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      <details id="toc" open><summary>Table of contents</summary><ul>
<li><a href="#trace-events">Trace events</a><ul>
<li><a href="#the-nodetrace_events-module">The <code>node:trace_events</code> module</a><ul>
<li><a href="#tracing-object"><code>Tracing</code> object</a><ul>
<li><a href="#tracingcategories"><code>tracing.categories</code></a></li>
<li><a href="#tracingdisable"><code>tracing.disable()</code></a></li>
<li><a href="#tracingenable"><code>tracing.enable()</code></a></li>
<li><a href="#tracingenabled"><code>tracing.enabled</code></a></li>
</ul>
</li>
<li><a href="#trace_eventscreatetracingoptions"><code>trace_events.createTracing(options)</code></a></li>
<li><a href="#trace_eventsgetenabledcategories"><code>trace_events.getEnabledCategories()</code></a></li>
</ul>
</li>
<li><a href="#examples">Examples</a><ul>
<li><a href="#collect-trace-events-data-by-inspector">Collect trace events data by inspector</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</details>

      <div id="apicontent">
        <h2>Trace events<span><a class="mark" href="#trace-events" id="trace-events">#</a></span><a aria-hidden="true" class="legacy" id="tracing_trace_events"></a></h2>
<!--introduced_in=v7.7.0-->

<blockquote>
<p>Stability: 1 - Experimental</p>
</blockquote>
<!-- source_link=lib/trace_events.js -->

<p>The <code>node:trace_events</code> module provides a mechanism to centralize tracing
information generated by V8, Node.js core, and userspace code.</p>
<p>Tracing can be enabled with the <code>--trace-event-categories</code> command-line flag
or by using the <code>node:trace_events</code> module. The <code>--trace-event-categories</code> flag
accepts a list of comma-separated category names.</p>
<p>The available categories are:</p>
<ul>
<li><code>node</code>: An empty placeholder.</li>
<li><code>node.async_hooks</code>: Enables capture of detailed <a href="async_hooks.html"><code>async_hooks</code></a> trace data.
The <a href="async_hooks.html"><code>async_hooks</code></a> events have a unique <code>asyncId</code> and a special <code>triggerId</code>
<code>triggerAsyncId</code> property.</li>
<li><code>node.bootstrap</code>: Enables capture of Node.js bootstrap milestones.</li>
<li><code>node.console</code>: Enables capture of <code>console.time()</code> and <code>console.count()</code>
output.</li>
<li><code>node.threadpoolwork.sync</code>: Enables capture of trace data for threadpool
synchronous operations, such as <code>blob</code>, <code>zlib</code>, <code>crypto</code> and <code>node_api</code>.</li>
<li><code>node.threadpoolwork.async</code>: Enables capture of trace data for threadpool
asynchronous operations, such as <code>blob</code>, <code>zlib</code>, <code>crypto</code> and <code>node_api</code>.</li>
<li><code>node.dns.native</code>: Enables capture of trace data for DNS queries.</li>
<li><code>node.net.native</code>: Enables capture of trace data for network.</li>
<li><code>node.environment</code>: Enables capture of Node.js Environment milestones.</li>
<li><code>node.fs.sync</code>: Enables capture of trace data for file system sync methods.</li>
<li><code>node.fs_dir.sync</code>: Enables capture of trace data for file system sync
directory methods.</li>
<li><code>node.fs.async</code>: Enables capture of trace data for file system async methods.</li>
<li><code>node.fs_dir.async</code>: Enables capture of trace data for file system async
directory methods.</li>
<li><code>node.perf</code>: Enables capture of <a href="perf_hooks.html">Performance API</a> measurements.<ul>
<li><code>node.perf.usertiming</code>: Enables capture of only Performance API User Timing
measures and marks.</li>
<li><code>node.perf.timerify</code>: Enables capture of only Performance API timerify
measurements.</li>
</ul>
</li>
<li><code>node.promises.rejections</code>: Enables capture of trace data tracking the number
of unhandled Promise rejections and handled-after-rejections.</li>
<li><code>node.vm.script</code>: Enables capture of trace data for the <code>node:vm</code> module&#39;s
<code>runInNewContext()</code>, <code>runInContext()</code>, and <code>runInThisContext()</code> methods.</li>
<li><code>v8</code>: The <a href="v8.html">V8</a> events are GC, compiling, and execution related.</li>
<li><code>node.http</code>: Enables capture of trace data for http request / response.</li>
</ul>
<p>By default the <code>node</code>, <code>node.async_hooks</code>, and <code>v8</code> categories are enabled.</p>
<pre><code class="language-bash">node --trace-event-categories v8,node,node.async_hooks server.js
</code></pre>
<p>Prior versions of Node.js required the use of the <code>--trace-events-enabled</code>
flag to enable trace events. This requirement has been removed. However, the
<code>--trace-events-enabled</code> flag <em>may</em> still be used and will enable the
<code>node</code>, <code>node.async_hooks</code>, and <code>v8</code> trace event categories by default.</p>
<pre><code class="language-bash">node --trace-events-enabled

# is equivalent to

node --trace-event-categories v8,node,node.async_hooks
</code></pre>
<p>Alternatively, trace events may be enabled using the <code>node:trace_events</code> module:</p>
<pre><code class="language-js">const trace_events = require(&#39;node:trace_events&#39;);
const tracing = trace_events.createTracing({ categories: [&#39;node.perf&#39;] });
tracing.enable();  // Enable trace event capture for the &#39;node.perf&#39; category

// do work

tracing.disable();  // Disable trace event capture for the &#39;node.perf&#39; category
</code></pre>
<p>Running Node.js with tracing enabled will produce log files that can be opened
in the <a href="https://www.chromium.org/developers/how-tos/trace-event-profiling-tool"><code>chrome://tracing</code></a>
tab of Chrome.</p>
<p>The logging file is by default called <code>node_trace.${rotation}.log</code>, where
<code>${rotation}</code> is an incrementing log-rotation id. The filepath pattern can
be specified with <code>--trace-event-file-pattern</code> that accepts a template
string that supports <code>${rotation}</code> and <code>${pid}</code>:</p>
<pre><code class="language-bash">node --trace-event-categories v8 --trace-event-file-pattern &#39;${pid}-${rotation}.log&#39; server.js
</code></pre>
<p>To guarantee that the log file is properly generated after signal events like
<code>SIGINT</code>, <code>SIGTERM</code>, or <code>SIGBREAK</code>, make sure to have the appropriate handlers
in your code, such as:</p>
<pre><code class="language-js">process.on(&#39;SIGINT&#39;, function onSigint() {
  console.info(&#39;Received SIGINT.&#39;);
  process.exit(130);  // Or applicable exit code depending on OS and signal
});
</code></pre>
<p>The tracing system uses the same time source
as the one used by <code>process.hrtime()</code>.
However the trace-event timestamps are expressed in microseconds,
unlike <code>process.hrtime()</code> which returns nanoseconds.</p>
<p>The features from this module are not available in <a href="worker_threads.html#class-worker"><code>Worker</code></a> threads.</p>
<section><h3>The <code>node:trace_events</code> module<span><a class="mark" href="#the-nodetrace_events-module" id="the-nodetrace_events-module">#</a></span><a aria-hidden="true" class="legacy" id="tracing_the_node_trace_events_module"></a></h3>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div><h4><code>Tracing</code> object<span><a class="mark" href="#tracing-object" id="tracing-object">#</a></span><a aria-hidden="true" class="legacy" id="tracing_tracing_object"></a></h4>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div><p>The <code>Tracing</code> object is used to enable or disable tracing for sets of
categories. Instances are created using the <code>trace_events.createTracing()</code>
method.</p>
<p>When created, the <code>Tracing</code> object is disabled. Calling the
<code>tracing.enable()</code> method adds the categories to the set of enabled trace event
categories. Calling <code>tracing.disable()</code> will remove the categories from the
set of enabled trace event categories.</p>
<h5><code>tracing.categories</code><span><a class="mark" href="#tracingcategories" id="tracingcategories">#</a></span><a aria-hidden="true" class="legacy" id="tracing_tracing_categories"></a></h5>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div><ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&lt;string&gt;</a></li>
</ul>
<p>A comma-separated list of the trace event categories covered by this
<code>Tracing</code> object.</p>
<h5><code>tracing.disable()</code><span><a class="mark" href="#tracingdisable" id="tracingdisable">#</a></span><a aria-hidden="true" class="legacy" id="tracing_tracing_disable"></a></h5>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div><p>Disables this <code>Tracing</code> object.</p>
<p>Only trace event categories <em>not</em> covered by other enabled <code>Tracing</code> objects
and <em>not</em> specified by the <code>--trace-event-categories</code> flag will be disabled.</p>
<pre><code class="language-js">const trace_events = require(&#39;node:trace_events&#39;);
const t1 = trace_events.createTracing({ categories: [&#39;node&#39;, &#39;v8&#39;] });
const t2 = trace_events.createTracing({ categories: [&#39;node.perf&#39;, &#39;node&#39;] });
t1.enable();
t2.enable();

// Prints &#39;node,node.perf,v8&#39;
console.log(trace_events.getEnabledCategories());

t2.disable(); // Will only disable emission of the &#39;node.perf&#39; category

// Prints &#39;node,v8&#39;
console.log(trace_events.getEnabledCategories());
</code></pre>
<h5><code>tracing.enable()</code><span><a class="mark" href="#tracingenable" id="tracingenable">#</a></span><a aria-hidden="true" class="legacy" id="tracing_tracing_enable"></a></h5>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div><p>Enables this <code>Tracing</code> object for the set of categories covered by the
<code>Tracing</code> object.</p>
<h5><code>tracing.enabled</code><span><a class="mark" href="#tracingenabled" id="tracingenabled">#</a></span><a aria-hidden="true" class="legacy" id="tracing_tracing_enabled"></a></h5>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div><ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type">&lt;boolean&gt;</a> <code>true</code> only if the <code>Tracing</code> object has been enabled.</li>
</ul>
<h4><code>trace_events.createTracing(options)</code><span><a class="mark" href="#trace_eventscreatetracingoptions" id="trace_eventscreatetracingoptions">#</a></span><a aria-hidden="true" class="legacy" id="tracing_trace_events_createtracing_options"></a></h4>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div><ul>
<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type">&lt;Object&gt;</a><ul>
<li><code>categories</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&lt;string[]&gt;</a> An array of trace category names. Values included
in the array are coerced to a string when possible. An error will be
thrown if the value cannot be coerced.</li>
</ul>
</li>
<li>Returns: <a href="tracing.html#tracing-object" class="type">&lt;Tracing&gt;</a>.</li>
</ul>
<p>Creates and returns a <code>Tracing</code> object for the given set of <code>categories</code>.</p>
<pre><code class="language-js">const trace_events = require(&#39;node:trace_events&#39;);
const categories = [&#39;node.perf&#39;, &#39;node.async_hooks&#39;];
const tracing = trace_events.createTracing({ categories });
tracing.enable();
// do stuff
tracing.disable();
</code></pre>
<h4><code>trace_events.getEnabledCategories()</code><span><a class="mark" href="#trace_eventsgetenabledcategories" id="trace_eventsgetenabledcategories">#</a></span><a aria-hidden="true" class="legacy" id="tracing_trace_events_getenabledcategories"></a></h4>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div><ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&lt;string&gt;</a></li>
</ul>
<p>Returns a comma-separated list of all currently-enabled trace event
categories. The current set of enabled trace event categories is determined
by the <em>union</em> of all currently-enabled <code>Tracing</code> objects and any categories
enabled using the <code>--trace-event-categories</code> flag.</p>
<p>Given the file <code>test.js</code> below, the command
<code>node --trace-event-categories node.perf test.js</code> will print
<code>&#39;node.async_hooks,node.perf&#39;</code> to the console.</p>
<pre><code class="language-js">const trace_events = require(&#39;node:trace_events&#39;);
const t1 = trace_events.createTracing({ categories: [&#39;node.async_hooks&#39;] });
const t2 = trace_events.createTracing({ categories: [&#39;node.perf&#39;] });
const t3 = trace_events.createTracing({ categories: [&#39;v8&#39;] });

t1.enable();
t2.enable();

console.log(trace_events.getEnabledCategories());
</code></pre>
</section><section><h3>Examples<span><a class="mark" href="#examples" id="examples">#</a></span><a aria-hidden="true" class="legacy" id="tracing_examples"></a></h3>
<h4>Collect trace events data by inspector<span><a class="mark" href="#collect-trace-events-data-by-inspector" id="collect-trace-events-data-by-inspector">#</a></span><a aria-hidden="true" class="legacy" id="tracing_collect_trace_events_data_by_inspector"></a></h4>
<pre><code class="language-js">&#39;use strict&#39;;

const { Session } = require(&#39;inspector&#39;);
const session = new Session();
session.connect();

function post(message, data) {
  return new Promise((resolve, reject) =&gt; {
    session.post(message, data, (err, result) =&gt; {
      if (err)
        reject(new Error(JSON.stringify(err)));
      else
        resolve(result);
    });
  });
}

async function collect() {
  const data = [];
  session.on(&#39;NodeTracing.dataCollected&#39;, (chunk) =&gt; data.push(chunk));
  session.on(&#39;NodeTracing.tracingComplete&#39;, () =&gt; {
    // done
  });
  const traceConfig = { includedCategories: [&#39;v8&#39;] };
  await post(&#39;NodeTracing.start&#39;, { traceConfig });
  // do something
  setTimeout(() =&gt; {
    post(&#39;NodeTracing.stop&#39;).then(() =&gt; {
      session.disconnect();
      console.log(data);
    });
  }, 1000);
}

collect();
</code></pre>
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