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154<li><a href="#debugger_execution_control">Execution control</a></li> 155<li><a href="#debugger_various">Various</a></li> 156</ul> 157</li> 158<li><a href="#debugger_advanced_usage">Advanced usage</a> 159<ul> 160<li><a href="#debugger_v8_inspector_integration_for_node_js">V8 inspector integration for Node.js</a></li> 161</ul> 162</li> 163</ul> 164</li> 165</ul></details> 166 167 <div id="apicontent"> 168 <h2>Debugger<span><a class="mark" href="#debugger_debugger" id="debugger_debugger">#</a></span></h2> 169 170<p></p><div class="api_stability api_stability_2"><a href="documentation.html#documentation_stability_index">Stability: 2</a> - Stable</div><p></p> 171 172<p>Node.js includes a command-line debugging utility. To use it, start Node.js 173with the <code>inspect</code> argument followed by the path to the script to debug.</p> 174<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-bash">node inspect myscript.js</span> 175< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8 176< For help, see: https://nodejs.org/en/docs/inspector 177< 178< Debugger attached. 179< 180 ok 181Break on start in myscript.js:2 182 1 // myscript.js 183<span class="hljs-meta">> </span><span class="language-bash">2 global.x = 5;</span> 184 3 setTimeout(() => { 185 4 debugger; 186<span class="hljs-meta">debug></span></code></pre> 187<p>The Node.js debugger client is not a full-featured debugger, but simple step and 188inspection are possible.</p> 189<p>Inserting the statement <code>debugger;</code> into the source code of a script will 190enable a breakpoint at that position in the code:</p> 191<!-- eslint-disable no-debugger --> 192<pre><code class="language-js"><span class="hljs-comment">// myscript.js</span> 193<span class="hljs-variable language_">global</span>.<span class="hljs-property">x</span> = <span class="hljs-number">5</span>; 194<span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 195 <span class="hljs-keyword">debugger</span>; 196 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'world'</span>); 197}, <span class="hljs-number">1000</span>); 198<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'hello'</span>);</code></pre> 199<p>Once the debugger is run, a breakpoint will occur at line 3:</p> 200<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-bash">node inspect myscript.js</span> 201< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8 202< For help, see: https://nodejs.org/en/docs/inspector 203< 204< Debugger attached. 205< 206 ok 207Break on start in myscript.js:2 208 1 // myscript.js 209<span class="hljs-meta">> </span><span class="language-bash">2 global.x = 5;</span> 210 3 setTimeout(() => { 211 4 debugger; 212<span class="hljs-meta">debug> </span><span class="language-bash">cont</span> 213< hello 214< 215break in myscript.js:4 216 2 global.x = 5; 217 3 setTimeout(() => { 218<span class="hljs-meta">> </span><span class="language-bash">4 debugger;</span> 219 5 console.log('world'); 220 6 }, 1000); 221<span class="hljs-meta">debug> </span><span class="language-bash">next</span> 222break in myscript.js:5 223 3 setTimeout(() => { 224 4 debugger; 225<span class="hljs-meta">> </span><span class="language-bash">5 console.log(<span class="hljs-string">'world'</span>);</span> 226 6 }, 1000); 227 7 console.log('hello'); 228<span class="hljs-meta">debug> </span><span class="language-bash">repl</span> 229Press Ctrl+C to leave debug repl 230<span class="hljs-meta">> </span><span class="language-bash">x</span> 2315 232<span class="hljs-meta">> </span><span class="language-bash">2 + 2</span> 2334 234<span class="hljs-meta">debug> </span><span class="language-bash">next</span> 235< world 236< 237break in myscript.js:6 238 4 debugger; 239 5 console.log('world'); 240<span class="hljs-meta">> </span><span class="language-bash">6 }, 1000);</span> 241 7 console.log('hello'); 242 8 243<span class="hljs-meta">debug> </span><span class="language-bash">.<span class="hljs-built_in">exit</span></span> 244<span class="hljs-meta">$</span></code></pre> 245<p>The <code>repl</code> command allows code to be evaluated remotely. The <code>next</code> command 246steps to the next line. Type <code>help</code> to see what other commands are available.</p> 247<p>Pressing <code>enter</code> without typing a command will repeat the previous debugger 248command.</p> 249<section><h3>Watchers<span><a class="mark" href="#debugger_watchers" id="debugger_watchers">#</a></span></h3> 250<p>It is possible to watch expression and variable values while debugging. On 251every breakpoint, each expression from the watchers list will be evaluated 252in the current context and displayed immediately before the breakpoint's 253source code listing.</p> 254<p>To begin watching an expression, type <code>watch('my_expression')</code>. The command 255<code>watchers</code> will print the active watchers. To remove a watcher, type 256<code>unwatch('my_expression')</code>.</p> 257</section><section><h3>Command reference<span><a class="mark" href="#debugger_command_reference" id="debugger_command_reference">#</a></span></h3> 258<h4>Stepping<span><a class="mark" href="#debugger_stepping" id="debugger_stepping">#</a></span></h4> 259<ul> 260<li><code>cont</code>, <code>c</code>: Continue execution</li> 261<li><code>next</code>, <code>n</code>: Step next</li> 262<li><code>step</code>, <code>s</code>: Step in</li> 263<li><code>out</code>, <code>o</code>: Step out</li> 264<li><code>pause</code>: Pause running code (like pause button in Developer Tools)</li> 265</ul> 266<h4>Breakpoints<span><a class="mark" href="#debugger_breakpoints" id="debugger_breakpoints">#</a></span></h4> 267<ul> 268<li><code>setBreakpoint()</code>, <code>sb()</code>: Set breakpoint on current line</li> 269<li><code>setBreakpoint(line)</code>, <code>sb(line)</code>: Set breakpoint on specific line</li> 270<li><code>setBreakpoint('fn()')</code>, <code>sb(...)</code>: Set breakpoint on a first statement in 271function's body</li> 272<li><code>setBreakpoint('script.js', 1)</code>, <code>sb(...)</code>: Set breakpoint on first line of 273<code>script.js</code></li> 274<li><code>setBreakpoint('script.js', 1, 'num < 4')</code>, <code>sb(...)</code>: Set conditional 275breakpoint on first line of <code>script.js</code> that only breaks when <code>num < 4</code> 276evaluates to <code>true</code></li> 277<li><code>clearBreakpoint('script.js', 1)</code>, <code>cb(...)</code>: Clear breakpoint in <code>script.js</code> 278on line 1</li> 279</ul> 280<p>It is also possible to set a breakpoint in a file (module) that 281is not loaded yet:</p> 282<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-bash">node inspect main.js</span> 283< Debugger listening on ws://127.0.0.1:9229/48a5b28a-550c-471b-b5e1-d13dd7165df9 284< For help, see: https://nodejs.org/en/docs/inspector 285< 286< Debugger attached. 287< 288 ok 289Break on start in main.js:1 290<span class="hljs-meta">> </span><span class="language-bash">1 const mod = require(<span class="hljs-string">'./mod.js'</span>);</span> 291 2 mod.hello(); 292 3 mod.hello(); 293<span class="hljs-meta">debug> </span><span class="language-bash">setBreakpoint(<span class="hljs-string">'mod.js'</span>, 22)</span> 294Warning: script 'mod.js' was not loaded yet. 295<span class="hljs-meta">debug> </span><span class="language-bash">c</span> 296break in mod.js:22 297 20 // USE OR OTHER DEALINGS IN THE SOFTWARE. 298 21 299<span class="hljs-meta">></span><span class="language-bash">22 exports.hello = <span class="hljs-function"><span class="hljs-title">function</span></span>() {</span> 300 23 return 'hello from module'; 301 24 }; 302<span class="hljs-meta">debug></span></code></pre> 303<p>It is also possible to set a conditional breakpoint that only breaks when a 304given expression evaluates to <code>true</code>:</p> 305<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-bash">node inspect main.js</span> 306< Debugger listening on ws://127.0.0.1:9229/ce24daa8-3816-44d4-b8ab-8273c8a66d35 307< For help, see: https://nodejs.org/en/docs/inspector 308< Debugger attached. 309Break on start in main.js:7 310 5 } 311 6 312<span class="hljs-meta">> </span><span class="language-bash">7 addOne(10);</span> 313 8 addOne(-1); 314 9 315<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> 316 1 'use strict'; 317 2 318 3 function addOne(num) { 319<span class="hljs-meta">> </span><span class="language-bash">4 <span class="hljs-built_in">return</span> num + 1;</span> 320 5 } 321 6 322 7 addOne(10); 323 8 addOne(-1); 324 9 325<span class="hljs-meta">debug> </span><span class="language-bash">cont</span> 326break in main.js:4 327 2 328 3 function addOne(num) { 329<span class="hljs-meta">> </span><span class="language-bash">4 <span class="hljs-built_in">return</span> num + 1;</span> 330 5 } 331 6 332<span class="hljs-meta">debug> </span><span class="language-bash"><span class="hljs-built_in">exec</span>(<span class="hljs-string">'num'</span>)</span> 333-1 334<span class="hljs-meta">debug></span></code></pre> 335<h4>Information<span><a class="mark" href="#debugger_information" id="debugger_information">#</a></span></h4> 336<ul> 337<li><code>backtrace</code>, <code>bt</code>: Print backtrace of current execution frame</li> 338<li><code>list(5)</code>: List scripts source code with 5 line context (5 lines before and 339after)</li> 340<li><code>watch(expr)</code>: Add expression to watch list</li> 341<li><code>unwatch(expr)</code>: Remove expression from watch list</li> 342<li><code>watchers</code>: List all watchers and their values (automatically listed on each 343breakpoint)</li> 344<li><code>repl</code>: Open debugger's repl for evaluation in debugging script's context</li> 345<li><code>exec expr</code>: Execute an expression in debugging script's context</li> 346</ul> 347<h4>Execution control<span><a class="mark" href="#debugger_execution_control" id="debugger_execution_control">#</a></span></h4> 348<ul> 349<li><code>run</code>: Run script (automatically runs on debugger's start)</li> 350<li><code>restart</code>: Restart script</li> 351<li><code>kill</code>: Kill script</li> 352</ul> 353<h4>Various<span><a class="mark" href="#debugger_various" id="debugger_various">#</a></span></h4> 354<ul> 355<li><code>scripts</code>: List all loaded scripts</li> 356<li><code>version</code>: Display V8's version</li> 357</ul> 358</section><section><h3>Advanced usage<span><a class="mark" href="#debugger_advanced_usage" id="debugger_advanced_usage">#</a></span></h3> 359<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> 360<p>V8 Inspector integration allows attaching Chrome DevTools to Node.js 361instances for debugging and profiling. It uses the 362<a href="https://chromedevtools.github.io/devtools-protocol/">Chrome DevTools Protocol</a>.</p> 363<p>V8 Inspector can be enabled by passing the <code>--inspect</code> flag when starting a 364Node.js application. It is also possible to supply a custom port with that flag, 365e.g. <code>--inspect=9222</code> will accept DevTools connections on port 9222.</p> 366<p>To break on the first line of the application code, pass the <code>--inspect-brk</code> 367flag instead of <code>--inspect</code>.</p> 368<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-bash">node --inspect index.js</span> 369Debugger listening on ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 370For help, see: https://nodejs.org/en/docs/inspector</code></pre> 371<p>(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 372at the end of the URL is generated on the fly, it varies in different 373debugging sessions.)</p> 374<p>If the Chrome browser is older than 66.0.3345.0, 375use <code>inspector.html</code> instead of <code>js_app.html</code> in the above URL.</p> 376<p>Chrome DevTools doesn't support debugging <a href="worker_threads.html">worker threads</a> yet. 377<a href="https://github.com/GoogleChromeLabs/ndb/">ndb</a> can be used to debug them.</p></section> 378 <!-- API END --> 379 </div> 380 </div> 381 </div> 382 <script> 383 'use strict'; 384 { 385 const kCustomPreference = 'customDarkTheme'; 386 const userSettings = sessionStorage.getItem(kCustomPreference); 387 const themeToggleButton = document.getElementById('theme-toggle-btn'); 388 if (userSettings === null && window.matchMedia) { 389 const mq = window.matchMedia('(prefers-color-scheme: dark)'); 390 if ('onchange' in mq) { 391 function mqChangeListener(e) { 392 document.body.classList.toggle('dark-mode', e.matches); 393 } 394 mq.addEventListener('change', mqChangeListener); 395 if (themeToggleButton) { 396 themeToggleButton.addEventListener('click', function() { 397 mq.removeEventListener('change', mqChangeListener); 398 }, { once: true }); 399 } 400 } 401 if (mq.matches) { 402 document.body.classList.add('dark-mode'); 403 } 404 } else if (userSettings === 'true') { 405 document.body.classList.add('dark-mode'); 406 } 407 if (themeToggleButton) { 408 themeToggleButton.hidden = false; 409 themeToggleButton.addEventListener('click', function() { 410 sessionStorage.setItem( 411 kCustomPreference, 412 document.body.classList.toggle('dark-mode') 413 ); 414 }); 415 } 416 } 417 </script> 418</body> 419</html> 420