <code>.then(undefined, handler)</code> 372handler on the <code>Promise</code>. This handler routes the exception 373asynchronously to the <a href="#events_emitter_symbol_for_nodejs_rejection_err_eventname_args"><code>Symbol.for('nodejs.rejection')</code></a> method 374if there is one, or to <a href="#events_error_events"><code>'error'</code></a> event handler if there is none.</p> 375<pre><code class="language-js"><span class="hljs-keyword">const</span> ee1 = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>({ <span class="hljs-attr">captureRejections</span>: <span class="hljs-literal">true</span> }); 376ee1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'something'</span>, <span class="hljs-keyword">async</span> (value) => { 377 <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(<span class="hljs-string">'kaboom'</span>); 378}); 379 380ee1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-variable language_">console</span>.<span class="hljs-property">log</span>); 381 382<span class="hljs-keyword">const</span> ee2 = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>({ <span class="hljs-attr">captureRejections</span>: <span class="hljs-literal">true</span> }); 383ee2.<span class="hljs-title function_">on</span>(<span class="hljs-string">'something'</span>, <span class="hljs-keyword">async</span> (value) => { 384 <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(<span class="hljs-string">'kaboom'</span>); 385}); 386 387ee2[<span class="hljs-title class_">Symbol</span>.<span class="hljs-title function_">for</span>(<span class="hljs-string">'nodejs.rejection'</span>)] = <span class="hljs-variable language_">console</span>.<span class="hljs-property">log</span>;</code></pre> 388<p>Setting <code>events.captureRejections = true</code> will change the default for all 389new instances of <code>EventEmitter</code>.</p> 390<pre><code class="language-js"><span class="hljs-keyword">const</span> events = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 391events.<span class="hljs-property">captureRejections</span> = <span class="hljs-literal">true</span>; 392<span class="hljs-keyword">const</span> ee1 = <span class="hljs-keyword">new</span> events.<span class="hljs-title class_">EventEmitter</span>(); 393ee1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'something'</span>, <span class="hljs-keyword">async</span> (value) => { 394 <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(<span class="hljs-string">'kaboom'</span>); 395}); 396 397ee1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-variable language_">console</span>.<span class="hljs-property">log</span>);</code></pre> 398<p>The <code>'error'</code> events that are generated by the <code>captureRejections</code> behavior 399do not have a catch handler to avoid infinite error loops: the 400recommendation is to <strong>not use <code>async</code> functions as <code>'error'</code> event handlers</strong>.</p> 401</section><section><h3>Class: <code>EventEmitter</code><span><a class="mark" href="#events_class_eventemitter" id="events_class_eventemitter">#</a></span></h3> 402<div class="api_metadata"> 403<details class="changelog"><summary>History</summary> 404<table> 405<tbody><tr><th>Version</th><th>Changes</th></tr> 406<tr><td>v13.4.0, v12.16.0</td> 407<td><p>Added captureRejections option.</p></td></tr> 408<tr><td>v0.1.26</td> 409<td><p><span>Added in: v0.1.26</span></p></td></tr> 410</tbody></table> 411</details> 412</div> 413<p>The <code>EventEmitter</code> class is defined and exposed by the <code>events</code> module:</p> 414<pre><code class="language-js"><span class="hljs-keyword">const</span> <span class="hljs-title class_">EventEmitter</span> = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>);</code></pre> 415<p>All <code>EventEmitter</code>s emit the event <code>'newListener'</code> when new listeners are 416added and <code>'removeListener'</code> when existing listeners are removed.</p> 417<p>It supports the following option:</p> 418<ul> 419<li><code>captureRejections</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> It enables 420<a href="#events_capture_rejections_of_promises">automatic capturing of promise rejection</a>. 421<strong>Default:</strong> <code>false</code>.</li> 422</ul> 423<h4>Event: <code>'newListener'</code><span><a class="mark" href="#events_event_newlistener" id="events_event_newlistener">#</a></span></h4> 424<div class="api_metadata"> 425<span>Added in: v0.1.26</span> 426</div> 427<ul> 428<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The name of the event being listened for</li> 429<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The event handler function</li> 430</ul> 431<p>The <code>EventEmitter</code> instance will emit its own <code>'newListener'</code> event <em>before</em> 432a listener is added to its internal array of listeners.</p> 433<p>Listeners registered for the <code>'newListener'</code> event are passed the event 434name and a reference to the listener being added.</p> 435<p>The fact that the event is triggered before adding the listener has a subtle 436but important side effect: any <em>additional</em> listeners registered to the same 437<code>name</code> <em>within</em> the <code>'newListener'</code> callback are inserted <em>before</em> the 438listener that is in the process of being added.</p> 439<pre><code class="language-js"><span class="hljs-keyword">class</span> <span class="hljs-title class_">MyEmitter</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">EventEmitter</span> {} 440 441<span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MyEmitter</span>(); 442<span class="hljs-comment">// Only do this once so we don't loop forever</span> 443myEmitter.<span class="hljs-title function_">once</span>(<span class="hljs-string">'newListener'</span>, <span class="hljs-function">(<span class="hljs-params">event, listener</span>) =></span> { 444 <span class="hljs-keyword">if</span> (event === <span class="hljs-string">'event'</span>) { 445 <span class="hljs-comment">// Insert a new listener in front</span> 446 myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-function">() =></span> { 447 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'B'</span>); 448 }); 449 } 450}); 451myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-function">() =></span> { 452 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'A'</span>); 453}); 454myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>); 455<span class="hljs-comment">// Prints:</span> 456<span class="hljs-comment">// B</span> 457<span class="hljs-comment">// A</span></code></pre> 458<h4>Event: <code>'removeListener'</code><span><a class="mark" href="#events_event_removelistener" id="events_event_removelistener">#</a></span></h4> 459<div class="api_metadata"> 460<details class="changelog"><summary>History</summary> 461<table> 462<tbody><tr><th>Version</th><th>Changes</th></tr> 463<tr><td>v6.1.0, v4.7.0</td> 464<td><p>For listeners attached using <code>.once()</code>, the <code>listener</code> argument now yields the original listener function.</p></td></tr> 465<tr><td>v0.9.3</td> 466<td><p><span>Added in: v0.9.3</span></p></td></tr> 467</tbody></table> 468</details> 469</div> 470<ul> 471<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The event name</li> 472<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The event handler function</li> 473</ul> 474<p>The <code>'removeListener'</code> event is emitted <em>after</em> the <code>listener</code> is removed.</p> 475<h4><code>emitter.addListener(eventName, listener)</code><span><a class="mark" href="#events_emitter_addlistener_eventname_listener" id="events_emitter_addlistener_eventname_listener">#</a></span></h4> 476<div class="api_metadata"> 477<span>Added in: v0.1.26</span> 478</div> 479<ul> 480<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 481<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> 482</ul> 483<p>Alias for <code>emitter.on(eventName, listener)</code>.</p> 484<h4><code>emitter.emit(eventName[, ...args])</code><span><a class="mark" href="#events_emitter_emit_eventname_args" id="events_emitter_emit_eventname_args">#</a></span></h4> 485<div class="api_metadata"> 486<span>Added in: v0.1.26</span> 487</div> 488<ul> 489<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 490<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 491<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> 492</ul> 493<p>Synchronously calls each of the listeners registered for the event named 494<code>eventName</code>, in the order they were registered, passing the supplied arguments 495to each.</p> 496<p>Returns <code>true</code> if the event had listeners, <code>false</code> otherwise.</p> 497<pre><code class="language-js"><span class="hljs-keyword">const</span> <span class="hljs-title class_">EventEmitter</span> = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 498<span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 499 500<span class="hljs-comment">// First listener</span> 501myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-keyword">function</span> <span class="hljs-title function_">firstListener</span>(<span class="hljs-params"></span>) { 502 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'Helloooo! first listener'</span>); 503}); 504<span class="hljs-comment">// Second listener</span> 505myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-keyword">function</span> <span class="hljs-title function_">secondListener</span>(<span class="hljs-params">arg1, arg2</span>) { 506 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`event with parameters <span class="hljs-subst">${arg1}</span>, <span class="hljs-subst">${arg2}</span> in second listener`</span>); 507}); 508<span class="hljs-comment">// Third listener</span> 509myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-keyword">function</span> <span class="hljs-title function_">thirdListener</span>(<span class="hljs-params">...args</span>) { 510 <span class="hljs-keyword">const</span> parameters = args.<span class="hljs-title function_">join</span>(<span class="hljs-string">', '</span>); 511 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`event with parameters <span class="hljs-subst">${parameters}</span> in third listener`</span>); 512}); 513 514<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(myEmitter.<span class="hljs-title function_">listeners</span>(<span class="hljs-string">'event'</span>)); 515 516myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>, <span class="hljs-number">4</span>, <span class="hljs-number">5</span>); 517 518<span class="hljs-comment">// Prints:</span> 519<span class="hljs-comment">// [</span> 520<span class="hljs-comment">// [Function: firstListener],</span> 521<span class="hljs-comment">// [Function: secondListener],</span> 522<span class="hljs-comment">// [Function: thirdListener]</span> 523<span class="hljs-comment">// ]</span> 524<span class="hljs-comment">// Helloooo! first listener</span> 525<span class="hljs-comment">// event with parameters 1, 2 in second listener</span> 526<span class="hljs-comment">// event with parameters 1, 2, 3, 4, 5 in third listener</span></code></pre> 527<h4><code>emitter.eventNames()</code><span><a class="mark" href="#events_emitter_eventnames" id="events_emitter_eventnames">#</a></span></h4> 528<div class="api_metadata"> 529<span>Added in: v6.0.0</span> 530</div> 531<ul> 532<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array" class="type"><Array></a></li> 533</ul> 534<p>Returns an array listing the events for which the emitter has registered 535listeners. The values in the array are strings or <code>Symbol</code>s.</p> 536<pre><code class="language-js"><span class="hljs-keyword">const</span> <span class="hljs-title class_">EventEmitter</span> = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 537<span class="hljs-keyword">const</span> myEE = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 538myEE.<span class="hljs-title function_">on</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-function">() =></span> {}); 539myEE.<span class="hljs-title function_">on</span>(<span class="hljs-string">'bar'</span>, <span class="hljs-function">() =></span> {}); 540 541<span class="hljs-keyword">const</span> sym = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'symbol'</span>); 542myEE.<span class="hljs-title function_">on</span>(sym, <span class="hljs-function">() =></span> {}); 543 544<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(myEE.<span class="hljs-title function_">eventNames</span>()); 545<span class="hljs-comment">// Prints: [ 'foo', 'bar', Symbol(symbol) ]</span></code></pre> 546<h4><code>emitter.getMaxListeners()</code><span><a class="mark" href="#events_emitter_getmaxlisteners" id="events_emitter_getmaxlisteners">#</a></span></h4> 547<div class="api_metadata"> 548<span>Added in: v1.0.0</span> 549</div> 550<ul> 551<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li> 552</ul> 553<p>Returns the current max listener value for the <code>EventEmitter</code> which is either 554set by <a href="#events_emitter_setmaxlisteners_n"><code>emitter.setMaxListeners(n)</code></a> or defaults to 555<a href="#events_events_defaultmaxlisteners"><code>events.defaultMaxListeners</code></a>.</p> 556<h4><code>emitter.listenerCount(eventName)</code><span><a class="mark" href="#events_emitter_listenercount_eventname" id="events_emitter_listenercount_eventname">#</a></span></h4> 557<div class="api_metadata"> 558<span>Added in: v3.2.0</span> 559</div> 560<ul> 561<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The name of the event being listened for</li> 562<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li> 563</ul> 564<p>Returns the number of listeners listening to the event named <code>eventName</code>.</p> 565<h4><code>emitter.listeners(eventName)</code><span><a class="mark" href="#events_emitter_listeners_eventname" id="events_emitter_listeners_eventname">#</a></span></h4> 566<div class="api_metadata"> 567<details class="changelog"><summary>History</summary> 568<table> 569<tbody><tr><th>Version</th><th>Changes</th></tr> 570<tr><td>v7.0.0</td> 571<td><p>For listeners attached using <code>.once()</code> this returns the original listeners instead of wrapper functions now.</p></td></tr> 572<tr><td>v0.1.26</td> 573<td><p><span>Added in: v0.1.26</span></p></td></tr> 574</tbody></table> 575</details> 576</div> 577<ul> 578<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 579<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function[]></a></li> 580</ul> 581<p>Returns a copy of the array of listeners for the event named <code>eventName</code>.</p> 582<pre><code class="language-js">server.<span class="hljs-title function_">on</span>(<span class="hljs-string">'connection'</span>, <span class="hljs-function">(<span class="hljs-params">stream</span>) =></span> { 583 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'someone connected!'</span>); 584}); 585<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(util.<span class="hljs-title function_">inspect</span>(server.<span class="hljs-title function_">listeners</span>(<span class="hljs-string">'connection'</span>))); 586<span class="hljs-comment">// Prints: [ [Function] ]</span></code></pre> 587<h4><code>emitter.off(eventName, listener)</code><span><a class="mark" href="#events_emitter_off_eventname_listener" id="events_emitter_off_eventname_listener">#</a></span></h4> 588<div class="api_metadata"> 589<span>Added in: v10.0.0</span> 590</div> 591<ul> 592<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 593<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> 594<li>Returns: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 595</ul> 596<p>Alias for <a href="#events_emitter_removelistener_eventname_listener"><code>emitter.removeListener()</code></a>.</p> 597<h4><code>emitter.on(eventName, listener)</code><span><a class="mark" href="#events_emitter_on_eventname_listener" id="events_emitter_on_eventname_listener">#</a></span></h4> 598<div class="api_metadata"> 599<span>Added in: v0.1.101</span> 600</div> 601<ul> 602<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The name of the event.</li> 603<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The callback function</li> 604<li>Returns: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 605</ul> 606<p>Adds the <code>listener</code> function to the end of the listeners array for the 607event named <code>eventName</code>. No checks are made to see if the <code>listener</code> has 608already been added. Multiple calls passing the same combination of <code>eventName</code> 609and <code>listener</code> will result in the <code>listener</code> being added, and called, multiple 610times.</p> 611<pre><code class="language-js">server.<span class="hljs-title function_">on</span>(<span class="hljs-string">'connection'</span>, <span class="hljs-function">(<span class="hljs-params">stream</span>) =></span> { 612 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'someone connected!'</span>); 613});</code></pre> 614<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p> 615<p>By default, event listeners are invoked in the order they are added. The 616<code>emitter.prependListener()</code> method can be used as an alternative to add the 617event listener to the beginning of the listeners array.</p> 618<pre><code class="language-js"><span class="hljs-keyword">const</span> myEE = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 619myEE.<span class="hljs-title function_">on</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-function">() =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'a'</span>)); 620myEE.<span class="hljs-title function_">prependListener</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-function">() =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'b'</span>)); 621myEE.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'foo'</span>); 622<span class="hljs-comment">// Prints:</span> 623<span class="hljs-comment">// b</span> 624<span class="hljs-comment">// a</span></code></pre> 625<h4><code>emitter.once(eventName, listener)</code><span><a class="mark" href="#events_emitter_once_eventname_listener" id="events_emitter_once_eventname_listener">#</a></span></h4> 626<div class="api_metadata"> 627<span>Added in: v0.3.0</span> 628</div> 629<ul> 630<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The name of the event.</li> 631<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The callback function</li> 632<li>Returns: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 633</ul> 634<p>Adds a <strong>one-time</strong> <code>listener</code> function for the event named <code>eventName</code>. The 635next time <code>eventName</code> is triggered, this listener is removed and then invoked.</p> 636<pre><code class="language-js">server.<span class="hljs-title function_">once</span>(<span class="hljs-string">'connection'</span>, <span class="hljs-function">(<span class="hljs-params">stream</span>) =></span> { 637 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'Ah, we have our first user!'</span>); 638});</code></pre> 639<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p> 640<p>By default, event listeners are invoked in the order they are added. The 641<code>emitter.prependOnceListener()</code> method can be used as an alternative to add the 642event listener to the beginning of the listeners array.</p> 643<pre><code class="language-js"><span class="hljs-keyword">const</span> myEE = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 644myEE.<span class="hljs-title function_">once</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-function">() =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'a'</span>)); 645myEE.<span class="hljs-title function_">prependOnceListener</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-function">() =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'b'</span>)); 646myEE.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'foo'</span>); 647<span class="hljs-comment">// Prints:</span> 648<span class="hljs-comment">// b</span> 649<span class="hljs-comment">// a</span></code></pre> 650<h4><code>emitter.prependListener(eventName, listener)</code><span><a class="mark" href="#events_emitter_prependlistener_eventname_listener" id="events_emitter_prependlistener_eventname_listener">#</a></span></h4> 651<div class="api_metadata"> 652<span>Added in: v6.0.0</span> 653</div> 654<ul> 655<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The name of the event.</li> 656<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The callback function</li> 657<li>Returns: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 658</ul> 659<p>Adds the <code>listener</code> function to the <em>beginning</em> of the listeners array for the 660event named <code>eventName</code>. No checks are made to see if the <code>listener</code> has 661already been added. Multiple calls passing the same combination of <code>eventName</code> 662and <code>listener</code> will result in the <code>listener</code> being added, and called, multiple 663times.</p> 664<pre><code class="language-js">server.<span class="hljs-title function_">prependListener</span>(<span class="hljs-string">'connection'</span>, <span class="hljs-function">(<span class="hljs-params">stream</span>) =></span> { 665 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'someone connected!'</span>); 666});</code></pre> 667<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p> 668<h4><code>emitter.prependOnceListener(eventName, listener)</code><span><a class="mark" href="#events_emitter_prependoncelistener_eventname_listener" id="events_emitter_prependoncelistener_eventname_listener">#</a></span></h4> 669<div class="api_metadata"> 670<span>Added in: v6.0.0</span> 671</div> 672<ul> 673<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The name of the event.</li> 674<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The callback function</li> 675<li>Returns: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 676</ul> 677<p>Adds a <strong>one-time</strong> <code>listener</code> function for the event named <code>eventName</code> to the 678<em>beginning</em> of the listeners array. The next time <code>eventName</code> is triggered, this 679listener is removed, and then invoked.</p> 680<pre><code class="language-js">server.<span class="hljs-title function_">prependOnceListener</span>(<span class="hljs-string">'connection'</span>, <span class="hljs-function">(<span class="hljs-params">stream</span>) =></span> { 681 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'Ah, we have our first user!'</span>); 682});</code></pre> 683<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p> 684<h4><code>emitter.removeAllListeners([eventName])</code><span><a class="mark" href="#events_emitter_removealllisteners_eventname" id="events_emitter_removealllisteners_eventname">#</a></span></h4> 685<div class="api_metadata"> 686<span>Added in: v0.1.26</span> 687</div> 688<ul> 689<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 690<li>Returns: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 691</ul> 692<p>Removes all listeners, or those of the specified <code>eventName</code>.</p> 693<p>It is bad practice to remove listeners added elsewhere in the code, 694particularly when the <code>EventEmitter</code> instance was created by some other 695component or module (e.g. sockets or file streams).</p> 696<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p> 697<h4><code>emitter.removeListener(eventName, listener)</code><span><a class="mark" href="#events_emitter_removelistener_eventname_listener" id="events_emitter_removelistener_eventname_listener">#</a></span></h4> 698<div class="api_metadata"> 699<span>Added in: v0.1.26</span> 700</div> 701<ul> 702<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 703<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> 704<li>Returns: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 705</ul> 706<p>Removes the specified <code>listener</code> from the listener array for the event named 707<code>eventName</code>.</p> 708<pre><code class="language-js"><span class="hljs-keyword">const</span> <span class="hljs-title function_">callback</span> = (<span class="hljs-params">stream</span>) => { 709 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'someone connected!'</span>); 710}; 711server.<span class="hljs-title function_">on</span>(<span class="hljs-string">'connection'</span>, callback); 712<span class="hljs-comment">// ...</span> 713server.<span class="hljs-title function_">removeListener</span>(<span class="hljs-string">'connection'</span>, callback);</code></pre> 714<p><code>removeListener()</code> will remove, at most, one instance of a listener from the 715listener array. If any single listener has been added multiple times to the 716listener array for the specified <code>eventName</code>, then <code>removeListener()</code> must be 717called multiple times to remove each instance.</p> 718<p>Once an event is emitted, all listeners attached to it at the 719time of emitting are called in order. This implies that any 720<code>removeListener()</code> or <code>removeAllListeners()</code> calls <em>after</em> emitting and 721<em>before</em> the last listener finishes execution will not remove them from 722<code>emit()</code> in progress. Subsequent events behave as expected.</p> 723<pre><code class="language-js"><span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MyEmitter</span>(); 724 725<span class="hljs-keyword">const</span> <span class="hljs-title function_">callbackA</span> = (<span class="hljs-params"></span>) => { 726 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'A'</span>); 727 myEmitter.<span class="hljs-title function_">removeListener</span>(<span class="hljs-string">'event'</span>, callbackB); 728}; 729 730<span class="hljs-keyword">const</span> <span class="hljs-title function_">callbackB</span> = (<span class="hljs-params"></span>) => { 731 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'B'</span>); 732}; 733 734myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, callbackA); 735 736myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, callbackB); 737 738<span class="hljs-comment">// callbackA removes listener callbackB but it will still be called.</span> 739<span class="hljs-comment">// Internal listener array at time of emit [callbackA, callbackB]</span> 740myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>); 741<span class="hljs-comment">// Prints:</span> 742<span class="hljs-comment">// A</span> 743<span class="hljs-comment">// B</span> 744 745<span class="hljs-comment">// callbackB is now removed.</span> 746<span class="hljs-comment">// Internal listener array [callbackA]</span> 747myEmitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'event'</span>); 748<span class="hljs-comment">// Prints:</span> 749<span class="hljs-comment">// A</span></code></pre> 750<p>Because listeners are managed using an internal array, calling this will 751change the position indices of any listener registered <em>after</em> the listener 752being removed. This will not impact the order in which listeners are called, 753but it means that any copies of the listener array as returned by 754the <code>emitter.listeners()</code> method will need to be recreated.</p> 755<p>When a single function has been added as a handler multiple times for a single 756event (as in the example below), <code>removeListener()</code> will remove the most 757recently added instance. In the example the <code>once('ping')</code> 758listener is removed:</p> 759<pre><code class="language-js"><span class="hljs-keyword">const</span> ee = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 760 761<span class="hljs-keyword">function</span> <span class="hljs-title function_">pong</span>(<span class="hljs-params"></span>) { 762 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'pong'</span>); 763} 764 765ee.<span class="hljs-title function_">on</span>(<span class="hljs-string">'ping'</span>, pong); 766ee.<span class="hljs-title function_">once</span>(<span class="hljs-string">'ping'</span>, pong); 767ee.<span class="hljs-title function_">removeListener</span>(<span class="hljs-string">'ping'</span>, pong); 768 769ee.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'ping'</span>); 770ee.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'ping'</span>);</code></pre> 771<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p> 772<h4><code>emitter.setMaxListeners(n)</code><span><a class="mark" href="#events_emitter_setmaxlisteners_n" id="events_emitter_setmaxlisteners_n">#</a></span></h4> 773<div class="api_metadata"> 774<span>Added in: v0.3.5</span> 775</div> 776<ul> 777<li><code>n</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li> 778<li>Returns: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 779</ul> 780<p>By default <code>EventEmitter</code>s will print a warning if more than <code>10</code> listeners are 781added for a particular event. This is a useful default that helps finding 782memory leaks. The <code>emitter.setMaxListeners()</code> method allows the limit to be 783modified for this specific <code>EventEmitter</code> instance. The value can be set to 784<code>Infinity</code> (or <code>0</code>) to indicate an unlimited number of listeners.</p> 785<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p> 786<h4><code>emitter.rawListeners(eventName)</code><span><a class="mark" href="#events_emitter_rawlisteners_eventname" id="events_emitter_rawlisteners_eventname">#</a></span></h4> 787<div class="api_metadata"> 788<span>Added in: v9.4.0</span> 789</div> 790<ul> 791<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 792<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function[]></a></li> 793</ul> 794<p>Returns a copy of the array of listeners for the event named <code>eventName</code>, 795including any wrappers (such as those created by <code>.once()</code>).</p> 796<pre><code class="language-js"><span class="hljs-keyword">const</span> emitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 797emitter.<span class="hljs-title function_">once</span>(<span class="hljs-string">'log'</span>, <span class="hljs-function">() =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'log once'</span>)); 798 799<span class="hljs-comment">// Returns a new Array with a function `onceWrapper` which has a property</span> 800<span class="hljs-comment">// `listener` which contains the original listener bound above</span> 801<span class="hljs-keyword">const</span> listeners = emitter.<span class="hljs-title function_">rawListeners</span>(<span class="hljs-string">'log'</span>); 802<span class="hljs-keyword">const</span> logFnWrapper = listeners[<span class="hljs-number">0</span>]; 803 804<span class="hljs-comment">// Logs "log once" to the console and does not unbind the `once` event</span> 805logFnWrapper.<span class="hljs-title function_">listener</span>(); 806 807<span class="hljs-comment">// Logs "log once" to the console and removes the listener</span> 808<span class="hljs-title function_">logFnWrapper</span>(); 809 810emitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'log'</span>, <span class="hljs-function">() =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'log persistently'</span>)); 811<span class="hljs-comment">// Will return a new Array with a single function bound by `.on()` above</span> 812<span class="hljs-keyword">const</span> newListeners = emitter.<span class="hljs-title function_">rawListeners</span>(<span class="hljs-string">'log'</span>); 813 814<span class="hljs-comment">// Logs "log persistently" twice</span> 815newListeners[<span class="hljs-number">0</span>](); 816emitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'log'</span>);</code></pre> 817<h4><code>emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])</code><span><a class="mark" href="#events_emitter_symbol_for_nodejs_rejection_err_eventname_args" id="events_emitter_symbol_for_nodejs_rejection_err_eventname_args">#</a></span></h4> 818<div class="api_metadata"> 819<span>Added in: v13.4.0, v12.16.0</span> 820</div> 821<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - captureRejections is experimental.</div><p></p> 822<ul> 823<li><code>err</code> Error</li> 824<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 825<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 826</ul> 827<p>The <code>Symbol.for('nodejs.rejection')</code> method is called in case a 828promise rejection happens when emitting an event and 829<a href="#events_capture_rejections_of_promises"><code>captureRejections</code></a> is enabled on the emitter. 830It is possible to use <a href="#events_events_capturerejectionsymbol"><code>events.captureRejectionSymbol</code></a> in 831place of <code>Symbol.for('nodejs.rejection')</code>.</p> 832<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">EventEmitter</span>, captureRejectionSymbol } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 833 834<span class="hljs-keyword">class</span> <span class="hljs-title class_">MyClass</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">EventEmitter</span> { 835 <span class="hljs-title function_">constructor</span>(<span class="hljs-params"></span>) { 836 <span class="hljs-variable language_">super</span>({ <span class="hljs-attr">captureRejections</span>: <span class="hljs-literal">true</span> }); 837 } 838 839 [captureRejectionSymbol](err, event, ...args) { 840 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'rejection happened for'</span>, event, <span class="hljs-string">'with'</span>, err, ...args); 841 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">destroy</span>(err); 842 } 843 844 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">err</span>) { 845 <span class="hljs-comment">// Tear the resource down here.</span> 846 } 847}</code></pre> 848</section><section><h3><code>events.defaultMaxListeners</code><span><a class="mark" href="#events_events_defaultmaxlisteners" id="events_events_defaultmaxlisteners">#</a></span></h3> 849<div class="api_metadata"> 850<span>Added in: v0.11.2</span> 851</div> 852<p>By default, a maximum of <code>10</code> listeners can be registered for any single 853event. This limit can be changed for individual <code>EventEmitter</code> instances 854using the <a href="#events_emitter_setmaxlisteners_n"><code>emitter.setMaxListeners(n)</code></a> method. To change the default 855for <em>all</em> <code>EventEmitter</code> instances, the <code>events.defaultMaxListeners</code> 856property can be used. If this value is not a positive number, a <code>RangeError</code> 857is thrown.</p> 858<p>Take caution when setting the <code>events.defaultMaxListeners</code> because the 859change affects <em>all</em> <code>EventEmitter</code> instances, including those created before 860the change is made. However, calling <a href="#events_emitter_setmaxlisteners_n"><code>emitter.setMaxListeners(n)</code></a> still has 861precedence over <code>events.defaultMaxListeners</code>.</p> 862<p>This is not a hard limit. The <code>EventEmitter</code> instance will allow 863more listeners to be added but will output a trace warning to stderr indicating 864that a "possible EventEmitter memory leak" has been detected. For any single 865<code>EventEmitter</code>, the <code>emitter.getMaxListeners()</code> and <code>emitter.setMaxListeners()</code> 866methods can be used to temporarily avoid this warning:</p> 867<pre><code class="language-js">emitter.<span class="hljs-title function_">setMaxListeners</span>(emitter.<span class="hljs-title function_">getMaxListeners</span>() + <span class="hljs-number">1</span>); 868emitter.<span class="hljs-title function_">once</span>(<span class="hljs-string">'event'</span>, <span class="hljs-function">() =></span> { 869 <span class="hljs-comment">// do stuff</span> 870 emitter.<span class="hljs-title function_">setMaxListeners</span>(<span class="hljs-title class_">Math</span>.<span class="hljs-title function_">max</span>(emitter.<span class="hljs-title function_">getMaxListeners</span>() - <span class="hljs-number">1</span>, <span class="hljs-number">0</span>)); 871});</code></pre> 872<p>The <a href="cli.html#cli_trace_warnings"><code>--trace-warnings</code></a> command-line flag can be used to display the 873stack trace for such warnings.</p> 874<p>The emitted warning can be inspected with <a href="process.html#process_event_warning"><code>process.on('warning')</code></a> and will 875have the additional <code>emitter</code>, <code>type</code> and <code>count</code> properties, referring to 876the event emitter instance, the event’s name and the number of attached 877listeners, respectively. 878Its <code>name</code> property is set to <code>'MaxListenersExceededWarning'</code>.</p> 879</section><section><h3><code>events.errorMonitor</code><span><a class="mark" href="#events_events_errormonitor" id="events_events_errormonitor">#</a></span></h3> 880<div class="api_metadata"> 881<span>Added in: v13.6.0, v12.17.0</span> 882</div> 883<p>This symbol shall be used to install a listener for only monitoring <code>'error'</code> 884events. Listeners installed using this symbol are called before the regular 885<code>'error'</code> listeners are called.</p> 886<p>Installing a listener using this symbol does not change the behavior once an 887<code>'error'</code> event is emitted, therefore the process will still crash if no 888regular <code>'error'</code> listener is installed.</p> 889</section><section><h3><code>events.getEventListeners(emitterOrTarget, eventName)</code><span><a class="mark" href="#events_events_geteventlisteners_emitterortarget_eventname" id="events_events_geteventlisteners_emitterortarget_eventname">#</a></span></h3> 890<div class="api_metadata"> 891<span>Added in: v14.17.0</span> 892</div> 893<ul> 894<li><code>emitterOrTarget</code> <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a> | <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a></li> 895<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 896<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function[]></a></li> 897</ul> 898<p>Returns a copy of the array of listeners for the event named <code>eventName</code>.</p> 899<p>For <code>EventEmitter</code>s this behaves exactly the same as calling <code>.listeners</code> on 900the emitter.</p> 901<p>For <code>EventTarget</code>s this is the only way to get the event listeners for the 902event target. This is useful for debugging and diagnostic purposes.</p> 903<pre><code class="language-js"><span class="hljs-keyword">const</span> { getEventListeners, <span class="hljs-title class_">EventEmitter</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 904 905{ 906 <span class="hljs-keyword">const</span> ee = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 907 <span class="hljs-keyword">const</span> <span class="hljs-title function_">listener</span> = (<span class="hljs-params"></span>) => <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'Events are fun'</span>); 908 ee.<span class="hljs-title function_">on</span>(<span class="hljs-string">'foo'</span>, listener); 909 <span class="hljs-title function_">getEventListeners</span>(ee, <span class="hljs-string">'foo'</span>); <span class="hljs-comment">// [listener]</span> 910} 911{ 912 <span class="hljs-keyword">const</span> et = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventTarget</span>(); 913 <span class="hljs-keyword">const</span> <span class="hljs-title function_">listener</span> = (<span class="hljs-params"></span>) => <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'Events are fun'</span>); 914 et.<span class="hljs-title function_">addEventListener</span>(<span class="hljs-string">'foo'</span>, listener); 915 <span class="hljs-title function_">getEventListeners</span>(et, <span class="hljs-string">'foo'</span>); <span class="hljs-comment">// [listener]</span> 916}</code></pre> 917</section><section><h3><code>events.once(emitter, name[, options])</code><span><a class="mark" href="#events_events_once_emitter_name_options" id="events_events_once_emitter_name_options">#</a></span></h3> 918<div class="api_metadata"> 919<details class="changelog"><summary>History</summary> 920<table> 921<tbody><tr><th>Version</th><th>Changes</th></tr> 922<tr><td>v14.17.0</td> 923<td><p>The <code>signal</code> option is supported now.</p></td></tr> 924<tr><td>v11.13.0, v10.16.0</td> 925<td><p><span>Added in: v11.13.0, v10.16.0</span></p></td></tr> 926</tbody></table> 927</details> 928</div> 929<ul> 930<li><code>emitter</code> <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 931<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> 932<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 933<ul> 934<li><code>signal</code> <a href="globals.html#globals_class_abortsignal" class="type"><AbortSignal></a> Can be used to cancel waiting for the event.</li> 935</ul> 936</li> 937<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a></li> 938</ul> 939<p>Creates a <code>Promise</code> that is fulfilled when the <code>EventEmitter</code> emits the given 940event or that is rejected if the <code>EventEmitter</code> emits <code>'error'</code> while waiting. 941The <code>Promise</code> will resolve with an array of all the arguments emitted to the 942given event.</p> 943<p>This method is intentionally generic and works with the web platform 944<a href="https://dom.spec.whatwg.org/#interface-eventtarget">EventTarget</a> interface, which has no special 945<code>'error'</code> event semantics and does not listen to the <code>'error'</code> event.</p> 946<pre><code class="language-js"><span class="hljs-keyword">const</span> { once, <span class="hljs-title class_">EventEmitter</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 947 948<span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">run</span>(<span class="hljs-params"></span>) { 949 <span class="hljs-keyword">const</span> ee = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 950 951 process.<span class="hljs-title function_">nextTick</span>(<span class="hljs-function">() =></span> { 952 ee.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'myevent'</span>, <span class="hljs-number">42</span>); 953 }); 954 955 <span class="hljs-keyword">const</span> [value] = <span class="hljs-keyword">await</span> <span class="hljs-title function_">once</span>(ee, <span class="hljs-string">'myevent'</span>); 956 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(value); 957 958 <span class="hljs-keyword">const</span> err = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(<span class="hljs-string">'kaboom'</span>); 959 process.<span class="hljs-title function_">nextTick</span>(<span class="hljs-function">() =></span> { 960 ee.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, err); 961 }); 962 963 <span class="hljs-keyword">try</span> { 964 <span class="hljs-keyword">await</span> <span class="hljs-title function_">once</span>(ee, <span class="hljs-string">'myevent'</span>); 965 } <span class="hljs-keyword">catch</span> (err) { 966 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'error happened'</span>, err); 967 } 968} 969 970<span class="hljs-title function_">run</span>();</code></pre> 971<p>The special handling of the <code>'error'</code> event is only used when <code>events.once()</code> 972is used to wait for another event. If <code>events.once()</code> is used to wait for the 973'<code>error'</code> event itself, then it is treated as any other kind of event without 974special handling:</p> 975<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">EventEmitter</span>, once } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 976 977<span class="hljs-keyword">const</span> ee = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 978 979<span class="hljs-title function_">once</span>(ee, <span class="hljs-string">'error'</span>) 980 .<span class="hljs-title function_">then</span>(<span class="hljs-function">(<span class="hljs-params">[err]</span>) =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'ok'</span>, err.<span class="hljs-property">message</span>)) 981 .<span class="hljs-title function_">catch</span>(<span class="hljs-function">(<span class="hljs-params">err</span>) =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'error'</span>, err.<span class="hljs-property">message</span>)); 982 983ee.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(<span class="hljs-string">'boom'</span>)); 984 985<span class="hljs-comment">// Prints: ok boom</span></code></pre> 986<p>An <a href="globals.html#globals_class_abortsignal" class="type"><AbortSignal></a> can be used to cancel waiting for the event:</p> 987<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">EventEmitter</span>, once } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 988 989<span class="hljs-keyword">const</span> ee = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 990<span class="hljs-keyword">const</span> ac = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AbortController</span>(); 991 992<span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">foo</span>(<span class="hljs-params">emitter, event, signal</span>) { 993 <span class="hljs-keyword">try</span> { 994 <span class="hljs-keyword">await</span> <span class="hljs-title function_">once</span>(emitter, event, { signal }); 995 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'event emitted!'</span>); 996 } <span class="hljs-keyword">catch</span> (error) { 997 <span class="hljs-keyword">if</span> (error.<span class="hljs-property">name</span> === <span class="hljs-string">'AbortError'</span>) { 998 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'Waiting for the event was canceled!'</span>); 999 } <span class="hljs-keyword">else</span> { 1000 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'There was an error'</span>, error.<span class="hljs-property">message</span>); 1001 } 1002 } 1003} 1004 1005<span class="hljs-title function_">foo</span>(ee, <span class="hljs-string">'foo'</span>, ac.<span class="hljs-property">signal</span>); 1006ac.<span class="hljs-title function_">abort</span>(); <span class="hljs-comment">// Abort waiting for the event</span> 1007ee.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'foo'</span>); <span class="hljs-comment">// Prints: Waiting for the event was canceled!</span></code></pre> 1008<h4>Awaiting multiple events emitted on <code>process.nextTick()</code><span><a class="mark" href="#events_awaiting_multiple_events_emitted_on_process_nexttick" id="events_awaiting_multiple_events_emitted_on_process_nexttick">#</a></span></h4> 1009<p>There is an edge case worth noting when using the <code>events.once()</code> function 1010to await multiple events emitted on in the same batch of <code>process.nextTick()</code> 1011operations, or whenever multiple events are emitted synchronously. Specifically, 1012because the <code>process.nextTick()</code> queue is drained before the <code>Promise</code> microtask 1013queue, and because <code>EventEmitter</code> emits all events synchronously, it is possible 1014for <code>events.once()</code> to miss an event.</p> 1015<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">EventEmitter</span>, once } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 1016 1017<span class="hljs-keyword">const</span> myEE = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 1018 1019<span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">foo</span>(<span class="hljs-params"></span>) { 1020 <span class="hljs-keyword">await</span> <span class="hljs-title function_">once</span>(myEE, <span class="hljs-string">'bar'</span>); 1021 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'bar'</span>); 1022 1023 <span class="hljs-comment">// This Promise will never resolve because the 'foo' event will</span> 1024 <span class="hljs-comment">// have already been emitted before the Promise is created.</span> 1025 <span class="hljs-keyword">await</span> <span class="hljs-title function_">once</span>(myEE, <span class="hljs-string">'foo'</span>); 1026 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'foo'</span>); 1027} 1028 1029process.<span class="hljs-title function_">nextTick</span>(<span class="hljs-function">() =></span> { 1030 myEE.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'bar'</span>); 1031 myEE.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'foo'</span>); 1032}); 1033 1034<span class="hljs-title function_">foo</span>().<span class="hljs-title function_">then</span>(<span class="hljs-function">() =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'done'</span>));</code></pre> 1035<p>To catch both events, create each of the Promises <em>before</em> awaiting either 1036of them, then it becomes possible to use <code>Promise.all()</code>, <code>Promise.race()</code>, 1037or <code>Promise.allSettled()</code>:</p> 1038<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">EventEmitter</span>, once } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 1039 1040<span class="hljs-keyword">const</span> myEE = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 1041 1042<span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">foo</span>(<span class="hljs-params"></span>) { 1043 <span class="hljs-keyword">await</span> <span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">all</span>([<span class="hljs-title function_">once</span>(myEE, <span class="hljs-string">'bar'</span>), <span class="hljs-title function_">once</span>(myEE, <span class="hljs-string">'foo'</span>)]); 1044 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-string">'bar'</span>); 1045} 1046 1047process.<span class="hljs-title function_">nextTick</span>(<span class="hljs-function">() =></span> { 1048 myEE.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'bar'</span>); 1049 myEE.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'foo'</span>); 1050}); 1051 1052<span class="hljs-title function_">foo</span>().<span class="hljs-title function_">then</span>(<span class="hljs-function">() =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'done'</span>));</code></pre> 1053</section><section><h3><code>events.captureRejections</code><span><a class="mark" href="#events_events_capturerejections" id="events_events_capturerejections">#</a></span></h3> 1054<div class="api_metadata"> 1055<span>Added in: v13.4.0, v12.16.0</span> 1056</div> 1057<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - captureRejections is experimental.</div><p></p> 1058<p>Value: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></p> 1059<p>Change the default <code>captureRejections</code> option on all new <code>EventEmitter</code> objects.</p> 1060</section><section><h3><code>events.captureRejectionSymbol</code><span><a class="mark" href="#events_events_capturerejectionsymbol" id="events_events_capturerejectionsymbol">#</a></span></h3> 1061<div class="api_metadata"> 1062<span>Added in: v13.4.0, v12.16.0</span> 1063</div> 1064<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - captureRejections is experimental.</div><p></p> 1065<p>Value: <code>Symbol.for('nodejs.rejection')</code></p> 1066<p>See how to write a custom <a href="#events_emitter_symbol_for_nodejs_rejection_err_eventname_args">rejection handler</a>.</p> 1067</section><section><h3><code>events.listenerCount(emitter, eventName)</code><span><a class="mark" href="#events_events_listenercount_emitter_eventname" id="events_events_listenercount_emitter_eventname">#</a></span></h3> 1068<div class="api_metadata"> 1069<span>Added in: v0.9.12</span><span>Deprecated since: v3.2.0</span> 1070</div> 1071<p></p><div class="api_stability api_stability_0"><a href="documentation.html#documentation_stability_index">Stability: 0</a> - Deprecated: Use <a href="#events_emitter_listenercount_eventname"><code>emitter.listenerCount()</code></a> instead.</div><p></p> 1072<ul> 1073<li><code>emitter</code> <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a> The emitter to query</li> 1074<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The event name</li> 1075</ul> 1076<p>A class method that returns the number of listeners for the given <code>eventName</code> 1077registered on the given <code>emitter</code>.</p> 1078<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">EventEmitter</span>, listenerCount } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 1079<span class="hljs-keyword">const</span> myEmitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 1080myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-function">() =></span> {}); 1081myEmitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'event'</span>, <span class="hljs-function">() =></span> {}); 1082<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">listenerCount</span>(myEmitter, <span class="hljs-string">'event'</span>)); 1083<span class="hljs-comment">// Prints: 2</span></code></pre> 1084</section><section><h3><code>events.on(emitter, eventName[, options])</code><span><a class="mark" href="#events_events_on_emitter_eventname_options" id="events_events_on_emitter_eventname_options">#</a></span></h3> 1085<div class="api_metadata"> 1086<span>Added in: v13.6.0, v12.16.0</span> 1087</div> 1088<ul> 1089<li><code>emitter</code> <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 1090<li><code>eventName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The name of the event being listened for</li> 1091<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 1092<ul> 1093<li><code>signal</code> <a href="globals.html#globals_class_abortsignal" class="type"><AbortSignal></a> Can be used to cancel awaiting events.</li> 1094</ul> 1095</li> 1096<li>Returns: <a href="https://tc39.github.io/ecma262/#sec-asynciterator-interface" class="type"><AsyncIterator></a> that iterates <code>eventName</code> events emitted by the <code>emitter</code></li> 1097</ul> 1098<pre><code class="language-js"><span class="hljs-keyword">const</span> { on, <span class="hljs-title class_">EventEmitter</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 1099 1100(<span class="hljs-keyword">async</span> () => { 1101 <span class="hljs-keyword">const</span> ee = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 1102 1103 <span class="hljs-comment">// Emit later on</span> 1104 process.<span class="hljs-title function_">nextTick</span>(<span class="hljs-function">() =></span> { 1105 ee.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-string">'bar'</span>); 1106 ee.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-number">42</span>); 1107 }); 1108 1109 <span class="hljs-keyword">for</span> <span class="hljs-keyword">await</span> (<span class="hljs-keyword">const</span> event <span class="hljs-keyword">of</span> <span class="hljs-title function_">on</span>(ee, <span class="hljs-string">'foo'</span>)) { 1110 <span class="hljs-comment">// The execution of this inner block is synchronous and it</span> 1111 <span class="hljs-comment">// processes one event at a time (even with await). Do not use</span> 1112 <span class="hljs-comment">// if concurrent execution is required.</span> 1113 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(event); <span class="hljs-comment">// prints ['bar'] [42]</span> 1114 } 1115 <span class="hljs-comment">// Unreachable here</span> 1116})();</code></pre> 1117<p>Returns an <code>AsyncIterator</code> that iterates <code>eventName</code> events. It will throw 1118if the <code>EventEmitter</code> emits <code>'error'</code>. It removes all listeners when 1119exiting the loop. The <code>value</code> returned by each iteration is an array 1120composed of the emitted event arguments.</p> 1121<p>An <a href="globals.html#globals_class_abortsignal" class="type"><AbortSignal></a> can be used to cancel waiting on events:</p> 1122<pre><code class="language-js"><span class="hljs-keyword">const</span> { on, <span class="hljs-title class_">EventEmitter</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 1123<span class="hljs-keyword">const</span> ac = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AbortController</span>(); 1124 1125(<span class="hljs-keyword">async</span> () => { 1126 <span class="hljs-keyword">const</span> ee = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 1127 1128 <span class="hljs-comment">// Emit later on</span> 1129 process.<span class="hljs-title function_">nextTick</span>(<span class="hljs-function">() =></span> { 1130 ee.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-string">'bar'</span>); 1131 ee.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-number">42</span>); 1132 }); 1133 1134 <span class="hljs-keyword">for</span> <span class="hljs-keyword">await</span> (<span class="hljs-keyword">const</span> event <span class="hljs-keyword">of</span> <span class="hljs-title function_">on</span>(ee, <span class="hljs-string">'foo'</span>, { <span class="hljs-attr">signal</span>: ac.<span class="hljs-property">signal</span> })) { 1135 <span class="hljs-comment">// The execution of this inner block is synchronous and it</span> 1136 <span class="hljs-comment">// processes one event at a time (even with await). Do not use</span> 1137 <span class="hljs-comment">// if concurrent execution is required.</span> 1138 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(event); <span class="hljs-comment">// prints ['bar'] [42]</span> 1139 } 1140 <span class="hljs-comment">// Unreachable here</span> 1141})(); 1142 1143process.<span class="hljs-title function_">nextTick</span>(<span class="hljs-function">() =></span> ac.<span class="hljs-title function_">abort</span>());</code></pre> 1144</section><section><h3><code>events.setMaxListeners(n[, ...eventTargets])</code><span><a class="mark" href="#events_events_setmaxlisteners_n_eventtargets" id="events_events_setmaxlisteners_n_eventtargets">#</a></span></h3> 1145<div class="api_metadata"> 1146<span>Added in: v14.17.0</span> 1147</div> 1148<ul> 1149<li><code>n</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> A non-negative number. The maximum number of listeners per 1150<code>EventTarget</code> event.</li> 1151<li><code>...eventsTargets</code> <a href="events.html#events_class_eventtarget" class="type"><EventTarget[]></a> | <a href="events.html#events_class_eventemitter" class="type"><EventEmitter[]></a> Zero or more <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> 1152or <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a> instances. If none are specified, <code>n</code> is set as the default 1153max for all newly created <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> and <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a> objects.</li> 1154</ul> 1155<pre><code class="language-js"><span class="hljs-keyword">const</span> { 1156 setMaxListeners, 1157 <span class="hljs-title class_">EventEmitter</span> 1158} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 1159 1160<span class="hljs-keyword">const</span> target = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventTarget</span>(); 1161<span class="hljs-keyword">const</span> emitter = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>(); 1162 1163<span class="hljs-title function_">setMaxListeners</span>(<span class="hljs-number">5</span>, target, emitter);</code></pre> 1164<p><a id="event-target-and-event-api"></a></p> 1165</section><section><h3><code>EventTarget</code> and <code>Event</code> API<span><a class="mark" href="#events_eventtarget_and_event_api" id="events_eventtarget_and_event_api">#</a></span></h3> 1166<div class="api_metadata"> 1167<span>Added in: v14.5.0</span> 1168</div> 1169<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p> 1170<p>The <code>EventTarget</code> and <code>Event</code> objects are a Node.js-specific implementation 1171of the <a href="https://dom.spec.whatwg.org/#eventtarget"><code>EventTarget</code> Web API</a> that are exposed by some Node.js core APIs. 1172Neither the <code>EventTarget</code> nor <code>Event</code> classes are available for end 1173user code to create.</p> 1174<pre><code class="language-js"><span class="hljs-keyword">const</span> target = <span class="hljs-title function_">getEventTargetSomehow</span>(); 1175 1176target.<span class="hljs-title function_">addEventListener</span>(<span class="hljs-string">'foo'</span>, <span class="hljs-function">(<span class="hljs-params">event</span>) =></span> { 1177 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'foo event happened!'</span>); 1178});</code></pre> 1179<h4>Node.js <code>EventTarget</code> vs. DOM <code>EventTarget</code><span><a class="mark" href="#events_node_js_eventtarget_vs_dom_eventtarget" id="events_node_js_eventtarget_vs_dom_eventtarget">#</a></span></h4> 1180<p>There are two key differences between the Node.js <code>EventTarget</code> and the 1181<a href="https://dom.spec.whatwg.org/#eventtarget"><code>EventTarget</code> Web API</a>:</p> 1182<ol> 1183<li>Whereas DOM <code>EventTarget</code> instances <em>may</em> be hierarchical, there is no 1184concept of hierarchy and event propagation in Node.js. That is, an event 1185dispatched to an <code>EventTarget</code> does not propagate through a hierarchy of 1186nested target objects that may each have their own set of handlers for the 1187event.</li> 1188<li>In the Node.js <code>EventTarget</code>, if an event listener is an async function 1189or returns a <code>Promise</code>, and the returned <code>Promise</code> rejects, the rejection 1190is automatically captured and handled the same way as a listener that 1191throws synchronously (see <a href="#events_eventtarget_error_handling"><code>EventTarget</code> error handling</a> for details).</li> 1192</ol> 1193<h4><code>NodeEventTarget</code> vs. <code>EventEmitter</code><span><a class="mark" href="#events_nodeeventtarget_vs_eventemitter" id="events_nodeeventtarget_vs_eventemitter">#</a></span></h4> 1194<p>The <code>NodeEventTarget</code> object implements a modified subset of the 1195<code>EventEmitter</code> API that allows it to closely <em>emulate</em> an <code>EventEmitter</code> in 1196certain situations. A <code>NodeEventTarget</code> is <em>not</em> an instance of <code>EventEmitter</code> 1197and cannot be used in place of an <code>EventEmitter</code> in most cases.</p> 1198<ol> 1199<li>Unlike <code>EventEmitter</code>, any given <code>listener</code> can be registered at most once 1200per event <code>type</code>. Attempts to register a <code>listener</code> multiple times are 1201ignored.</li> 1202<li>The <code>NodeEventTarget</code> does not emulate the full <code>EventEmitter</code> API. 1203Specifically the <code>prependListener()</code>, <code>prependOnceListener()</code>, 1204<code>rawListeners()</code>, <code>setMaxListeners()</code>, <code>getMaxListeners()</code>, and 1205<code>errorMonitor</code> APIs are not emulated. The <code>'newListener'</code> and 1206<code>'removeListener'</code> events will also not be emitted.</li> 1207<li>The <code>NodeEventTarget</code> does not implement any special default behavior 1208for events with type <code>'error'</code>.</li> 1209<li>The <code>NodeEventTarget</code> supports <code>EventListener</code> objects as well as 1210functions as handlers for all event types.</li> 1211</ol> 1212<h4>Event listener<span><a class="mark" href="#events_event_listener" id="events_event_listener">#</a></span></h4> 1213<p>Event listeners registered for an event <code>type</code> may either be JavaScript 1214functions or objects with a <code>handleEvent</code> property whose value is a function.</p> 1215<p>In either case, the handler function is invoked with the <code>event</code> argument 1216passed to the <code>eventTarget.dispatchEvent()</code> function.</p> 1217<p>Async functions may be used as event listeners. If an async handler function 1218rejects, the rejection is captured and handled as described in 1219<a href="#events_eventtarget_error_handling"><code>EventTarget</code> error handling</a>.</p> 1220<p>An error thrown by one handler function does not prevent the other handlers 1221from being invoked.</p> 1222<p>The return value of a handler function is ignored.</p> 1223<p>Handlers are always invoked in the order they were added.</p> 1224<p>Handler functions may mutate the <code>event</code> object.</p> 1225<pre><code class="language-js"><span class="hljs-keyword">function</span> <span class="hljs-title function_">handler1</span>(<span class="hljs-params">event</span>) { 1226 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(event.<span class="hljs-property">type</span>); <span class="hljs-comment">// Prints 'foo'</span> 1227 event.<span class="hljs-property">a</span> = <span class="hljs-number">1</span>; 1228} 1229 1230<span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">handler2</span>(<span class="hljs-params">event</span>) { 1231 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(event.<span class="hljs-property">type</span>); <span class="hljs-comment">// Prints 'foo'</span> 1232 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(event.<span class="hljs-property">a</span>); <span class="hljs-comment">// Prints 1</span> 1233} 1234 1235<span class="hljs-keyword">const</span> handler3 = { 1236 <span class="hljs-title function_">handleEvent</span>(<span class="hljs-params">event</span>) { 1237 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(event.<span class="hljs-property">type</span>); <span class="hljs-comment">// Prints 'foo'</span> 1238 } 1239}; 1240 1241<span class="hljs-keyword">const</span> handler4 = { 1242 <span class="hljs-keyword">async</span> <span class="hljs-title function_">handleEvent</span>(<span class="hljs-params">event</span>) { 1243 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(event.<span class="hljs-property">type</span>); <span class="hljs-comment">// Prints 'foo'</span> 1244 } 1245}; 1246 1247<span class="hljs-keyword">const</span> target = <span class="hljs-title function_">getEventTargetSomehow</span>(); 1248 1249target.<span class="hljs-title function_">addEventListener</span>(<span class="hljs-string">'foo'</span>, handler1); 1250target.<span class="hljs-title function_">addEventListener</span>(<span class="hljs-string">'foo'</span>, handler2); 1251target.<span class="hljs-title function_">addEventListener</span>(<span class="hljs-string">'foo'</span>, handler3); 1252target.<span class="hljs-title function_">addEventListener</span>(<span class="hljs-string">'foo'</span>, handler4, { <span class="hljs-attr">once</span>: <span class="hljs-literal">true</span> });</code></pre> 1253<h4><code>EventTarget</code> error handling<span><a class="mark" href="#events_eventtarget_error_handling" id="events_eventtarget_error_handling">#</a></span></h4> 1254<p>When a registered event listener throws (or returns a Promise that rejects), 1255by default the error is treated as an uncaught exception on 1256<code>process.nextTick()</code>. This means uncaught exceptions in <code>EventTarget</code>s will 1257terminate the Node.js process by default.</p> 1258<p>Throwing within an event listener will <em>not</em> stop the other registered handlers 1259from being invoked.</p> 1260<p>The <code>EventTarget</code> does not implement any special default handling for <code>'error'</code> 1261type events like <code>EventEmitter</code>.</p> 1262<p>Currently errors are first forwarded to the <code>process.on('error')</code> event 1263before reaching <code>process.on('uncaughtException')</code>. This behavior is 1264deprecated and will change in a future release to align <code>EventTarget</code> with 1265other Node.js APIs. Any code relying on the <code>process.on('error')</code> event should 1266be aligned with the new behavior.</p> 1267<h4>Class: <code>Event</code><span><a class="mark" href="#events_class_event" id="events_class_event">#</a></span></h4> 1268<div class="api_metadata"> 1269<span>Added in: v14.5.0</span> 1270</div> 1271<p>The <code>Event</code> object is an adaptation of the <a href="https://dom.spec.whatwg.org/#event"><code>Event</code> Web API</a>. Instances 1272are created internally by Node.js.</p> 1273<h5><code>event.bubbles</code><span><a class="mark" href="#events_event_bubbles" id="events_event_bubbles">#</a></span></h5> 1274<div class="api_metadata"> 1275<span>Added in: v14.5.0</span> 1276</div> 1277<ul> 1278<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> Always returns <code>false</code>.</li> 1279</ul> 1280<p>This is not used in Node.js and is provided purely for completeness.</p> 1281<h5><code>event.cancelBubble()</code><span><a class="mark" href="#events_event_cancelbubble" id="events_event_cancelbubble">#</a></span></h5> 1282<div class="api_metadata"> 1283<span>Added in: v14.5.0</span> 1284</div> 1285<p>Alias for <code>event.stopPropagation()</code>. This is not used in Node.js and is 1286provided purely for completeness.</p> 1287<h5><code>event.cancelable</code><span><a class="mark" href="#events_event_cancelable" id="events_event_cancelable">#</a></span></h5> 1288<div class="api_metadata"> 1289<span>Added in: v14.5.0</span> 1290</div> 1291<ul> 1292<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> True if the event was created with the <code>cancelable</code> option.</li> 1293</ul> 1294<h5><code>event.composed</code><span><a class="mark" href="#events_event_composed" id="events_event_composed">#</a></span></h5> 1295<div class="api_metadata"> 1296<span>Added in: v14.5.0</span> 1297</div> 1298<ul> 1299<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> Always returns <code>false</code>.</li> 1300</ul> 1301<p>This is not used in Node.js and is provided purely for completeness.</p> 1302<h5><code>event.composedPath()</code><span><a class="mark" href="#events_event_composedpath" id="events_event_composedpath">#</a></span></h5> 1303<div class="api_metadata"> 1304<span>Added in: v14.5.0</span> 1305</div> 1306<p>Returns an array containing the current <code>EventTarget</code> as the only entry or 1307empty if the event is not being dispatched. This is not used in 1308Node.js and is provided purely for completeness.</p> 1309<h5><code>event.currentTarget</code><span><a class="mark" href="#events_event_currenttarget" id="events_event_currenttarget">#</a></span></h5> 1310<div class="api_metadata"> 1311<span>Added in: v14.5.0</span> 1312</div> 1313<ul> 1314<li>Type: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> The <code>EventTarget</code> dispatching the event.</li> 1315</ul> 1316<p>Alias for <code>event.target</code>.</p> 1317<h5><code>event.defaultPrevented</code><span><a class="mark" href="#events_event_defaultprevented" id="events_event_defaultprevented">#</a></span></h5> 1318<div class="api_metadata"> 1319<span>Added in: v14.5.0</span> 1320</div> 1321<ul> 1322<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> 1323</ul> 1324<p>Is <code>true</code> if <code>cancelable</code> is <code>true</code> and <code>event.preventDefault()</code> has been 1325called.</p> 1326<h5><code>event.eventPhase</code><span><a class="mark" href="#events_event_eventphase" id="events_event_eventphase">#</a></span></h5> 1327<div class="api_metadata"> 1328<span>Added in: v14.5.0</span> 1329</div> 1330<ul> 1331<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> Returns <code>0</code> while an event is not being dispatched, <code>2</code> while 1332it is being dispatched.</li> 1333</ul> 1334<p>This is not used in Node.js and is provided purely for completeness.</p> 1335<h5><code>event.isTrusted</code><span><a class="mark" href="#events_event_istrusted" id="events_event_istrusted">#</a></span></h5> 1336<div class="api_metadata"> 1337<span>Added in: v14.5.0</span> 1338</div> 1339<ul> 1340<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> 1341</ul> 1342<p>The <a href="globals.html#globals_class_abortsignal" class="type"><AbortSignal></a> <code>"abort"</code> event is emitted with <code>isTrusted</code> set to <code>true</code>. The 1343value is <code>false</code> in all other cases.</p> 1344<h5><code>event.preventDefault()</code><span><a class="mark" href="#events_event_preventdefault" id="events_event_preventdefault">#</a></span></h5> 1345<div class="api_metadata"> 1346<span>Added in: v14.5.0</span> 1347</div> 1348<p>Sets the <code>defaultPrevented</code> property to <code>true</code> if <code>cancelable</code> is <code>true</code>.</p> 1349<h5><code>event.returnValue</code><span><a class="mark" href="#events_event_returnvalue" id="events_event_returnvalue">#</a></span></h5> 1350<div class="api_metadata"> 1351<span>Added in: v14.5.0</span> 1352</div> 1353<ul> 1354<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> True if the event has not been canceled.</li> 1355</ul> 1356<p>This is not used in Node.js and is provided purely for completeness.</p> 1357<h5><code>event.srcElement</code><span><a class="mark" href="#events_event_srcelement" id="events_event_srcelement">#</a></span></h5> 1358<div class="api_metadata"> 1359<span>Added in: v14.5.0</span> 1360</div> 1361<ul> 1362<li>Type: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> The <code>EventTarget</code> dispatching the event.</li> 1363</ul> 1364<p>Alias for <code>event.target</code>.</p> 1365<h5><code>event.stopImmediatePropagation()</code><span><a class="mark" href="#events_event_stopimmediatepropagation" id="events_event_stopimmediatepropagation">#</a></span></h5> 1366<div class="api_metadata"> 1367<span>Added in: v14.5.0</span> 1368</div> 1369<p>Stops the invocation of event listeners after the current one completes.</p> 1370<h5><code>event.stopPropagation()</code><span><a class="mark" href="#events_event_stoppropagation" id="events_event_stoppropagation">#</a></span></h5> 1371<div class="api_metadata"> 1372<span>Added in: v14.5.0</span> 1373</div> 1374<p>This is not used in Node.js and is provided purely for completeness.</p> 1375<h5><code>event.target</code><span><a class="mark" href="#events_event_target" id="events_event_target">#</a></span></h5> 1376<div class="api_metadata"> 1377<span>Added in: v14.5.0</span> 1378</div> 1379<ul> 1380<li>Type: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> The <code>EventTarget</code> dispatching the event.</li> 1381</ul> 1382<h5><code>event.timeStamp</code><span><a class="mark" href="#events_event_timestamp" id="events_event_timestamp">#</a></span></h5> 1383<div class="api_metadata"> 1384<span>Added in: v14.5.0</span> 1385</div> 1386<ul> 1387<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 1388</ul> 1389<p>The millisecond timestamp when the <code>Event</code> object was created.</p> 1390<h5><code>event.type</code><span><a class="mark" href="#events_event_type" id="events_event_type">#</a></span></h5> 1391<div class="api_metadata"> 1392<span>Added in: v14.5.0</span> 1393</div> 1394<ul> 1395<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> 1396</ul> 1397<p>The event type identifier.</p> 1398<h4>Class: <code>EventTarget</code><span><a class="mark" href="#events_class_eventtarget" id="events_class_eventtarget">#</a></span></h4> 1399<div class="api_metadata"> 1400<span>Added in: v14.5.0</span> 1401</div> 1402<h5><code>eventTarget.addEventListener(type, listener[, options])</code><span><a class="mark" href="#events_eventtarget_addeventlistener_type_listener_options" id="events_eventtarget_addeventlistener_type_listener_options">#</a></span></h5> 1403<div class="api_metadata"> 1404<span>Added in: v14.5.0</span> 1405</div> 1406<ul> 1407<li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> 1408<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="events.html#events_event_listener" class="type"><EventListener></a></li> 1409<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 1410<ul> 1411<li><code>once</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> When <code>true</code>, the listener is automatically removed 1412when it is first invoked. <strong>Default:</strong> <code>false</code>.</li> 1413<li><code>passive</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> When <code>true</code>, serves as a hint that the listener will 1414not call the <code>Event</code> object's <code>preventDefault()</code> method. 1415<strong>Default:</strong> <code>false</code>.</li> 1416<li><code>capture</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> Not directly used by Node.js. Added for API 1417completeness. <strong>Default:</strong> <code>false</code>.</li> 1418</ul> 1419</li> 1420</ul> 1421<p>Adds a new handler for the <code>type</code> event. Any given <code>listener</code> is added 1422only once per <code>type</code> and per <code>capture</code> option value.</p> 1423<p>If the <code>once</code> option is <code>true</code>, the <code>listener</code> is removed after the 1424next time a <code>type</code> event is dispatched.</p> 1425<p>The <code>capture</code> option is not used by Node.js in any functional way other than 1426tracking registered event listeners per the <code>EventTarget</code> specification. 1427Specifically, the <code>capture</code> option is used as part of the key when registering 1428a <code>listener</code>. Any individual <code>listener</code> may be added once with 1429<code>capture = false</code>, and once with <code>capture = true</code>.</p> 1430<pre><code class="language-js"><span class="hljs-keyword">function</span> <span class="hljs-title function_">handler</span>(<span class="hljs-params">event</span>) {} 1431 1432<span class="hljs-keyword">const</span> target = <span class="hljs-title function_">getEventTargetSomehow</span>(); 1433target.<span class="hljs-title function_">addEventListener</span>(<span class="hljs-string">'foo'</span>, handler, { <span class="hljs-attr">capture</span>: <span class="hljs-literal">true</span> }); <span class="hljs-comment">// first</span> 1434target.<span class="hljs-title function_">addEventListener</span>(<span class="hljs-string">'foo'</span>, handler, { <span class="hljs-attr">capture</span>: <span class="hljs-literal">false</span> }); <span class="hljs-comment">// second</span> 1435 1436<span class="hljs-comment">// Removes the second instance of handler</span> 1437target.<span class="hljs-title function_">removeEventListener</span>(<span class="hljs-string">'foo'</span>, handler); 1438 1439<span class="hljs-comment">// Removes the first instance of handler</span> 1440target.<span class="hljs-title function_">removeEventListener</span>(<span class="hljs-string">'foo'</span>, handler, { <span class="hljs-attr">capture</span>: <span class="hljs-literal">true</span> });</code></pre> 1441<h5><code>eventTarget.dispatchEvent(event)</code><span><a class="mark" href="#events_eventtarget_dispatchevent_event" id="events_eventtarget_dispatchevent_event">#</a></span></h5> 1442<div class="api_metadata"> 1443<span>Added in: v14.5.0</span> 1444</div> 1445<ul> 1446<li><code>event</code> <a href="events.html#events_class_event" class="type"><Event></a></li> 1447<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> <code>true</code> if either event’s <code>cancelable</code> attribute value is 1448false or its <code>preventDefault()</code> method was not invoked, otherwise <code>false</code>.</li> 1449</ul> 1450<p>Dispatches the <code>event</code> to the list of handlers for <code>event.type</code>.</p> 1451<p>The registered event listeners is synchronously invoked in the order they 1452were registered.</p> 1453<h5><code>eventTarget.removeEventListener(type, listener)</code><span><a class="mark" href="#events_eventtarget_removeeventlistener_type_listener" id="events_eventtarget_removeeventlistener_type_listener">#</a></span></h5> 1454<div class="api_metadata"> 1455<span>Added in: v14.5.0</span> 1456</div> 1457<ul> 1458<li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> 1459<li><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="events.html#events_event_listener" class="type"><EventListener></a></li> 1460<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 1461<ul> 1462<li><code>capture</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> 1463</ul> 1464</li> 1465</ul> 1466<p>Removes the <code>listener</code> from the list of handlers for event <code>type</code>.</p> 1467<h4>Class: <code>NodeEventTarget</code><span><a class="mark" href="#events_class_nodeeventtarget" id="events_class_nodeeventtarget">#</a></span></h4> 1468<div class="api_metadata"> 1469<span>Added in: v14.5.0</span> 1470</div> 1471<ul> 1472<li>Extends: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a></li> 1473</ul> 1474<p>The <code>NodeEventTarget</code> is a Node.js-specific extension to <code>EventTarget</code> 1475that emulates a subset of the <code>EventEmitter</code> API.</p> 1476<h5><code>nodeEventTarget.addListener(type, listener[, options])</code><span><a class="mark" href="#events_nodeeventtarget_addlistener_type_listener_options" id="events_nodeeventtarget_addlistener_type_listener_options">#</a></span></h5> 1477<div class="api_metadata"> 1478<span>Added in: v14.5.0</span> 1479</div> 1480<ul> 1481<li> 1482<p><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></p> 1483</li> 1484<li> 1485<p><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="events.html#events_event_listener" class="type"><EventListener></a></p> 1486</li> 1487<li> 1488<p><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></p> 1489<ul> 1490<li><code>once</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> 1491</ul> 1492</li> 1493<li> 1494<p>Returns: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> this</p> 1495</li> 1496</ul> 1497<p>Node.js-specific extension to the <code>EventTarget</code> class that emulates the 1498equivalent <code>EventEmitter</code> API. The only difference between <code>addListener()</code> and 1499<code>addEventListener()</code> is that <code>addListener()</code> will return a reference to the 1500<code>EventTarget</code>.</p> 1501<h5><code>nodeEventTarget.eventNames()</code><span><a class="mark" href="#events_nodeeventtarget_eventnames" id="events_nodeeventtarget_eventnames">#</a></span></h5> 1502<div class="api_metadata"> 1503<span>Added in: v14.5.0</span> 1504</div> 1505<ul> 1506<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a></li> 1507</ul> 1508<p>Node.js-specific extension to the <code>EventTarget</code> class that returns an array 1509of event <code>type</code> names for which event listeners are registered.</p> 1510<h5><code>nodeEventTarget.listenerCount(type)</code><span><a class="mark" href="#events_nodeeventtarget_listenercount_type" id="events_nodeeventtarget_listenercount_type">#</a></span></h5> 1511<div class="api_metadata"> 1512<span>Added in: v14.5.0</span> 1513</div> 1514<ul> 1515<li> 1516<p><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></p> 1517</li> 1518<li> 1519<p>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></p> 1520</li> 1521</ul> 1522<p>Node.js-specific extension to the <code>EventTarget</code> class that returns the number 1523of event listeners registered for the <code>type</code>.</p> 1524<h5><code>nodeEventTarget.off(type, listener)</code><span><a class="mark" href="#events_nodeeventtarget_off_type_listener" id="events_nodeeventtarget_off_type_listener">#</a></span></h5> 1525<div class="api_metadata"> 1526<span>Added in: v14.5.0</span> 1527</div> 1528<ul> 1529<li> 1530<p><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></p> 1531</li> 1532<li> 1533<p><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="events.html#events_event_listener" class="type"><EventListener></a></p> 1534</li> 1535<li> 1536<p>Returns: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> this</p> 1537</li> 1538</ul> 1539<p>Node.js-specific alias for <code>eventTarget.removeListener()</code>.</p> 1540<h5><code>nodeEventTarget.on(type, listener[, options])</code><span><a class="mark" href="#events_nodeeventtarget_on_type_listener_options" id="events_nodeeventtarget_on_type_listener_options">#</a></span></h5> 1541<div class="api_metadata"> 1542<span>Added in: v14.5.0</span> 1543</div> 1544<ul> 1545<li> 1546<p><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></p> 1547</li> 1548<li> 1549<p><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="events.html#events_event_listener" class="type"><EventListener></a></p> 1550</li> 1551<li> 1552<p><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></p> 1553<ul> 1554<li><code>once</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> 1555</ul> 1556</li> 1557<li> 1558<p>Returns: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> this</p> 1559</li> 1560</ul> 1561<p>Node.js-specific alias for <code>eventTarget.addListener()</code>.</p> 1562<h5><code>nodeEventTarget.once(type, listener[, options])</code><span><a class="mark" href="#events_nodeeventtarget_once_type_listener_options" id="events_nodeeventtarget_once_type_listener_options">#</a></span></h5> 1563<div class="api_metadata"> 1564<span>Added in: v14.5.0</span> 1565</div> 1566<ul> 1567<li> 1568<p><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></p> 1569</li> 1570<li> 1571<p><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="events.html#events_event_listener" class="type"><EventListener></a></p> 1572</li> 1573<li> 1574<p><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></p> 1575</li> 1576<li> 1577<p>Returns: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> this</p> 1578</li> 1579</ul> 1580<p>Node.js-specific extension to the <code>EventTarget</code> class that adds a <code>once</code> 1581listener for the given event <code>type</code>. This is equivalent to calling <code>on</code> 1582with the <code>once</code> option set to <code>true</code>.</p> 1583<h5><code>nodeEventTarget.removeAllListeners([type])</code><span><a class="mark" href="#events_nodeeventtarget_removealllisteners_type" id="events_nodeeventtarget_removealllisteners_type">#</a></span></h5> 1584<div class="api_metadata"> 1585<span>Added in: v14.5.0</span> 1586</div> 1587<ul> 1588<li> 1589<p><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></p> 1590</li> 1591<li> 1592<p>Returns: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> this</p> 1593</li> 1594</ul> 1595<p>Node.js-specific extension to the <code>EventTarget</code> class. If <code>type</code> is specified, 1596removes all registered listeners for <code>type</code>, otherwise removes all registered 1597listeners.</p> 1598<h5><code>nodeEventTarget.removeListener(type, listener)</code><span><a class="mark" href="#events_nodeeventtarget_removelistener_type_listener" id="events_nodeeventtarget_removelistener_type_listener">#</a></span></h5> 1599<div class="api_metadata"> 1600<span>Added in: v14.5.0</span> 1601</div> 1602<ul> 1603<li> 1604<p><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></p> 1605</li> 1606<li> 1607<p><code>listener</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="events.html#events_event_listener" class="type"><EventListener></a></p> 1608</li> 1609<li> 1610<p>Returns: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a> this</p> 1611</li> 1612</ul> 1613<p>Node.js-specific extension to the <code>EventTarget</code> class that removes the 1614<code>listener</code> for the given <code>type</code>. The only difference between <code>removeListener()</code> 1615and <code>removeEventListener()</code> is that <code>removeListener()</code> will return a reference 1616to the <code>EventTarget</code>.</p></section> 1617 <!-- API END --> 1618 </div> 1619 </div> 1620 </div> 1621 <script> 1622 'use strict'; 1623 { 1624 const kCustomPreference = 'customDarkTheme'; 1625 const userSettings = sessionStorage.getItem(kCustomPreference); 1626 const themeToggleButton = document.getElementById('theme-toggle-btn'); 1627 if (userSettings === null && window.matchMedia) { 1628 const mq = window.matchMedia('(prefers-color-scheme: dark)'); 1629 if ('onchange' in mq) { 1630 function mqChangeListener(e) { 1631 document.body.classList.toggle('dark-mode', e.matches); 1632 } 1633 mq.addEventListener('change', mqChangeListener); 1634 if (themeToggleButton) { 1635 themeToggleButton.addEventListener('click', function() { 1636 mq.removeEventListener('change', mqChangeListener); 1637 }, { once: true }); 1638 } 1639 } 1640 if (mq.matches) { 1641 document.body.classList.add('dark-mode'); 1642 } 1643 } else if (userSettings === 'true') { 1644 document.body.classList.add('dark-mode'); 1645 } 1646 if (themeToggleButton) { 1647 themeToggleButton.hidden = false; 1648 themeToggleButton.addEventListener('click', function() { 1649 sessionStorage.setItem( 1650 kCustomPreference, 1651 document.body.classList.toggle('dark-mode') 1652 ); 1653 }); 1654 } 1655 } 1656 </script> 1657</body> 1658</html> 1659