1<!DOCTYPE html> 2<html lang="en"> 3<head> 4 <meta charset="utf-8"> 5 <meta name="viewport" content="width=device-width"> 6 <meta name="nodejs.org:node-version" content="v18.20.1"> 7 <title>Debugger | Node.js v18.20.1 Documentation</title> 8 <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Lato:400,700,400italic&display=fallback"> 9 <link rel="stylesheet" href="assets/style.css"> 10 <link rel="stylesheet" href="assets/hljs.css"> 11 <link rel="canonical" href="https://nodejs.org/api/debugger.html"> 12 <script async defer src="assets/api.js" type="text/javascript"></script> 13 14</head> 15<body class="alt apidoc" id="api-section-debugger"> 16 <div id="content" class="clearfix"> 17 <div id="column2" class="interior"> 18 <div id="intro" class="interior"> 19 <a href="/" title="Go back to the home page"> 20 Node.js 21 </a> 22 </div> 23 <ul> 24<li><a href="documentation.html" class="nav-documentation">About this documentation</a></li> 25<li><a href="synopsis.html" 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171<li><a href="cluster.html" class="nav-cluster">Cluster</a></li> 172<li><a href="cli.html" class="nav-cli">Command-line options</a></li> 173<li><a href="console.html" class="nav-console">Console</a></li> 174<li><a href="corepack.html" class="nav-corepack">Corepack</a></li> 175<li><a href="crypto.html" class="nav-crypto">Crypto</a></li> 176<li><a href="debugger.html" class="nav-debugger active">Debugger</a></li> 177<li><a href="deprecations.html" class="nav-deprecations">Deprecated APIs</a></li> 178<li><a href="diagnostics_channel.html" class="nav-diagnostics_channel">Diagnostics Channel</a></li> 179<li><a href="dns.html" class="nav-dns">DNS</a></li> 180<li><a href="domain.html" class="nav-domain">Domain</a></li> 181<li><a href="errors.html" class="nav-errors">Errors</a></li> 182<li><a href="events.html" class="nav-events">Events</a></li> 183<li><a href="fs.html" class="nav-fs">File system</a></li> 184<li><a href="globals.html" class="nav-globals">Globals</a></li> 185<li><a 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class="nav-tty">TTY</a></li> 213<li><a href="dgram.html" class="nav-dgram">UDP/datagram</a></li> 214<li><a href="url.html" class="nav-url">URL</a></li> 215<li><a href="util.html" class="nav-util">Utilities</a></li> 216<li><a href="v8.html" class="nav-v8">V8</a></li> 217<li><a href="vm.html" class="nav-vm">VM</a></li> 218<li><a href="wasi.html" class="nav-wasi">WASI</a></li> 219<li><a href="webcrypto.html" class="nav-webcrypto">Web Crypto API</a></li> 220<li><a href="webstreams.html" class="nav-webstreams">Web Streams API</a></li> 221<li><a href="worker_threads.html" class="nav-worker_threads">Worker threads</a></li> 222<li><a href="zlib.html" class="nav-zlib">Zlib</a></li> 223</ul> 224<hr class="line"> 225<ul> 226<li><a href="https://github.com/nodejs/node" class="nav-https-github-com-nodejs-node">Code repository and issue tracker</a></li> 227</ul></div> 228 </li> 229 230 231 <li class="picker-header"> 232 <a href="#"> 233 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 234 Other versions 235 </a> 236 <div class="picker"><ol id="alt-docs"><li><a href="https://nodejs.org/docs/latest-v21.x/api/debugger.html">21.x</a></li> 237<li><a href="https://nodejs.org/docs/latest-v20.x/api/debugger.html">20.x <b>LTS</b></a></li> 238<li><a href="https://nodejs.org/docs/latest-v19.x/api/debugger.html">19.x</a></li> 239<li><a href="https://nodejs.org/docs/latest-v18.x/api/debugger.html">18.x <b>LTS</b></a></li> 240<li><a href="https://nodejs.org/docs/latest-v17.x/api/debugger.html">17.x</a></li> 241<li><a href="https://nodejs.org/docs/latest-v16.x/api/debugger.html">16.x</a></li> 242<li><a href="https://nodejs.org/docs/latest-v15.x/api/debugger.html">15.x</a></li> 243<li><a href="https://nodejs.org/docs/latest-v14.x/api/debugger.html">14.x</a></li> 244<li><a href="https://nodejs.org/docs/latest-v13.x/api/debugger.html">13.x</a></li> 245<li><a href="https://nodejs.org/docs/latest-v12.x/api/debugger.html">12.x</a></li> 246<li><a href="https://nodejs.org/docs/latest-v11.x/api/debugger.html">11.x</a></li> 247<li><a href="https://nodejs.org/docs/latest-v10.x/api/debugger.html">10.x</a></li> 248<li><a href="https://nodejs.org/docs/latest-v9.x/api/debugger.html">9.x</a></li> 249<li><a href="https://nodejs.org/docs/latest-v8.x/api/debugger.html">8.x</a></li> 250<li><a href="https://nodejs.org/docs/latest-v7.x/api/debugger.html">7.x</a></li> 251<li><a href="https://nodejs.org/docs/latest-v6.x/api/debugger.html">6.x</a></li> 252<li><a href="https://nodejs.org/docs/latest-v5.x/api/debugger.html">5.x</a></li> 253<li><a href="https://nodejs.org/docs/latest-v4.x/api/debugger.html">4.x</a></li> 254<li><a href="https://nodejs.org/docs/latest-v0.12.x/api/debugger.html">0.12.x</a></li> 255<li><a href="https://nodejs.org/docs/latest-v0.10.x/api/debugger.html">0.10.x</a></li></ol></div> 256 </li> 257 258 <li class="picker-header"> 259 <a href="#"> 260 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 261 Options 262 </a> 263 264 <div class="picker"> 265 <ul> 266 <li> 267 <a href="all.html">View on single page</a> 268 </li> 269 <li> 270 <a href="debugger.json">View as JSON</a> 271 </li> 272 <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/main/doc/api/debugger.md">Edit on GitHub</a></li> 273 </ul> 274 </div> 275 </li> 276 </ul> 277 </div> 278 <hr> 279 </header> 280 281 <details id="toc" open><summary>Table of contents</summary><ul> 282<li><span class="stability_2"><a href="#debugger">Debugger</a></span> 283<ul> 284<li><a href="#watchers">Watchers</a></li> 285<li><a href="#command-reference">Command reference</a> 286<ul> 287<li><a href="#stepping">Stepping</a></li> 288<li><a href="#breakpoints">Breakpoints</a></li> 289<li><a href="#information">Information</a></li> 290<li><a href="#execution-control">Execution control</a></li> 291<li><a href="#various">Various</a></li> 292</ul> 293</li> 294<li><a href="#advanced-usage">Advanced usage</a> 295<ul> 296<li><a href="#v8-inspector-integration-for-nodejs">V8 inspector integration for Node.js</a></li> 297</ul> 298</li> 299</ul> 300</li> 301</ul></details> 302 303 <div id="apicontent"> 304 <h2>Debugger<span><a class="mark" href="#debugger" id="debugger">#</a></span><a aria-hidden="true" class="legacy" id="debugger_debugger"></a></h2> 305 306<p></p><div class="api_stability api_stability_2"><a href="documentation.html#stability-index">Stability: 2</a> - Stable</div><p></p> 307 308<p>Node.js includes a command-line debugging utility. The Node.js debugger client 309is not a full-featured debugger, but simple stepping and inspection are 310possible.</p> 311<p>To use it, start Node.js with the <code>inspect</code> argument followed by the path to the 312script to debug.</p> 313<pre><code class="language-console"><span class="hljs-meta prompt_">$ </span><span class="language-bash">node inspect myscript.js</span> 314< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8 315< For help, see: https://nodejs.org/en/docs/inspector 316< 317connecting to 127.0.0.1:9229 ... ok 318< Debugger attached. 319< 320 ok 321Break on start in myscript.js:2 322 1 // myscript.js 323<span class="hljs-meta prompt_">> </span><span class="language-bash">2 global.x = 5;</span> 324 3 setTimeout(() => { 325 4 debugger; 326<span class="hljs-meta prompt_">debug></span></code> <button class="copy-button">copy</button></pre> 327<p>The debugger automatically breaks on the first executable line. To instead 328run until the first breakpoint (specified by a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/debugger"><code>debugger</code></a> statement), set 329the <code>NODE_INSPECT_RESUME_ON_START</code> environment variable to <code>1</code>.</p> 330<pre><code class="language-console"><span class="hljs-meta prompt_">$ </span><span class="language-bash"><span class="hljs-built_in">cat</span> myscript.js</span> 331// myscript.js 332global.x = 5; 333setTimeout(() => { 334 debugger; 335 console.log('world'); 336}, 1000); 337console.log('hello'); 338<span class="hljs-meta prompt_">$ </span><span class="language-bash">NODE_INSPECT_RESUME_ON_START=1 node inspect myscript.js</span> 339< Debugger listening on ws://127.0.0.1:9229/f1ed133e-7876-495b-83ae-c32c6fc319c2 340< For help, see: https://nodejs.org/en/docs/inspector 341< 342connecting to 127.0.0.1:9229 ... ok 343< Debugger attached. 344< 345< hello 346< 347break in myscript.js:4 348 2 global.x = 5; 349 3 setTimeout(() => { 350<span class="hljs-meta prompt_">> </span><span class="language-bash">4 debugger;</span> 351 5 console.log('world'); 352 6 }, 1000); 353<span class="hljs-meta prompt_">debug> </span><span class="language-bash">next</span> 354break in myscript.js:5 355 3 setTimeout(() => { 356 4 debugger; 357<span class="hljs-meta prompt_">> </span><span class="language-bash">5 console.log(<span class="hljs-string">'world'</span>);</span> 358 6 }, 1000); 359 7 console.log('hello'); 360<span class="hljs-meta prompt_">debug> </span><span class="language-bash">repl</span> 361Press Ctrl+C to leave debug repl 362<span class="hljs-meta prompt_">> </span><span class="language-bash">x</span> 3635 364<span class="hljs-meta prompt_">> </span><span class="language-bash">2 + 2</span> 3654 366<span class="hljs-meta prompt_">debug> </span><span class="language-bash">next</span> 367< world 368< 369break in myscript.js:6 370 4 debugger; 371 5 console.log('world'); 372<span class="hljs-meta prompt_">> </span><span class="language-bash">6 }, 1000);</span> 373 7 console.log('hello'); 374 8 375<span class="hljs-meta prompt_">debug> </span><span class="language-bash">.<span class="hljs-built_in">exit</span></span> 376<span class="hljs-meta prompt_">$</span></code> <button class="copy-button">copy</button></pre> 377<p>The <code>repl</code> command allows code to be evaluated remotely. The <code>next</code> command 378steps to the next line. Type <code>help</code> to see what other commands are available.</p> 379<p>Pressing <code>enter</code> without typing a command will repeat the previous debugger 380command.</p> 381<section><h3>Watchers<span><a class="mark" href="#watchers" id="watchers">#</a></span><a aria-hidden="true" class="legacy" id="debugger_watchers"></a></h3> 382<p>It is possible to watch expression and variable values while debugging. On 383every breakpoint, each expression from the watchers list will be evaluated 384in the current context and displayed immediately before the breakpoint's 385source code listing.</p> 386<p>To begin watching an expression, type <code>watch('my_expression')</code>. The command 387<code>watchers</code> will print the active watchers. To remove a watcher, type 388<code>unwatch('my_expression')</code>.</p> 389</section><section><h3>Command reference<span><a class="mark" href="#command-reference" id="command-reference">#</a></span><a aria-hidden="true" class="legacy" id="debugger_command_reference"></a></h3> 390<h4>Stepping<span><a class="mark" href="#stepping" id="stepping">#</a></span><a aria-hidden="true" class="legacy" id="debugger_stepping"></a></h4> 391<ul> 392<li><code>cont</code>, <code>c</code>: Continue execution</li> 393<li><code>next</code>, <code>n</code>: Step next</li> 394<li><code>step</code>, <code>s</code>: Step in</li> 395<li><code>out</code>, <code>o</code>: Step out</li> 396<li><code>pause</code>: Pause running code (like pause button in Developer Tools)</li> 397</ul> 398<h4>Breakpoints<span><a class="mark" href="#breakpoints" id="breakpoints">#</a></span><a aria-hidden="true" class="legacy" id="debugger_breakpoints"></a></h4> 399<ul> 400<li><code>setBreakpoint()</code>, <code>sb()</code>: Set breakpoint on current line</li> 401<li><code>setBreakpoint(line)</code>, <code>sb(line)</code>: Set breakpoint on specific line</li> 402<li><code>setBreakpoint('fn()')</code>, <code>sb(...)</code>: Set breakpoint on a first statement in 403function's body</li> 404<li><code>setBreakpoint('script.js', 1)</code>, <code>sb(...)</code>: Set breakpoint on first line of 405<code>script.js</code></li> 406<li><code>setBreakpoint('script.js', 1, 'num < 4')</code>, <code>sb(...)</code>: Set conditional 407breakpoint on first line of <code>script.js</code> that only breaks when <code>num < 4</code> 408evaluates to <code>true</code></li> 409<li><code>clearBreakpoint('script.js', 1)</code>, <code>cb(...)</code>: Clear breakpoint in <code>script.js</code> 410on line 1</li> 411</ul> 412<p>It is also possible to set a breakpoint in a file (module) that 413is not loaded yet:</p> 414<pre><code class="language-console"><span class="hljs-meta prompt_">$ </span><span class="language-bash">node inspect main.js</span> 415< Debugger listening on ws://127.0.0.1:9229/48a5b28a-550c-471b-b5e1-d13dd7165df9 416< For help, see: https://nodejs.org/en/docs/inspector 417< 418connecting to 127.0.0.1:9229 ... ok 419< Debugger attached. 420< 421Break on start in main.js:1 422<span class="hljs-meta prompt_">> </span><span class="language-bash">1 const mod = require(<span class="hljs-string">'./mod.js'</span>);</span> 423 2 mod.hello(); 424 3 mod.hello(); 425<span class="hljs-meta prompt_">debug> </span><span class="language-bash">setBreakpoint(<span class="hljs-string">'mod.js'</span>, 22)</span> 426Warning: script 'mod.js' was not loaded yet. 427<span class="hljs-meta prompt_">debug> </span><span class="language-bash">c</span> 428break in mod.js:22 429 20 // USE OR OTHER DEALINGS IN THE SOFTWARE. 430 21 431<span class="hljs-meta prompt_">></span><span class="language-bash">22 exports.hello = <span class="hljs-function"><span class="hljs-title">function</span></span>() {</span> 432 23 return 'hello from module'; 433 24 }; 434<span class="hljs-meta prompt_">debug></span></code> <button class="copy-button">copy</button></pre> 435<p>It is also possible to set a conditional breakpoint that only breaks when a 436given expression evaluates to <code>true</code>:</p> 437<pre><code class="language-console"><span class="hljs-meta prompt_">$ </span><span class="language-bash">node inspect main.js</span> 438< Debugger listening on ws://127.0.0.1:9229/ce24daa8-3816-44d4-b8ab-8273c8a66d35 439< For help, see: https://nodejs.org/en/docs/inspector 440< 441connecting to 127.0.0.1:9229 ... ok 442< Debugger attached. 443Break on start in main.js:7 444 5 } 445 6 446<span class="hljs-meta prompt_">> </span><span class="language-bash">7 addOne(10);</span> 447 8 addOne(-1); 448 9 449<span class="hljs-meta prompt_">debug> </span><span class="language-bash">setBreakpoint(<span class="hljs-string">'main.js'</span>, 4, <span class="hljs-string">'num < 0'</span>)</span> 450 1 'use strict'; 451 2 452 3 function addOne(num) { 453<span class="hljs-meta prompt_">> </span><span class="language-bash">4 <span class="hljs-built_in">return</span> num + 1;</span> 454 5 } 455 6 456 7 addOne(10); 457 8 addOne(-1); 458 9 459<span class="hljs-meta prompt_">debug> </span><span class="language-bash">cont</span> 460break in main.js:4 461 2 462 3 function addOne(num) { 463<span class="hljs-meta prompt_">> </span><span class="language-bash">4 <span class="hljs-built_in">return</span> num + 1;</span> 464 5 } 465 6 466<span class="hljs-meta prompt_">debug> </span><span class="language-bash"><span class="hljs-built_in">exec</span>(<span class="hljs-string">'num'</span>)</span> 467-1 468<span class="hljs-meta prompt_">debug></span></code> <button class="copy-button">copy</button></pre> 469<h4>Information<span><a class="mark" href="#information" id="information">#</a></span><a aria-hidden="true" class="legacy" id="debugger_information"></a></h4> 470<ul> 471<li><code>backtrace</code>, <code>bt</code>: Print backtrace of current execution frame</li> 472<li><code>list(5)</code>: List scripts source code with 5 line context (5 lines before and 473after)</li> 474<li><code>watch(expr)</code>: Add expression to watch list</li> 475<li><code>unwatch(expr)</code>: Remove expression from watch list</li> 476<li><code>unwatch(index)</code>: Remove expression at specific index from watch list</li> 477<li><code>watchers</code>: List all watchers and their values (automatically listed on each 478breakpoint)</li> 479<li><code>repl</code>: Open debugger's repl for evaluation in debugging script's context</li> 480<li><code>exec expr</code>, <code>p expr</code>: Execute an expression in debugging script's context and 481print its value</li> 482<li><code>profile</code>: Start CPU profiling session</li> 483<li><code>profileEnd</code>: Stop current CPU profiling session</li> 484<li><code>profiles</code>: List all completed CPU profiling sessions</li> 485<li><code>profiles[n].save(filepath = 'node.cpuprofile')</code>: Save CPU profiling session 486to disk as JSON</li> 487<li><code>takeHeapSnapshot(filepath = 'node.heapsnapshot')</code>: Take a heap snapshot 488and save to disk as JSON</li> 489</ul> 490<h4>Execution control<span><a class="mark" href="#execution-control" id="execution-control">#</a></span><a aria-hidden="true" class="legacy" id="debugger_execution_control"></a></h4> 491<ul> 492<li><code>run</code>: Run script (automatically runs on debugger's start)</li> 493<li><code>restart</code>: Restart script</li> 494<li><code>kill</code>: Kill script</li> 495</ul> 496<h4>Various<span><a class="mark" href="#various" id="various">#</a></span><a aria-hidden="true" class="legacy" id="debugger_various"></a></h4> 497<ul> 498<li><code>scripts</code>: List all loaded scripts</li> 499<li><code>version</code>: Display V8's version</li> 500</ul> 501</section><section><h3>Advanced usage<span><a class="mark" href="#advanced-usage" id="advanced-usage">#</a></span><a aria-hidden="true" class="legacy" id="debugger_advanced_usage"></a></h3> 502<h4>V8 inspector integration for Node.js<span><a class="mark" href="#v8-inspector-integration-for-nodejs" id="v8-inspector-integration-for-nodejs">#</a></span><a aria-hidden="true" class="legacy" id="debugger_v8_inspector_integration_for_node_js"></a></h4> 503<p>V8 Inspector integration allows attaching Chrome DevTools to Node.js 504instances for debugging and profiling. It uses the 505<a href="https://chromedevtools.github.io/devtools-protocol/">Chrome DevTools Protocol</a>.</p> 506<p>V8 Inspector can be enabled by passing the <code>--inspect</code> flag when starting a 507Node.js application. It is also possible to supply a custom port with that flag, 508e.g. <code>--inspect=9222</code> will accept DevTools connections on port 9222.</p> 509<p>To break on the first line of the application code, pass the <code>--inspect-brk</code> 510flag instead of <code>--inspect</code>.</p> 511<pre><code class="language-console"><span class="hljs-meta prompt_">$ </span><span class="language-bash">node --inspect index.js</span> 512Debugger listening on ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 513For help, see: https://nodejs.org/en/docs/inspector</code> <button class="copy-button">copy</button></pre> 514<p>(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 515at the end of the URL is generated on the fly, it varies in different 516debugging sessions.)</p> 517<p>If the Chrome browser is older than 66.0.3345.0, 518use <code>inspector.html</code> instead of <code>js_app.html</code> in the above URL.</p> 519<p>Chrome DevTools doesn't support debugging <a href="worker_threads.html">worker threads</a> yet. 520<a href="https://github.com/GoogleChromeLabs/ndb/">ndb</a> can be used to debug them.</p></section> 521 <!-- API END --> 522 </div> 523 </div> 524 </div> 525</body> 526</html> 527