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href="#debugger_advanced_usage">Advanced usage</a> 157<ul> 158<li><a href="#debugger_v8_inspector_integration_for_node_js">V8 inspector integration for Node.js</a></li> 159</ul> 160</li> 161</ul> 162</li> 163</ul></details> 164 165 <div id="apicontent"> 166 <h2>Debugger<span><a class="mark" href="#debugger_debugger" id="debugger_debugger">#</a></span></h2> 167 168<p></p><div class="api_stability api_stability_2"><a href="documentation.html#documentation_stability_index">Stability: 2</a> - Stable</div><p></p> 169 170<p>Node.js includes a command-line debugging utility. To use it, start Node.js 171with the <code>inspect</code> argument followed by the path to the script to debug.</p> 172<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-bash">node inspect myscript.js</span> 173< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8 174< For help, see: https://nodejs.org/en/docs/inspector 175< 176< Debugger attached. 177< 178 ok 179Break on start in myscript.js:2 180 1 // myscript.js 181<span class="hljs-meta">> </span><span class="language-bash">2 global.x = 5;</span> 182 3 setTimeout(() => { 183 4 debugger; 184<span class="hljs-meta">debug></span></code></pre> 185<p>The Node.js debugger client is not a full-featured debugger, but simple step and 186inspection are possible.</p> 187<p>Inserting the statement <code>debugger;</code> into the source code of a script will 188enable a breakpoint at that position in the code:</p> 189<!-- eslint-disable no-debugger --> 190<pre><code class="language-js"><span class="hljs-comment">// myscript.js</span> 191<span class="hljs-variable language_">global</span>.<span class="hljs-property">x</span> = <span class="hljs-number">5</span>; 192<span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 193 <span class="hljs-keyword">debugger</span>; 194 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'world'</span>); 195}, <span class="hljs-number">1000</span>); 196<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'hello'</span>);</code></pre> 197<p>Once the debugger is run, a breakpoint will occur at line 3:</p> 198<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-bash">node inspect myscript.js</span> 199< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8 200< For help, see: https://nodejs.org/en/docs/inspector 201< 202< Debugger attached. 203< 204 ok 205Break on start in myscript.js:2 206 1 // myscript.js 207<span class="hljs-meta">> </span><span class="language-bash">2 global.x = 5;</span> 208 3 setTimeout(() => { 209 4 debugger; 210<span class="hljs-meta">debug> </span><span class="language-bash">cont</span> 211< hello 212< 213break in myscript.js:4 214 2 global.x = 5; 215 3 setTimeout(() => { 216<span class="hljs-meta">> </span><span class="language-bash">4 debugger;</span> 217 5 console.log('world'); 218 6 }, 1000); 219<span class="hljs-meta">debug> </span><span class="language-bash">next</span> 220break in myscript.js:5 221 3 setTimeout(() => { 222 4 debugger; 223<span class="hljs-meta">> </span><span class="language-bash">5 console.log(<span class="hljs-string">'world'</span>);</span> 224 6 }, 1000); 225 7 console.log('hello'); 226<span class="hljs-meta">debug> </span><span class="language-bash">repl</span> 227Press Ctrl+C to leave debug repl 228<span class="hljs-meta">> </span><span class="language-bash">x</span> 2295 230<span class="hljs-meta">> </span><span class="language-bash">2 + 2</span> 2314 232<span class="hljs-meta">debug> </span><span class="language-bash">next</span> 233< world 234< 235break in myscript.js:6 236 4 debugger; 237 5 console.log('world'); 238<span class="hljs-meta">> </span><span class="language-bash">6 }, 1000);</span> 239 7 console.log('hello'); 240 8 241<span class="hljs-meta">debug> </span><span class="language-bash">.<span class="hljs-built_in">exit</span></span> 242<span class="hljs-meta">$</span></code></pre> 243<p>The <code>repl</code> command allows code to be evaluated remotely. The <code>next</code> command 244steps to the next line. Type <code>help</code> to see what other commands are available.</p> 245<p>Pressing <code>enter</code> without typing a command will repeat the previous debugger 246command.</p> 247<section><h3>Watchers<span><a class="mark" href="#debugger_watchers" id="debugger_watchers">#</a></span></h3> 248<p>It is possible to watch expression and variable values while debugging. On 249every breakpoint, each expression from the watchers list will be evaluated 250in the current context and displayed immediately before the breakpoint's 251source code listing.</p> 252<p>To begin watching an expression, type <code>watch('my_expression')</code>. The command 253<code>watchers</code> will print the active watchers. To remove a watcher, type 254<code>unwatch('my_expression')</code>.</p> 255</section><section><h3>Command reference<span><a class="mark" href="#debugger_command_reference" id="debugger_command_reference">#</a></span></h3> 256<h4>Stepping<span><a class="mark" href="#debugger_stepping" id="debugger_stepping">#</a></span></h4> 257<ul> 258<li><code>cont</code>, <code>c</code>: Continue execution</li> 259<li><code>next</code>, <code>n</code>: Step next</li> 260<li><code>step</code>, <code>s</code>: Step in</li> 261<li><code>out</code>, <code>o</code>: Step out</li> 262<li><code>pause</code>: Pause running code (like pause button in Developer Tools)</li> 263</ul> 264<h4>Breakpoints<span><a class="mark" href="#debugger_breakpoints" id="debugger_breakpoints">#</a></span></h4> 265<ul> 266<li><code>setBreakpoint()</code>, <code>sb()</code>: Set breakpoint on current line</li> 267<li><code>setBreakpoint(line)</code>, <code>sb(line)</code>: Set breakpoint on specific line</li> 268<li><code>setBreakpoint('fn()')</code>, <code>sb(...)</code>: Set breakpoint on a first statement in 269function's body</li> 270<li><code>setBreakpoint('script.js', 1)</code>, <code>sb(...)</code>: Set breakpoint on first line of 271<code>script.js</code></li> 272<li><code>setBreakpoint('script.js', 1, 'num < 4')</code>, <code>sb(...)</code>: Set conditional 273breakpoint on first line of <code>script.js</code> that only breaks when <code>num < 4</code> 274evaluates to <code>true</code></li> 275<li><code>clearBreakpoint('script.js', 1)</code>, <code>cb(...)</code>: Clear breakpoint in <code>script.js</code> 276on line 1</li> 277</ul> 278<p>It is also possible to set a breakpoint in a file (module) that 279is not loaded yet:</p> 280<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-bash">node inspect main.js</span> 281< Debugger listening on ws://127.0.0.1:9229/48a5b28a-550c-471b-b5e1-d13dd7165df9 282< For help, see: https://nodejs.org/en/docs/inspector 283< 284< Debugger attached. 285< 286 ok 287Break on start in main.js:1 288<span class="hljs-meta">> </span><span class="language-bash">1 const mod = require(<span class="hljs-string">'./mod.js'</span>);</span> 289 2 mod.hello(); 290 3 mod.hello(); 291<span class="hljs-meta">debug> </span><span class="language-bash">setBreakpoint(<span class="hljs-string">'mod.js'</span>, 22)</span> 292Warning: script 'mod.js' was not loaded yet. 293<span class="hljs-meta">debug> </span><span class="language-bash">c</span> 294break in mod.js:22 295 20 // USE OR OTHER DEALINGS IN THE SOFTWARE. 296 21 297<span class="hljs-meta">></span><span class="language-bash">22 exports.hello = <span class="hljs-function"><span class="hljs-title">function</span></span>() {</span> 298 23 return 'hello from module'; 299 24 }; 300<span class="hljs-meta">debug></span></code></pre> 301<p>It is also possible to set a conditional breakpoint that only breaks when a 302given expression evaluates to <code>true</code>:</p> 303<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-bash">node inspect main.js</span> 304< Debugger listening on ws://127.0.0.1:9229/ce24daa8-3816-44d4-b8ab-8273c8a66d35 305< For help, see: https://nodejs.org/en/docs/inspector 306< Debugger attached. 307Break on start in main.js:7 308 5 } 309 6 310<span class="hljs-meta">> </span><span class="language-bash">7 addOne(10);</span> 311 8 addOne(-1); 312 9 313<span class="hljs-meta">debug> </span><span class="language-bash">setBreakpoint(<span class="hljs-string">'main.js'</span>, 4, <span class="hljs-string">'num < 0'</span>)</span> 314 1 'use strict'; 315 2 316 3 function addOne(num) { 317<span class="hljs-meta">> </span><span class="language-bash">4 <span class="hljs-built_in">return</span> num + 1;</span> 318 5 } 319 6 320 7 addOne(10); 321 8 addOne(-1); 322 9 323<span class="hljs-meta">debug> </span><span class="language-bash">cont</span> 324break in main.js:4 325 2 326 3 function addOne(num) { 327<span class="hljs-meta">> </span><span class="language-bash">4 <span class="hljs-built_in">return</span> num + 1;</span> 328 5 } 329 6 330<span class="hljs-meta">debug> </span><span class="language-bash"><span class="hljs-built_in">exec</span>(<span class="hljs-string">'num'</span>)</span> 331-1 332<span class="hljs-meta">debug></span></code></pre> 333<h4>Information<span><a class="mark" href="#debugger_information" id="debugger_information">#</a></span></h4> 334<ul> 335<li><code>backtrace</code>, <code>bt</code>: Print backtrace of current execution frame</li> 336<li><code>list(5)</code>: List scripts source code with 5 line context (5 lines before and 337after)</li> 338<li><code>watch(expr)</code>: Add expression to watch list</li> 339<li><code>unwatch(expr)</code>: Remove expression from watch list</li> 340<li><code>watchers</code>: List all watchers and their values (automatically listed on each 341breakpoint)</li> 342<li><code>repl</code>: Open debugger's repl for evaluation in debugging script's context</li> 343<li><code>exec expr</code>: Execute an expression in debugging script's context</li> 344</ul> 345<h4>Execution control<span><a class="mark" href="#debugger_execution_control" id="debugger_execution_control">#</a></span></h4> 346<ul> 347<li><code>run</code>: Run script (automatically runs on debugger's start)</li> 348<li><code>restart</code>: Restart script</li> 349<li><code>kill</code>: Kill script</li> 350</ul> 351<h4>Various<span><a class="mark" href="#debugger_various" id="debugger_various">#</a></span></h4> 352<ul> 353<li><code>scripts</code>: List all loaded scripts</li> 354<li><code>version</code>: Display V8's version</li> 355</ul> 356</section><section><h3>Advanced usage<span><a class="mark" href="#debugger_advanced_usage" id="debugger_advanced_usage">#</a></span></h3> 357<h4>V8 inspector integration for Node.js<span><a class="mark" href="#debugger_v8_inspector_integration_for_node_js" id="debugger_v8_inspector_integration_for_node_js">#</a></span></h4> 358<p>V8 Inspector integration allows attaching Chrome DevTools to Node.js 359instances for debugging and profiling. It uses the 360<a href="https://chromedevtools.github.io/devtools-protocol/">Chrome DevTools Protocol</a>.</p> 361<p>V8 Inspector can be enabled by passing the <code>--inspect</code> flag when starting a 362Node.js application. It is also possible to supply a custom port with that flag, 363e.g. <code>--inspect=9222</code> will accept DevTools connections on port 9222.</p> 364<p>To break on the first line of the application code, pass the <code>--inspect-brk</code> 365flag instead of <code>--inspect</code>.</p> 366<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-bash">node --inspect index.js</span> 367Debugger listening on ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 368For help, see: https://nodejs.org/en/docs/inspector</code></pre> 369<p>(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 370at the end of the URL is generated on the fly, it varies in different 371debugging sessions.)</p> 372<p>If the Chrome browser is older than 66.0.3345.0, 373use <code>inspector.html</code> instead of <code>js_app.html</code> in the above URL.</p> 374<p>Chrome DevTools doesn't support debugging <a href="worker_threads.html">worker threads</a> yet. 375<a href="https://github.com/GoogleChromeLabs/ndb/">ndb</a> can be used to debug them.</p></section> 376 <!-- API END --> 377 </div> 378 </div> 379 </div> 380 <script> 381 'use strict'; 382 { 383 const kCustomPreference = 'customDarkTheme'; 384 const userSettings = sessionStorage.getItem(kCustomPreference); 385 const themeToggleButton = document.getElementById('theme-toggle-btn'); 386 if (userSettings === null && window.matchMedia) { 387 const mq = window.matchMedia('(prefers-color-scheme: dark)'); 388 if ('onchange' in mq) { 389 function mqChangeListener(e) { 390 document.body.classList.toggle('dark-mode', e.matches); 391 } 392 mq.addEventListener('change', mqChangeListener); 393 if (themeToggleButton) { 394 themeToggleButton.addEventListener('click', function() { 395 mq.removeEventListener('change', mqChangeListener); 396 }, { once: true }); 397 } 398 } 399 if (mq.matches) { 400 document.body.classList.add('dark-mode'); 401 } 402 } else if (userSettings === 'true') { 403 document.body.classList.add('dark-mode'); 404 } 405 if (themeToggleButton) { 406 themeToggleButton.hidden = false; 407 themeToggleButton.addEventListener('click', function() { 408 sessionStorage.setItem( 409 kCustomPreference, 410 document.body.classList.toggle('dark-mode') 411 ); 412 }); 413 } 414 } 415 </script> 416</body> 417</html> 418