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152<li><a href="#domain_class_domain">Class: <code>Domain</code></a> 153<ul> 154<li><a href="#domain_domain_members"><code>domain.members</code></a></li> 155<li><a href="#domain_domain_add_emitter"><code>domain.add(emitter)</code></a></li> 156<li><a href="#domain_domain_bind_callback"><code>domain.bind(callback)</code></a></li> 157<li><a href="#domain_domain_enter"><code>domain.enter()</code></a></li> 158<li><a href="#domain_domain_exit"><code>domain.exit()</code></a></li> 159<li><a href="#domain_domain_intercept_callback"><code>domain.intercept(callback)</code></a></li> 160<li><a href="#domain_domain_remove_emitter"><code>domain.remove(emitter)</code></a></li> 161<li><a href="#domain_domain_run_fn_args"><code>domain.run(fn[, ...args])</code></a></li> 162</ul> 163</li> 164<li><a href="#domain_domains_and_promises">Domains and promises</a></li> 165</ul> 166</li> 167</ul></details> 168 169 <div id="apicontent"> 170 <h2>Domain<span><a class="mark" href="#domain_domain" id="domain_domain">#</a></span></h2> 171<div class="api_metadata"> 172<details class="changelog"><summary>History</summary> 173<table> 174<tbody><tr><th>Version</th><th>Changes</th></tr> 175<tr><td>v8.8.0</td> 176<td><p>Any <code>Promise</code>s created in VM contexts no longer have a <code>.domain</code> property. Their handlers are still executed in the proper domain, however, and <code>Promise</code>s created in the main context still possess a <code>.domain</code> property.</p></td></tr> 177<tr><td>v8.0.0</td> 178<td><p>Handlers for <code>Promise</code>s are now invoked in the domain in which the first promise of a chain was created.</p></td></tr> 179<tr><td>v1.4.2</td> 180<td><p><span>Deprecated since: v1.4.2</span></p></td></tr> 181</tbody></table> 182</details> 183</div> 184 185<p></p><div class="api_stability api_stability_0"><a href="documentation.html#documentation_stability_index">Stability: 0</a> - Deprecated</div><p></p> 186<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v14.20.1/lib/domain.js">lib/domain.js</a></p> 187<p><strong>This module is pending deprecation</strong>. Once a replacement API has been 188finalized, this module will be fully deprecated. Most developers should 189<strong>not</strong> have cause to use this module. Users who absolutely must have 190the functionality that domains provide may rely on it for the time being 191but should expect to have to migrate to a different solution 192in the future.</p> 193<p>Domains provide a way to handle multiple different IO operations as a 194single group. If any of the event emitters or callbacks registered to a 195domain emit an <code>'error'</code> event, or throw an error, then the domain object 196will be notified, rather than losing the context of the error in the 197<code>process.on('uncaughtException')</code> handler, or causing the program to 198exit immediately with an error code.</p> 199<section><h3>Warning: Don't ignore errors!<span><a class="mark" href="#domain_warning_don_t_ignore_errors" id="domain_warning_don_t_ignore_errors">#</a></span></h3> 200 201<p>Domain error handlers are not a substitute for closing down a 202process when an error occurs.</p> 203<p>By the very nature of how <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/throw"><code>throw</code></a> works in JavaScript, there is almost 204never any way to safely "pick up where it left off", without leaking 205references, or creating some other sort of undefined brittle state.</p> 206<p>The safest way to respond to a thrown error is to shut down the 207process. Of course, in a normal web server, there may be many 208open connections, and it is not reasonable to abruptly shut those down 209because an error was triggered by someone else.</p> 210<p>The better approach is to send an error response to the request that 211triggered the error, while letting the others finish in their normal 212time, and stop listening for new requests in that worker.</p> 213<p>In this way, <code>domain</code> usage goes hand-in-hand with the cluster module, 214since the master process can fork a new worker when a worker 215encounters an error. For Node.js programs that scale to multiple 216machines, the terminating proxy or service registry can take note of 217the failure, and react accordingly.</p> 218<p>For example, this is not a good idea:</p> 219<pre><code class="language-js"><span class="hljs-comment">// XXX WARNING! BAD IDEA!</span> 220 221<span class="hljs-keyword">const</span> d = <span class="hljs-built_in">require</span>(<span class="hljs-string">'domain'</span>).<span class="hljs-title function_">create</span>(); 222d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 223 <span class="hljs-comment">// The error won't crash the process, but what it does is worse!</span> 224 <span class="hljs-comment">// Though we've prevented abrupt process restarting, we are leaking</span> 225 <span class="hljs-comment">// a lot of resources if this ever happens.</span> 226 <span class="hljs-comment">// This is no better than process.on('uncaughtException')!</span> 227 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`error, but oh well <span class="hljs-subst">${er.message}</span>`</span>); 228}); 229d.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 230 <span class="hljs-built_in">require</span>(<span class="hljs-string">'http'</span>).<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { 231 <span class="hljs-title function_">handleRequest</span>(req, res); 232 }).<span class="hljs-title function_">listen</span>(<span class="hljs-variable constant_">PORT</span>); 233});</code></pre> 234<p>By using the context of a domain, and the resilience of separating our 235program into multiple worker processes, we can react more 236appropriately, and handle errors with much greater safety.</p> 237<pre><code class="language-js"><span class="hljs-comment">// Much better!</span> 238 239<span class="hljs-keyword">const</span> cluster = <span class="hljs-built_in">require</span>(<span class="hljs-string">'cluster'</span>); 240<span class="hljs-keyword">const</span> <span class="hljs-variable constant_">PORT</span> = +process.<span class="hljs-property">env</span>.<span class="hljs-property">PORT</span> || <span class="hljs-number">1337</span>; 241 242<span class="hljs-keyword">if</span> (cluster.<span class="hljs-property">isMaster</span>) { 243 <span class="hljs-comment">// A more realistic scenario would have more than 2 workers,</span> 244 <span class="hljs-comment">// and perhaps not put the master and worker in the same file.</span> 245 <span class="hljs-comment">//</span> 246 <span class="hljs-comment">// It is also possible to get a bit fancier about logging, and</span> 247 <span class="hljs-comment">// implement whatever custom logic is needed to prevent DoS</span> 248 <span class="hljs-comment">// attacks and other bad behavior.</span> 249 <span class="hljs-comment">//</span> 250 <span class="hljs-comment">// See the options in the cluster documentation.</span> 251 <span class="hljs-comment">//</span> 252 <span class="hljs-comment">// The important thing is that the master does very little,</span> 253 <span class="hljs-comment">// increasing our resilience to unexpected errors.</span> 254 255 cluster.<span class="hljs-title function_">fork</span>(); 256 cluster.<span class="hljs-title function_">fork</span>(); 257 258 cluster.<span class="hljs-title function_">on</span>(<span class="hljs-string">'disconnect'</span>, <span class="hljs-function">(<span class="hljs-params">worker</span>) =></span> { 259 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'disconnect!'</span>); 260 cluster.<span class="hljs-title function_">fork</span>(); 261 }); 262 263} <span class="hljs-keyword">else</span> { 264 <span class="hljs-comment">// the worker</span> 265 <span class="hljs-comment">//</span> 266 <span class="hljs-comment">// This is where we put our bugs!</span> 267 268 <span class="hljs-keyword">const</span> domain = <span class="hljs-built_in">require</span>(<span class="hljs-string">'domain'</span>); 269 270 <span class="hljs-comment">// See the cluster documentation for more details about using</span> 271 <span class="hljs-comment">// worker processes to serve requests. How it works, caveats, etc.</span> 272 273 <span class="hljs-keyword">const</span> server = <span class="hljs-built_in">require</span>(<span class="hljs-string">'http'</span>).<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { 274 <span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 275 d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 276 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">`error <span class="hljs-subst">${er.stack}</span>`</span>); 277 278 <span class="hljs-comment">// We're in dangerous territory!</span> 279 <span class="hljs-comment">// By definition, something unexpected occurred,</span> 280 <span class="hljs-comment">// which we probably didn't want.</span> 281 <span class="hljs-comment">// Anything can happen now! Be very careful!</span> 282 283 <span class="hljs-keyword">try</span> { 284 <span class="hljs-comment">// Make sure we close down within 30 seconds</span> 285 <span class="hljs-keyword">const</span> killtimer = <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 286 process.<span class="hljs-title function_">exit</span>(<span class="hljs-number">1</span>); 287 }, <span class="hljs-number">30000</span>); 288 <span class="hljs-comment">// But don't keep the process open just for that!</span> 289 killtimer.<span class="hljs-title function_">unref</span>(); 290 291 <span class="hljs-comment">// Stop taking new requests.</span> 292 server.<span class="hljs-title function_">close</span>(); 293 294 <span class="hljs-comment">// Let the master know we're dead. This will trigger a</span> 295 <span class="hljs-comment">// 'disconnect' in the cluster master, and then it will fork</span> 296 <span class="hljs-comment">// a new worker.</span> 297 cluster.<span class="hljs-property">worker</span>.<span class="hljs-title function_">disconnect</span>(); 298 299 <span class="hljs-comment">// Try to send an error to the request that triggered the problem</span> 300 res.<span class="hljs-property">statusCode</span> = <span class="hljs-number">500</span>; 301 res.<span class="hljs-title function_">setHeader</span>(<span class="hljs-string">'content-type'</span>, <span class="hljs-string">'text/plain'</span>); 302 res.<span class="hljs-title function_">end</span>(<span class="hljs-string">'Oops, there was a problem!\n'</span>); 303 } <span class="hljs-keyword">catch</span> (er2) { 304 <span class="hljs-comment">// Oh well, not much we can do at this point.</span> 305 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">`Error sending 500! <span class="hljs-subst">${er2.stack}</span>`</span>); 306 } 307 }); 308 309 <span class="hljs-comment">// Because req and res were created before this domain existed,</span> 310 <span class="hljs-comment">// we need to explicitly add them.</span> 311 <span class="hljs-comment">// See the explanation of implicit vs explicit binding below.</span> 312 d.<span class="hljs-title function_">add</span>(req); 313 d.<span class="hljs-title function_">add</span>(res); 314 315 <span class="hljs-comment">// Now run the handler function in the domain.</span> 316 d.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 317 <span class="hljs-title function_">handleRequest</span>(req, res); 318 }); 319 }); 320 server.<span class="hljs-title function_">listen</span>(<span class="hljs-variable constant_">PORT</span>); 321} 322 323<span class="hljs-comment">// This part is not important. Just an example routing thing.</span> 324<span class="hljs-comment">// Put fancy application logic here.</span> 325<span class="hljs-keyword">function</span> <span class="hljs-title function_">handleRequest</span>(<span class="hljs-params">req, res</span>) { 326 <span class="hljs-keyword">switch</span> (req.<span class="hljs-property">url</span>) { 327 <span class="hljs-keyword">case</span> <span class="hljs-string">'/error'</span>: 328 <span class="hljs-comment">// We do some async stuff, and then...</span> 329 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 330 <span class="hljs-comment">// Whoops!</span> 331 flerb.<span class="hljs-title function_">bark</span>(); 332 }, timeout); 333 <span class="hljs-keyword">break</span>; 334 <span class="hljs-attr">default</span>: 335 res.<span class="hljs-title function_">end</span>(<span class="hljs-string">'ok'</span>); 336 } 337}</code></pre> 338</section><section><h3>Additions to <code>Error</code> objects<span><a class="mark" href="#domain_additions_to_error_objects" id="domain_additions_to_error_objects">#</a></span></h3> 339 340<p>Any time an <code>Error</code> object is routed through a domain, a few extra fields 341are added to it.</p> 342<ul> 343<li><code>error.domain</code> The domain that first handled the error.</li> 344<li><code>error.domainEmitter</code> The event emitter that emitted an <code>'error'</code> event 345with the error object.</li> 346<li><code>error.domainBound</code> The callback function which was bound to the 347domain, and passed an error as its first argument.</li> 348<li><code>error.domainThrown</code> A boolean indicating whether the error was 349thrown, emitted, or passed to a bound callback function.</li> 350</ul> 351</section><section><h3>Implicit binding<span><a class="mark" href="#domain_implicit_binding" id="domain_implicit_binding">#</a></span></h3> 352 353<p>If domains are in use, then all <strong>new</strong> <code>EventEmitter</code> objects (including 354Stream objects, requests, responses, etc.) will be implicitly bound to 355the active domain at the time of their creation.</p> 356<p>Additionally, callbacks passed to lowlevel event loop requests (such as 357to <code>fs.open()</code>, or other callback-taking methods) will automatically be 358bound to the active domain. If they throw, then the domain will catch 359the error.</p> 360<p>In order to prevent excessive memory usage, <code>Domain</code> objects themselves 361are not implicitly added as children of the active domain. If they 362were, then it would be too easy to prevent request and response objects 363from being properly garbage collected.</p> 364<p>To nest <code>Domain</code> objects as children of a parent <code>Domain</code> they must be 365explicitly added.</p> 366<p>Implicit binding routes thrown errors and <code>'error'</code> events to the 367<code>Domain</code>'s <code>'error'</code> event, but does not register the <code>EventEmitter</code> on the 368<code>Domain</code>. 369Implicit binding only takes care of thrown errors and <code>'error'</code> events.</p> 370</section><section><h3>Explicit binding<span><a class="mark" href="#domain_explicit_binding" id="domain_explicit_binding">#</a></span></h3> 371 372<p>Sometimes, the domain in use is not the one that ought to be used for a 373specific event emitter. Or, the event emitter could have been created 374in the context of one domain, but ought to instead be bound to some 375other domain.</p> 376<p>For example, there could be one domain in use for an HTTP server, but 377perhaps we would like to have a separate domain to use for each request.</p> 378<p>That is possible via explicit binding.</p> 379<pre><code class="language-js"><span class="hljs-comment">// Create a top-level domain for the server</span> 380<span class="hljs-keyword">const</span> domain = <span class="hljs-built_in">require</span>(<span class="hljs-string">'domain'</span>); 381<span class="hljs-keyword">const</span> http = <span class="hljs-built_in">require</span>(<span class="hljs-string">'http'</span>); 382<span class="hljs-keyword">const</span> serverDomain = domain.<span class="hljs-title function_">create</span>(); 383 384serverDomain.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 385 <span class="hljs-comment">// Server is created in the scope of serverDomain</span> 386 http.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { 387 <span class="hljs-comment">// Req and res are also created in the scope of serverDomain</span> 388 <span class="hljs-comment">// however, we'd prefer to have a separate domain for each request.</span> 389 <span class="hljs-comment">// create it first thing, and add req and res to it.</span> 390 <span class="hljs-keyword">const</span> reqd = domain.<span class="hljs-title function_">create</span>(); 391 reqd.<span class="hljs-title function_">add</span>(req); 392 reqd.<span class="hljs-title function_">add</span>(res); 393 reqd.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 394 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'Error'</span>, er, req.<span class="hljs-property">url</span>); 395 <span class="hljs-keyword">try</span> { 396 res.<span class="hljs-title function_">writeHead</span>(<span class="hljs-number">500</span>); 397 res.<span class="hljs-title function_">end</span>(<span class="hljs-string">'Error occurred, sorry.'</span>); 398 } <span class="hljs-keyword">catch</span> (er2) { 399 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'Error sending 500'</span>, er2, req.<span class="hljs-property">url</span>); 400 } 401 }); 402 }).<span class="hljs-title function_">listen</span>(<span class="hljs-number">1337</span>); 403});</code></pre> 404</section><section><h3><code>domain.create()</code><span><a class="mark" href="#domain_domain_create" id="domain_domain_create">#</a></span></h3> 405<ul> 406<li>Returns: <a href="domain.html#domain_class_domain" class="type"><Domain></a></li> 407</ul> 408</section><section><h3>Class: <code>Domain</code><span><a class="mark" href="#domain_class_domain" id="domain_class_domain">#</a></span></h3> 409<ul> 410<li>Extends: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 411</ul> 412<p>The <code>Domain</code> class encapsulates the functionality of routing errors and 413uncaught exceptions to the active <code>Domain</code> object.</p> 414<p>To handle the errors that it catches, listen to its <code>'error'</code> event.</p> 415<h4><code>domain.members</code><span><a class="mark" href="#domain_domain_members" id="domain_domain_members">#</a></span></h4> 416<ul> 417<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array" class="type"><Array></a></li> 418</ul> 419<p>An array of timers and event emitters that have been explicitly added 420to the domain.</p> 421<h4><code>domain.add(emitter)</code><span><a class="mark" href="#domain_domain_add_emitter" id="domain_domain_add_emitter">#</a></span></h4> 422<ul> 423<li><code>emitter</code> <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a> | <a href="timers.html#timers_timers" class="type"><Timer></a> emitter or timer to be added to the domain</li> 424</ul> 425<p>Explicitly adds an emitter to the domain. If any event handlers called by 426the emitter throw an error, or if the emitter emits an <code>'error'</code> event, it 427will be routed to the domain's <code>'error'</code> event, just like with implicit 428binding.</p> 429<p>This also works with timers that are returned from <a href="timers.html#timers_setinterval_callback_delay_args"><code>setInterval()</code></a> and 430<a href="timers.html#timers_settimeout_callback_delay_args"><code>setTimeout()</code></a>. If their callback function throws, it will be caught by 431the domain <code>'error'</code> handler.</p> 432<p>If the Timer or <code>EventEmitter</code> was already bound to a domain, it is removed 433from that one, and bound to this one instead.</p> 434<h4><code>domain.bind(callback)</code><span><a class="mark" href="#domain_domain_bind_callback" id="domain_domain_bind_callback">#</a></span></h4> 435<ul> 436<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The callback function</li> 437<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The bound function</li> 438</ul> 439<p>The returned function will be a wrapper around the supplied callback 440function. When the returned function is called, any errors that are 441thrown will be routed to the domain's <code>'error'</code> event.</p> 442<pre><code class="language-js"><span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 443 444<span class="hljs-keyword">function</span> <span class="hljs-title function_">readSomeFile</span>(<span class="hljs-params">filename, cb</span>) { 445 fs.<span class="hljs-title function_">readFile</span>(filename, <span class="hljs-string">'utf8'</span>, d.<span class="hljs-title function_">bind</span>(<span class="hljs-function">(<span class="hljs-params">er, data</span>) =></span> { 446 <span class="hljs-comment">// If this throws, it will also be passed to the domain.</span> 447 <span class="hljs-keyword">return</span> <span class="hljs-title function_">cb</span>(er, data ? <span class="hljs-variable constant_">JSON</span>.<span class="hljs-title function_">parse</span>(data) : <span class="hljs-literal">null</span>); 448 })); 449} 450 451d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 452 <span class="hljs-comment">// An error occurred somewhere. If we throw it now, it will crash the program</span> 453 <span class="hljs-comment">// with the normal line number and stack message.</span> 454});</code></pre> 455<h4><code>domain.enter()</code><span><a class="mark" href="#domain_domain_enter" id="domain_domain_enter">#</a></span></h4> 456<p>The <code>enter()</code> method is plumbing used by the <code>run()</code>, <code>bind()</code>, and 457<code>intercept()</code> methods to set the active domain. It sets <code>domain.active</code> and 458<code>process.domain</code> to the domain, and implicitly pushes the domain onto the domain 459stack managed by the domain module (see <a href="#domain_domain_exit"><code>domain.exit()</code></a> for details on the 460domain stack). The call to <code>enter()</code> delimits the beginning of a chain of 461asynchronous calls and I/O operations bound to a domain.</p> 462<p>Calling <code>enter()</code> changes only the active domain, and does not alter the domain 463itself. <code>enter()</code> and <code>exit()</code> can be called an arbitrary number of times on a 464single domain.</p> 465<h4><code>domain.exit()</code><span><a class="mark" href="#domain_domain_exit" id="domain_domain_exit">#</a></span></h4> 466<p>The <code>exit()</code> method exits the current domain, popping it off the domain stack. 467Any time execution is going to switch to the context of a different chain of 468asynchronous calls, it's important to ensure that the current domain is exited. 469The call to <code>exit()</code> delimits either the end of or an interruption to the chain 470of asynchronous calls and I/O operations bound to a domain.</p> 471<p>If there are multiple, nested domains bound to the current execution context, 472<code>exit()</code> will exit any domains nested within this domain.</p> 473<p>Calling <code>exit()</code> changes only the active domain, and does not alter the domain 474itself. <code>enter()</code> and <code>exit()</code> can be called an arbitrary number of times on a 475single domain.</p> 476<h4><code>domain.intercept(callback)</code><span><a class="mark" href="#domain_domain_intercept_callback" id="domain_domain_intercept_callback">#</a></span></h4> 477<ul> 478<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The callback function</li> 479<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The intercepted function</li> 480</ul> 481<p>This method is almost identical to <a href="#domain_domain_bind_callback"><code>domain.bind(callback)</code></a>. However, in 482addition to catching thrown errors, it will also intercept <a href="errors.html#errors_class_error"><code>Error</code></a> 483objects sent as the first argument to the function.</p> 484<p>In this way, the common <code>if (err) return callback(err);</code> pattern can be replaced 485with a single error handler in a single place.</p> 486<pre><code class="language-js"><span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 487 488<span class="hljs-keyword">function</span> <span class="hljs-title function_">readSomeFile</span>(<span class="hljs-params">filename, cb</span>) { 489 fs.<span class="hljs-title function_">readFile</span>(filename, <span class="hljs-string">'utf8'</span>, d.<span class="hljs-title function_">intercept</span>(<span class="hljs-function">(<span class="hljs-params">data</span>) =></span> { 490 <span class="hljs-comment">// Note, the first argument is never passed to the</span> 491 <span class="hljs-comment">// callback since it is assumed to be the 'Error' argument</span> 492 <span class="hljs-comment">// and thus intercepted by the domain.</span> 493 494 <span class="hljs-comment">// If this throws, it will also be passed to the domain</span> 495 <span class="hljs-comment">// so the error-handling logic can be moved to the 'error'</span> 496 <span class="hljs-comment">// event on the domain instead of being repeated throughout</span> 497 <span class="hljs-comment">// the program.</span> 498 <span class="hljs-keyword">return</span> <span class="hljs-title function_">cb</span>(<span class="hljs-literal">null</span>, <span class="hljs-variable constant_">JSON</span>.<span class="hljs-title function_">parse</span>(data)); 499 })); 500} 501 502d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 503 <span class="hljs-comment">// An error occurred somewhere. If we throw it now, it will crash the program</span> 504 <span class="hljs-comment">// with the normal line number and stack message.</span> 505});</code></pre> 506<h4><code>domain.remove(emitter)</code><span><a class="mark" href="#domain_domain_remove_emitter" id="domain_domain_remove_emitter">#</a></span></h4> 507<ul> 508<li><code>emitter</code> <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a> | <a href="timers.html#timers_timers" class="type"><Timer></a> emitter or timer to be removed from the domain</li> 509</ul> 510<p>The opposite of <a href="#domain_domain_add_emitter"><code>domain.add(emitter)</code></a>. Removes domain handling from the 511specified emitter.</p> 512<h4><code>domain.run(fn[, ...args])</code><span><a class="mark" href="#domain_domain_run_fn_args" id="domain_domain_run_fn_args">#</a></span></h4> 513<ul> 514<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> 515<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 516</ul> 517<p>Run the supplied function in the context of the domain, implicitly 518binding all event emitters, timers, and lowlevel requests that are 519created in that context. Optionally, arguments can be passed to 520the function.</p> 521<p>This is the most basic way to use a domain.</p> 522<pre><code class="language-js"><span class="hljs-keyword">const</span> domain = <span class="hljs-built_in">require</span>(<span class="hljs-string">'domain'</span>); 523<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'fs'</span>); 524<span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 525d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 526 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'Caught error!'</span>, er); 527}); 528d.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 529 process.<span class="hljs-title function_">nextTick</span>(<span class="hljs-function">() =></span> { 530 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { <span class="hljs-comment">// Simulating some various async stuff</span> 531 fs.<span class="hljs-title function_">open</span>(<span class="hljs-string">'non-existent file'</span>, <span class="hljs-string">'r'</span>, <span class="hljs-function">(<span class="hljs-params">er, fd</span>) =></span> { 532 <span class="hljs-keyword">if</span> (er) <span class="hljs-keyword">throw</span> er; 533 <span class="hljs-comment">// proceed...</span> 534 }); 535 }, <span class="hljs-number">100</span>); 536 }); 537});</code></pre> 538<p>In this example, the <code>d.on('error')</code> handler will be triggered, rather 539than crashing the program.</p> 540</section><section><h3>Domains and promises<span><a class="mark" href="#domain_domains_and_promises" id="domain_domains_and_promises">#</a></span></h3> 541<p>As of Node.js 8.0.0, the handlers of promises are run inside the domain in 542which the call to <code>.then()</code> or <code>.catch()</code> itself was made:</p> 543<pre><code class="language-js"><span class="hljs-keyword">const</span> d1 = domain.<span class="hljs-title function_">create</span>(); 544<span class="hljs-keyword">const</span> d2 = domain.<span class="hljs-title function_">create</span>(); 545 546<span class="hljs-keyword">let</span> p; 547d1.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 548 p = <span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">42</span>); 549}); 550 551d2.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 552 p.<span class="hljs-title function_">then</span>(<span class="hljs-function">(<span class="hljs-params">v</span>) =></span> { 553 <span class="hljs-comment">// running in d2</span> 554 }); 555});</code></pre> 556<p>A callback may be bound to a specific domain using <a href="#domain_domain_bind_callback"><code>domain.bind(callback)</code></a>:</p> 557<pre><code class="language-js"><span class="hljs-keyword">const</span> d1 = domain.<span class="hljs-title function_">create</span>(); 558<span class="hljs-keyword">const</span> d2 = domain.<span class="hljs-title function_">create</span>(); 559 560<span class="hljs-keyword">let</span> p; 561d1.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 562 p = <span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">42</span>); 563}); 564 565d2.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 566 p.<span class="hljs-title function_">then</span>(p.<span class="hljs-property">domain</span>.<span class="hljs-title function_">bind</span>(<span class="hljs-function">(<span class="hljs-params">v</span>) =></span> { 567 <span class="hljs-comment">// running in d1</span> 568 })); 569});</code></pre> 570<p>Domains will not interfere with the error handling mechanisms for 571promises. In other words, no <code>'error'</code> event will be emitted for unhandled 572<code>Promise</code> rejections.</p></section> 573 <!-- API END --> 574 </div> 575 </div> 576 </div> 577 <script> 578 'use strict'; 579 { 580 const kCustomPreference = 'customDarkTheme'; 581 const userSettings = sessionStorage.getItem(kCustomPreference); 582 const themeToggleButton = document.getElementById('theme-toggle-btn'); 583 if (userSettings === null && window.matchMedia) { 584 const mq = window.matchMedia('(prefers-color-scheme: dark)'); 585 if ('onchange' in mq) { 586 function mqChangeListener(e) { 587 document.body.classList.toggle('dark-mode', e.matches); 588 } 589 mq.addEventListener('change', mqChangeListener); 590 if (themeToggleButton) { 591 themeToggleButton.addEventListener('click', function() { 592 mq.removeEventListener('change', mqChangeListener); 593 }, { once: true }); 594 } 595 } 596 if (mq.matches) { 597 document.body.classList.add('dark-mode'); 598 } 599 } else if (userSettings === 'true') { 600 document.body.classList.add('dark-mode'); 601 } 602 if (themeToggleButton) { 603 themeToggleButton.hidden = false; 604 themeToggleButton.addEventListener('click', function() { 605 sessionStorage.setItem( 606 kCustomPreference, 607 document.body.classList.toggle('dark-mode') 608 ); 609 }); 610 } 611 } 612 </script> 613</body> 614</html> 615