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<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width"> <meta name="nodejs.org:node-version" content="v16.20.0"> <title>Inspector | Node.js v16.20.0 Documentation</title> <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Lato:400,700,400italic&display=fallback"> <link rel="stylesheet" href="assets/style.css"> <link rel="stylesheet" href="assets/hljs.css"> <link rel="canonical" href="https://nodejs.org/api/inspector.html"> <script async defer src="assets/api.js" type="text/javascript"></script> </head> <body class="alt apidoc" id="api-section-inspector"> <div id="content" class="clearfix"> <div id="column2" class="interior"> <div id="intro" class="interior"> <a href="/" title="Go back to the home page"> Node.js </a> </div> <ul> <li><a href="documentation.html" class="nav-documentation">About this documentation</a></li> <li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li> </ul> <hr 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href="#inspectoropenport-host-wait"><code>inspector.open([port[, host[, wait]]])</code></a></li> <li><a href="#inspectorurl"><code>inspector.url()</code></a></li> <li><a href="#inspectorwaitfordebugger"><code>inspector.waitForDebugger()</code></a></li> <li><a href="#class-inspectorsession">Class: <code>inspector.Session</code></a> <ul> <li><a href="#new-inspectorsession"><code>new inspector.Session()</code></a></li> <li><a href="#event-inspectornotification">Event: <code>'inspectorNotification'</code></a></li> <li><a href="#event-inspector-protocol-method">Event: <code><inspector-protocol-method></code>;</a></li> <li><a href="#sessionconnect"><code>session.connect()</code></a></li> <li><a href="#sessionconnecttomainthread"><code>session.connectToMainThread()</code></a></li> <li><a href="#sessiondisconnect"><code>session.disconnect()</code></a></li> <li><a href="#sessionpostmethod-params-callback"><code>session.post(method[, params][, callback])</code></a></li> </ul> </li> <li><a href="#example-usage">Example usage</a> <ul> <li><a href="#cpu-profiler">CPU profiler</a></li> <li><a href="#heap-profiler">Heap profiler</a></li> </ul> </li> </ul> </li> </ul></div></div> </li> <li class="picker-header"> <a href="#"> <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> Index </a> <div class="picker"><ul> <li><a href="documentation.html" class="nav-documentation">About this documentation</a></li> <li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li> <li> <a href="index.html">Index</a> </li> </ul> <hr class="line"> <ul> <li><a href="assert.html" class="nav-assert">Assertion testing</a></li> <li><a href="async_context.html" class="nav-async_context">Asynchronous context tracking</a></li> <li><a href="async_hooks.html" class="nav-async_hooks">Async hooks</a></li> <li><a href="buffer.html" class="nav-buffer">Buffer</a></li> <li><a href="addons.html" class="nav-addons">C++ addons</a></li> <li><a href="n-api.html" 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class="nav-dgram">UDP/datagram</a></li> <li><a href="url.html" class="nav-url">URL</a></li> <li><a href="util.html" class="nav-util">Utilities</a></li> <li><a href="v8.html" class="nav-v8">V8</a></li> <li><a href="vm.html" class="nav-vm">VM</a></li> <li><a href="wasi.html" class="nav-wasi">WASI</a></li> <li><a href="webcrypto.html" class="nav-webcrypto">Web Crypto API</a></li> <li><a href="webstreams.html" class="nav-webstreams">Web Streams API</a></li> <li><a href="worker_threads.html" class="nav-worker_threads">Worker threads</a></li> <li><a href="zlib.html" class="nav-zlib">Zlib</a></li> </ul> <hr class="line"> <ul> <li><a href="https://github.com/nodejs/node" class="nav-https-github-com-nodejs-node">Code repository and issue tracker</a></li> </ul></div> </li> <li class="picker-header"> <a href="#"> <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> Other versions </a> <div class="picker"><ol id="alt-docs"><li><a href="https://nodejs.org/docs/latest-v19.x/api/inspector.html">19.x</a></li> <li><a href="https://nodejs.org/docs/latest-v18.x/api/inspector.html">18.x <b>LTS</b></a></li> <li><a href="https://nodejs.org/docs/latest-v17.x/api/inspector.html">17.x</a></li> <li><a href="https://nodejs.org/docs/latest-v16.x/api/inspector.html">16.x <b>LTS</b></a></li> <li><a href="https://nodejs.org/docs/latest-v15.x/api/inspector.html">15.x</a></li> <li><a href="https://nodejs.org/docs/latest-v14.x/api/inspector.html">14.x <b>LTS</b></a></li> <li><a href="https://nodejs.org/docs/latest-v13.x/api/inspector.html">13.x</a></li> <li><a href="https://nodejs.org/docs/latest-v12.x/api/inspector.html">12.x</a></li> <li><a href="https://nodejs.org/docs/latest-v11.x/api/inspector.html">11.x</a></li> <li><a href="https://nodejs.org/docs/latest-v10.x/api/inspector.html">10.x</a></li> <li><a href="https://nodejs.org/docs/latest-v9.x/api/inspector.html">9.x</a></li> <li><a href="https://nodejs.org/docs/latest-v8.x/api/inspector.html">8.x</a></li></ol></div> </li> <li class="picker-header"> <a href="#"> <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> Options </a> <div class="picker"> <ul> <li> <a href="all.html">View on single page</a> </li> <li> <a href="inspector.json">View as JSON</a> </li> <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/main/doc/api/inspector.md">Edit on GitHub</a></li> </ul> </div> </li> </ul> </div> <hr> </header> <details id="toc" open><summary>Table of contents</summary><ul> <li><span class="stability_2"><a href="#inspector">Inspector</a></span> <ul> <li><a href="#inspectorclose"><code>inspector.close()</code></a></li> <li><a href="#inspectorconsole"><code>inspector.console</code></a></li> <li><a href="#inspectoropenport-host-wait"><code>inspector.open([port[, host[, wait]]])</code></a></li> <li><a href="#inspectorurl"><code>inspector.url()</code></a></li> <li><a href="#inspectorwaitfordebugger"><code>inspector.waitForDebugger()</code></a></li> <li><a href="#class-inspectorsession">Class: <code>inspector.Session</code></a> <ul> <li><a href="#new-inspectorsession"><code>new inspector.Session()</code></a></li> <li><a href="#event-inspectornotification">Event: <code>'inspectorNotification'</code></a></li> <li><a href="#event-inspector-protocol-method">Event: <code><inspector-protocol-method></code>;</a></li> <li><a href="#sessionconnect"><code>session.connect()</code></a></li> <li><a href="#sessionconnecttomainthread"><code>session.connectToMainThread()</code></a></li> <li><a href="#sessiondisconnect"><code>session.disconnect()</code></a></li> <li><a href="#sessionpostmethod-params-callback"><code>session.post(method[, params][, callback])</code></a></li> </ul> </li> <li><a href="#example-usage">Example usage</a> <ul> <li><a href="#cpu-profiler">CPU profiler</a></li> <li><a href="#heap-profiler">Heap profiler</a></li> </ul> </li> </ul> </li> </ul></details> <div id="apicontent"> <h2>Inspector<span><a class="mark" href="#inspector" id="inspector">#</a></span><a aria-hidden="true" class="legacy" id="inspector_inspector"></a></h2> <p></p><div class="api_stability api_stability_2"><a href="documentation.html#stability-index">Stability: 2</a> - Stable</div><p></p> <p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v16.20.0/lib/inspector.js">lib/inspector.js</a></p> <p>The <code>node:inspector</code> module provides an API for interacting with the V8 inspector.</p> <p>It can be accessed using:</p> <pre><code class="language-js"><span class="hljs-keyword">const</span> inspector = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:inspector'</span>);</code></pre> <section><h3><code>inspector.close()</code><span><a class="mark" href="#inspectorclose" id="inspectorclose">#</a></span><a aria-hidden="true" class="legacy" id="inspector_inspector_close"></a></h3> <div class="api_metadata"> <details class="changelog"><summary>History</summary> <table> <tbody><tr><th>Version</th><th>Changes</th></tr> <tr><td>v16.18.0</td> <td><p>The API is exposed in the worker threads.</p></td></tr> <tr><td>v9.0.0</td> <td><p><span>Added in: v9.0.0</span></p></td></tr> </tbody></table> </details> </div> <p>Deactivate the inspector. Blocks until there are no active connections.</p> </section><section><h3><code>inspector.console</code><span><a class="mark" href="#inspectorconsole" id="inspectorconsole">#</a></span><a aria-hidden="true" class="legacy" id="inspector_inspector_console"></a></h3> <ul> <li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> An object to send messages to the remote inspector console.</li> </ul> <pre><code class="language-js"><span class="hljs-built_in">require</span>(<span class="hljs-string">'node:inspector'</span>).<span class="hljs-property">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'a message'</span>);</code></pre> <p>The inspector console does not have API parity with Node.js console.</p> </section><section><h3><code>inspector.open([port[, host[, wait]]])</code><span><a class="mark" href="#inspectoropenport-host-wait" id="inspectoropenport-host-wait">#</a></span><a aria-hidden="true" class="legacy" id="inspector_inspector_open_port_host_wait"></a></h3> <ul> <li><code>port</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> Port to listen on for inspector connections. Optional. <strong>Default:</strong> what was specified on the CLI.</li> <li><code>host</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> Host to listen on for inspector connections. Optional. <strong>Default:</strong> what was specified on the CLI.</li> <li><code>wait</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> Block until a client has connected. Optional. <strong>Default:</strong> <code>false</code>.</li> </ul> <p>Activate inspector on host and port. Equivalent to <code>node --inspect=[[host:]port]</code>, but can be done programmatically after node has started.</p> <p>If wait is <code>true</code>, will block until a client has connected to the inspect port and flow control has been passed to the debugger client.</p> <p>See the <a href="cli.html#warning-binding-inspector-to-a-public-ipport-combination-is-insecure">security warning</a> regarding the <code>host</code> parameter usage.</p> </section><section><h3><code>inspector.url()</code><span><a class="mark" href="#inspectorurl" id="inspectorurl">#</a></span><a aria-hidden="true" class="legacy" id="inspector_inspector_url"></a></h3> <ul> <li>Returns: <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#Undefined_type" class="type"><undefined></a></li> </ul> <p>Return the URL of the active inspector, or <code>undefined</code> if there is none.</p> <pre><code class="language-console"><span class="hljs-meta prompt_">$ </span><span class="language-bash">node --inspect -p <span class="hljs-string">'inspector.url()'</span></span> Debugger listening on ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34 For help, see: https://nodejs.org/en/docs/inspector ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34 <span class="hljs-meta prompt_"> $ </span><span class="language-bash">node --inspect=localhost:3000 -p <span class="hljs-string">'inspector.url()'</span></span> Debugger listening on ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a For help, see: https://nodejs.org/en/docs/inspector ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a <span class="hljs-meta prompt_"> $ </span><span class="language-bash">node -p <span class="hljs-string">'inspector.url()'</span></span> undefined</code></pre> </section><section><h3><code>inspector.waitForDebugger()</code><span><a class="mark" href="#inspectorwaitfordebugger" id="inspectorwaitfordebugger">#</a></span><a aria-hidden="true" class="legacy" id="inspector_inspector_waitfordebugger"></a></h3> <div class="api_metadata"> <span>Added in: v12.7.0</span> </div> <p>Blocks until a client (existing or connected later) has sent <code>Runtime.runIfWaitingForDebugger</code> command.</p> <p>An exception will be thrown if there is no active inspector.</p> </section><section><h3>Class: <code>inspector.Session</code><span><a class="mark" href="#class-inspectorsession" id="class-inspectorsession">#</a></span><a aria-hidden="true" class="legacy" id="inspector_class_inspector_session"></a></h3> <ul> <li>Extends: <a href="events.html#class-eventemitter" class="type"><EventEmitter></a></li> </ul> <p>The <code>inspector.Session</code> is used for dispatching messages to the V8 inspector back-end and receiving message responses and notifications.</p> <h4><code>new inspector.Session()</code><span><a class="mark" href="#new-inspectorsession" id="new-inspectorsession">#</a></span><a aria-hidden="true" class="legacy" id="inspector_new_inspector_session"></a></h4> <div class="api_metadata"> <span>Added in: v8.0.0</span> </div> <p>Create a new instance of the <code>inspector.Session</code> class. The inspector session needs to be connected through <a href="#sessionconnect"><code>session.connect()</code></a> before the messages can be dispatched to the inspector backend.</p> <h4>Event: <code>'inspectorNotification'</code><span><a class="mark" href="#event-inspectornotification" id="event-inspectornotification">#</a></span><a aria-hidden="true" class="legacy" id="inspector_event_inspectornotification"></a></h4> <div class="api_metadata"> <span>Added in: v8.0.0</span> </div> <ul> <li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> The notification message object</li> </ul> <p>Emitted when any notification from the V8 Inspector is received.</p> <pre><code class="language-js">session.<span class="hljs-title function_">on</span>(<span class="hljs-string">'inspectorNotification'</span>, <span class="hljs-function">(<span class="hljs-params">message</span>) =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(message.<span class="hljs-property">method</span>)); <span class="hljs-comment">// Debugger.paused</span> <span class="hljs-comment">// Debugger.resumed</span></code></pre> <p>It is also possible to subscribe only to notifications with specific method:</p> <h4>Event: <code><inspector-protocol-method></code>;<span><a class="mark" href="#event-inspector-protocol-method" id="event-inspector-protocol-method">#</a></span><a aria-hidden="true" class="legacy" id="inspector_event_inspector_protocol_method"></a></h4> <div class="api_metadata"> <span>Added in: v8.0.0</span> </div> <ul> <li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> The notification message object</li> </ul> <p>Emitted when an inspector notification is received that has its method field set to the <code><inspector-protocol-method></code> value.</p> <p>The following snippet installs a listener on the <a href="https://chromedevtools.github.io/devtools-protocol/v8/Debugger#event-paused"><code>'Debugger.paused'</code></a> event, and prints the reason for program suspension whenever program execution is suspended (through breakpoints, for example):</p> <pre><code class="language-js">session.<span class="hljs-title function_">on</span>(<span class="hljs-string">'Debugger.paused'</span>, <span class="hljs-function">(<span class="hljs-params">{ params }</span>) =></span> { <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(params.<span class="hljs-property">hitBreakpoints</span>); }); <span class="hljs-comment">// [ '/the/file/that/has/the/breakpoint.js:11:0' ]</span></code></pre> <h4><code>session.connect()</code><span><a class="mark" href="#sessionconnect" id="sessionconnect">#</a></span><a aria-hidden="true" class="legacy" id="inspector_session_connect"></a></h4> <div class="api_metadata"> <span>Added in: v8.0.0</span> </div> <p>Connects a session to the inspector back-end.</p> <h4><code>session.connectToMainThread()</code><span><a class="mark" href="#sessionconnecttomainthread" id="sessionconnecttomainthread">#</a></span><a aria-hidden="true" class="legacy" id="inspector_session_connecttomainthread"></a></h4> <div class="api_metadata"> <span>Added in: v12.11.0</span> </div> <p>Connects a session to the main thread inspector back-end. An exception will be thrown if this API was not called on a Worker thread.</p> <h4><code>session.disconnect()</code><span><a class="mark" href="#sessiondisconnect" id="sessiondisconnect">#</a></span><a aria-hidden="true" class="legacy" id="inspector_session_disconnect"></a></h4> <div class="api_metadata"> <span>Added in: v8.0.0</span> </div> <p>Immediately close the session. All pending message callbacks will be called with an error. <a href="#sessionconnect"><code>session.connect()</code></a> will need to be called to be able to send messages again. Reconnected session will lose all inspector state, such as enabled agents or configured breakpoints.</p> <h4><code>session.post(method[, params][, callback])</code><span><a class="mark" href="#sessionpostmethod-params-callback" id="sessionpostmethod-params-callback">#</a></span><a aria-hidden="true" class="legacy" id="inspector_session_post_method_params_callback"></a></h4> <div class="api_metadata"> <span>Added in: v8.0.0</span> </div> <ul> <li><code>method</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> <li><code>params</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li> <li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> </ul> <p>Posts a message to the inspector back-end. <code>callback</code> will be notified when a response is received. <code>callback</code> is a function that accepts two optional arguments: error and message-specific result.</p> <pre><code class="language-js">session.<span class="hljs-title function_">post</span>(<span class="hljs-string">'Runtime.evaluate'</span>, { <span class="hljs-attr">expression</span>: <span class="hljs-string">'2 + 2'</span> }, <span class="hljs-function">(<span class="hljs-params">error, { result }</span>) =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(result)); <span class="hljs-comment">// Output: { type: 'number', value: 4, description: '4' }</span></code></pre> <p>The latest version of the V8 inspector protocol is published on the <a href="https://chromedevtools.github.io/devtools-protocol/v8/">Chrome DevTools Protocol Viewer</a>.</p> <p>Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.</p> </section><section><h3>Example usage<span><a class="mark" href="#example-usage" id="example-usage">#</a></span><a aria-hidden="true" class="legacy" id="inspector_example_usage"></a></h3> <p>Apart from the debugger, various V8 Profilers are available through the DevTools protocol.</p> <h4>CPU profiler<span><a class="mark" href="#cpu-profiler" id="cpu-profiler">#</a></span><a aria-hidden="true" class="legacy" id="inspector_cpu_profiler"></a></h4> <p>Here's an example showing how to use the <a href="https://chromedevtools.github.io/devtools-protocol/v8/Profiler">CPU Profiler</a>:</p> <pre><code class="language-js"><span class="hljs-keyword">const</span> inspector = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:inspector'</span>); <span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); <span class="hljs-keyword">const</span> session = <span class="hljs-keyword">new</span> inspector.<span class="hljs-title class_">Session</span>(); session.<span class="hljs-title function_">connect</span>(); session.<span class="hljs-title function_">post</span>(<span class="hljs-string">'Profiler.enable'</span>, <span class="hljs-function">() =></span> { session.<span class="hljs-title function_">post</span>(<span class="hljs-string">'Profiler.start'</span>, <span class="hljs-function">() =></span> { <span class="hljs-comment">// Invoke business logic under measurement here...</span> <span class="hljs-comment">// some time later...</span> session.<span class="hljs-title function_">post</span>(<span class="hljs-string">'Profiler.stop'</span>, <span class="hljs-function">(<span class="hljs-params">err, { profile }</span>) =></span> { <span class="hljs-comment">// Write profile to disk, upload, etc.</span> <span class="hljs-keyword">if</span> (!err) { fs.<span class="hljs-title function_">writeFileSync</span>(<span class="hljs-string">'./profile.cpuprofile'</span>, <span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">stringify</span>(profile)); } }); }); });</code></pre> <h4>Heap profiler<span><a class="mark" href="#heap-profiler" id="heap-profiler">#</a></span><a aria-hidden="true" class="legacy" id="inspector_heap_profiler"></a></h4> <p>Here's an example showing how to use the <a href="https://chromedevtools.github.io/devtools-protocol/v8/HeapProfiler">Heap Profiler</a>:</p> <pre><code class="language-js"><span class="hljs-keyword">const</span> inspector = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:inspector'</span>); <span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); <span class="hljs-keyword">const</span> session = <span class="hljs-keyword">new</span> inspector.<span class="hljs-title class_">Session</span>(); <span class="hljs-keyword">const</span> fd = fs.<span class="hljs-title function_">openSync</span>(<span class="hljs-string">'profile.heapsnapshot'</span>, <span class="hljs-string">'w'</span>); session.<span class="hljs-title function_">connect</span>(); session.<span class="hljs-title function_">on</span>(<span class="hljs-string">'HeapProfiler.addHeapSnapshotChunk'</span>, <span class="hljs-function">(<span class="hljs-params">m</span>) =></span> { fs.<span class="hljs-title function_">writeSync</span>(fd, m.<span class="hljs-property">params</span>.<span class="hljs-property">chunk</span>); }); session.<span class="hljs-title function_">post</span>(<span class="hljs-string">'HeapProfiler.takeHeapSnapshot'</span>, <span class="hljs-literal">null</span>, <span class="hljs-function">(<span class="hljs-params">err, r</span>) =></span> { <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'HeapProfiler.takeHeapSnapshot done:'</span>, err, r); session.<span class="hljs-title function_">disconnect</span>(); fs.<span class="hljs-title function_">closeSync</span>(fd); });</code></pre></section> <!-- API END --> </div> </div> </div> </body> </html>
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