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open><summary>Table of contents</summary><ul> 140<li><span class="stability_1"><a href="#tracing_trace_events">Trace events</a></span> 141<ul> 142<li><a href="#tracing_the_trace_events_module">The <code>trace_events</code> module</a> 143<ul> 144<li><a href="#tracing_tracing_object"><code>Tracing</code> object</a> 145<ul> 146<li><a href="#tracing_tracing_categories"><code>tracing.categories</code></a></li> 147<li><a href="#tracing_tracing_disable"><code>tracing.disable()</code></a></li> 148<li><a href="#tracing_tracing_enable"><code>tracing.enable()</code></a></li> 149<li><a href="#tracing_tracing_enabled"><code>tracing.enabled</code></a></li> 150</ul> 151</li> 152<li><a href="#tracing_trace_events_createtracing_options"><code>trace_events.createTracing(options)</code></a></li> 153<li><a href="#tracing_trace_events_getenabledcategories"><code>trace_events.getEnabledCategories()</code></a></li> 154</ul> 155</li> 156</ul> 157</li> 158</ul></details> 159 160 <div id="apicontent"> 161 <h2>Trace events<span><a class="mark" href="#tracing_trace_events" id="tracing_trace_events">#</a></span></h2> 162 163<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p> 164<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v14.20.1/lib/trace_events.js">lib/trace_events.js</a></p> 165<p>The <code>trace_events</code> module provides a mechanism to centralize tracing information 166generated by V8, Node.js core, and userspace code.</p> 167<p>Tracing can be enabled with the <code>--trace-event-categories</code> command-line flag 168or by using the <code>trace_events</code> module. The <code>--trace-event-categories</code> flag 169accepts a list of comma-separated category names.</p> 170<p>The available categories are:</p> 171<ul> 172<li><code>node</code>: An empty placeholder.</li> 173<li><code>node.async_hooks</code>: Enables capture of detailed <a href="async_hooks.html"><code>async_hooks</code></a> trace data. 174The <a href="async_hooks.html"><code>async_hooks</code></a> events have a unique <code>asyncId</code> and a special <code>triggerId</code> 175<code>triggerAsyncId</code> property.</li> 176<li><code>node.bootstrap</code>: Enables capture of Node.js bootstrap milestones.</li> 177<li><code>node.console</code>: Enables capture of <code>console.time()</code> and <code>console.count()</code> 178output.</li> 179<li><code>node.dns.native</code>: Enables capture of trace data for DNS queries.</li> 180<li><code>node.environment</code>: Enables capture of Node.js Environment milestones.</li> 181<li><code>node.fs.sync</code>: Enables capture of trace data for file system sync methods.</li> 182<li><code>node.perf</code>: Enables capture of <a href="perf_hooks.html">Performance API</a> measurements. 183<ul> 184<li><code>node.perf.usertiming</code>: Enables capture of only Performance API User Timing 185measures and marks.</li> 186<li><code>node.perf.timerify</code>: Enables capture of only Performance API timerify 187measurements.</li> 188</ul> 189</li> 190<li><code>node.promises.rejections</code>: Enables capture of trace data tracking the number 191of unhandled Promise rejections and handled-after-rejections.</li> 192<li><code>node.vm.script</code>: Enables capture of trace data for the <code>vm</code> module's 193<code>runInNewContext()</code>, <code>runInContext()</code>, and <code>runInThisContext()</code> methods.</li> 194<li><code>v8</code>: The <a href="v8.html">V8</a> events are GC, compiling, and execution related.</li> 195</ul> 196<p>By default the <code>node</code>, <code>node.async_hooks</code>, and <code>v8</code> categories are enabled.</p> 197<pre><code class="language-bash">node --trace-event-categories v8,node,node.async_hooks server.js</code></pre> 198<p>Prior versions of Node.js required the use of the <code>--trace-events-enabled</code> 199flag to enable trace events. This requirement has been removed. However, the 200<code>--trace-events-enabled</code> flag <em>may</em> still be used and will enable the 201<code>node</code>, <code>node.async_hooks</code>, and <code>v8</code> trace event categories by default.</p> 202<pre><code class="language-bash">node --trace-events-enabled 203 204<span class="hljs-comment"># is equivalent to</span> 205 206node --trace-event-categories v8,node,node.async_hooks</code></pre> 207<p>Alternatively, trace events may be enabled using the <code>trace_events</code> module:</p> 208<pre><code class="language-js"><span class="hljs-keyword">const</span> trace_events = <span class="hljs-built_in">require</span>(<span class="hljs-string">'trace_events'</span>); 209<span class="hljs-keyword">const</span> tracing = trace_events.<span class="hljs-title function_">createTracing</span>({ <span class="hljs-attr">categories</span>: [<span class="hljs-string">'node.perf'</span>] }); 210tracing.<span class="hljs-title function_">enable</span>(); <span class="hljs-comment">// Enable trace event capture for the 'node.perf' category</span> 211 212<span class="hljs-comment">// do work</span> 213 214tracing.<span class="hljs-title function_">disable</span>(); <span class="hljs-comment">// Disable trace event capture for the 'node.perf' category</span></code></pre> 215<p>Running Node.js with tracing enabled will produce log files that can be opened 216in the <a href="https://www.chromium.org/developers/how-tos/trace-event-profiling-tool"><code>chrome://tracing</code></a> 217tab of Chrome.</p> 218<p>The logging file is by default called <code>node_trace.${rotation}.log</code>, where 219<code>${rotation}</code> is an incrementing log-rotation id. The filepath pattern can 220be specified with <code>--trace-event-file-pattern</code> that accepts a template 221string that supports <code>${rotation}</code> and <code>${pid}</code>:</p> 222<pre><code class="language-bash">node --trace-event-categories v8 --trace-event-file-pattern <span class="hljs-string">'${pid}-${rotation}.log'</span> server.js</code></pre> 223<p>The tracing system uses the same time source 224as the one used by <code>process.hrtime()</code>. 225However the trace-event timestamps are expressed in microseconds, 226unlike <code>process.hrtime()</code> which returns nanoseconds.</p> 227<p>The features from this module are not available in <a href="worker_threads.html#worker_threads_class_worker"><code>Worker</code></a> threads.</p> 228<section><h3>The <code>trace_events</code> module<span><a class="mark" href="#tracing_the_trace_events_module" id="tracing_the_trace_events_module">#</a></span></h3> 229<div class="api_metadata"> 230<span>Added in: v10.0.0</span> 231</div> 232<h4><code>Tracing</code> object<span><a class="mark" href="#tracing_tracing_object" id="tracing_tracing_object">#</a></span></h4> 233<div class="api_metadata"> 234<span>Added in: v10.0.0</span> 235</div> 236<p>The <code>Tracing</code> object is used to enable or disable tracing for sets of 237categories. Instances are created using the <code>trace_events.createTracing()</code> 238method.</p> 239<p>When created, the <code>Tracing</code> object is disabled. Calling the 240<code>tracing.enable()</code> method adds the categories to the set of enabled trace event 241categories. Calling <code>tracing.disable()</code> will remove the categories from the 242set of enabled trace event categories.</p> 243<h5><code>tracing.categories</code><span><a class="mark" href="#tracing_tracing_categories" id="tracing_tracing_categories">#</a></span></h5> 244<div class="api_metadata"> 245<span>Added in: v10.0.0</span> 246</div> 247<ul> 248<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> 249</ul> 250<p>A comma-separated list of the trace event categories covered by this 251<code>Tracing</code> object.</p> 252<h5><code>tracing.disable()</code><span><a class="mark" href="#tracing_tracing_disable" id="tracing_tracing_disable">#</a></span></h5> 253<div class="api_metadata"> 254<span>Added in: v10.0.0</span> 255</div> 256<p>Disables this <code>Tracing</code> object.</p> 257<p>Only trace event categories <em>not</em> covered by other enabled <code>Tracing</code> objects 258and <em>not</em> specified by the <code>--trace-event-categories</code> flag will be disabled.</p> 259<pre><code class="language-js"><span class="hljs-keyword">const</span> trace_events = <span class="hljs-built_in">require</span>(<span class="hljs-string">'trace_events'</span>); 260<span class="hljs-keyword">const</span> t1 = trace_events.<span class="hljs-title function_">createTracing</span>({ <span class="hljs-attr">categories</span>: [<span class="hljs-string">'node'</span>, <span class="hljs-string">'v8'</span>] }); 261<span class="hljs-keyword">const</span> t2 = trace_events.<span class="hljs-title function_">createTracing</span>({ <span class="hljs-attr">categories</span>: [<span class="hljs-string">'node.perf'</span>, <span class="hljs-string">'node'</span>] }); 262t1.<span class="hljs-title function_">enable</span>(); 263t2.<span class="hljs-title function_">enable</span>(); 264 265<span class="hljs-comment">// Prints 'node,node.perf,v8'</span> 266<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(trace_events.<span class="hljs-title function_">getEnabledCategories</span>()); 267 268t2.<span class="hljs-title function_">disable</span>(); <span class="hljs-comment">// Will only disable emission of the 'node.perf' category</span> 269 270<span class="hljs-comment">// Prints 'node,v8'</span> 271<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(trace_events.<span class="hljs-title function_">getEnabledCategories</span>());</code></pre> 272<h5><code>tracing.enable()</code><span><a class="mark" href="#tracing_tracing_enable" id="tracing_tracing_enable">#</a></span></h5> 273<div class="api_metadata"> 274<span>Added in: v10.0.0</span> 275</div> 276<p>Enables this <code>Tracing</code> object for the set of categories covered by the 277<code>Tracing</code> object.</p> 278<h5><code>tracing.enabled</code><span><a class="mark" href="#tracing_tracing_enabled" id="tracing_tracing_enabled">#</a></span></h5> 279<div class="api_metadata"> 280<span>Added in: v10.0.0</span> 281</div> 282<ul> 283<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> <code>true</code> only if the <code>Tracing</code> object has been enabled.</li> 284</ul> 285<h4><code>trace_events.createTracing(options)</code><span><a class="mark" href="#tracing_trace_events_createtracing_options" id="tracing_trace_events_createtracing_options">#</a></span></h4> 286<div class="api_metadata"> 287<span>Added in: v10.0.0</span> 288</div> 289<ul> 290<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 291<ul> 292<li><code>categories</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a> An array of trace category names. Values included 293in the array are coerced to a string when possible. An error will be 294thrown if the value cannot be coerced.</li> 295</ul> 296</li> 297<li>Returns: <a href="tracing.html#tracing_tracing_object" class="type"><Tracing></a>.</li> 298</ul> 299<p>Creates and returns a <code>Tracing</code> object for the given set of <code>categories</code>.</p> 300<pre><code class="language-js"><span class="hljs-keyword">const</span> trace_events = <span class="hljs-built_in">require</span>(<span class="hljs-string">'trace_events'</span>); 301<span class="hljs-keyword">const</span> categories = [<span class="hljs-string">'node.perf'</span>, <span class="hljs-string">'node.async_hooks'</span>]; 302<span class="hljs-keyword">const</span> tracing = trace_events.<span class="hljs-title function_">createTracing</span>({ categories }); 303tracing.<span class="hljs-title function_">enable</span>(); 304<span class="hljs-comment">// do stuff</span> 305tracing.<span class="hljs-title function_">disable</span>();</code></pre> 306<h4><code>trace_events.getEnabledCategories()</code><span><a class="mark" href="#tracing_trace_events_getenabledcategories" id="tracing_trace_events_getenabledcategories">#</a></span></h4> 307<div class="api_metadata"> 308<span>Added in: v10.0.0</span> 309</div> 310<ul> 311<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> 312</ul> 313<p>Returns a comma-separated list of all currently-enabled trace event 314categories. The current set of enabled trace event categories is determined 315by the <em>union</em> of all currently-enabled <code>Tracing</code> objects and any categories 316enabled using the <code>--trace-event-categories</code> flag.</p> 317<p>Given the file <code>test.js</code> below, the command 318<code>node --trace-event-categories node.perf test.js</code> will print 319<code>'node.async_hooks,node.perf'</code> to the console.</p> 320<pre><code class="language-js"><span class="hljs-keyword">const</span> trace_events = <span class="hljs-built_in">require</span>(<span class="hljs-string">'trace_events'</span>); 321<span class="hljs-keyword">const</span> t1 = trace_events.<span class="hljs-title function_">createTracing</span>({ <span class="hljs-attr">categories</span>: [<span class="hljs-string">'node.async_hooks'</span>] }); 322<span class="hljs-keyword">const</span> t2 = trace_events.<span class="hljs-title function_">createTracing</span>({ <span class="hljs-attr">categories</span>: [<span class="hljs-string">'node.perf'</span>] }); 323<span class="hljs-keyword">const</span> t3 = trace_events.<span class="hljs-title function_">createTracing</span>({ <span class="hljs-attr">categories</span>: [<span class="hljs-string">'v8'</span>] }); 324 325t1.<span class="hljs-title function_">enable</span>(); 326t2.<span class="hljs-title function_">enable</span>(); 327 328<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(trace_events.<span class="hljs-title function_">getEnabledCategories</span>());</code></pre></section> 329 <!-- API END --> 330 </div> 331 </div> 332 </div> 333 <script> 334 'use strict'; 335 { 336 const kCustomPreference = 'customDarkTheme'; 337 const userSettings = sessionStorage.getItem(kCustomPreference); 338 const themeToggleButton = document.getElementById('theme-toggle-btn'); 339 if (userSettings === null && window.matchMedia) { 340 const mq = window.matchMedia('(prefers-color-scheme: dark)'); 341 if ('onchange' in mq) { 342 function mqChangeListener(e) { 343 document.body.classList.toggle('dark-mode', e.matches); 344 } 345 mq.addEventListener('change', mqChangeListener); 346 if (themeToggleButton) { 347 themeToggleButton.addEventListener('click', function() { 348 mq.removeEventListener('change', mqChangeListener); 349 }, { once: true }); 350 } 351 } 352 if (mq.matches) { 353 document.body.classList.add('dark-mode'); 354 } 355 } else if (userSettings === 'true') { 356 document.body.classList.add('dark-mode'); 357 } 358 if (themeToggleButton) { 359 themeToggleButton.hidden = false; 360 themeToggleButton.addEventListener('click', function() { 361 sessionStorage.setItem( 362 kCustomPreference, 363 document.body.classList.toggle('dark-mode') 364 ); 365 }); 366 } 367 } 368 </script> 369</body> 370</html> 371