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class="picker-header"> 117 <a href="#"> 118 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 119 Table of contents 120 </a> 121 122 <div class="picker"><div class="toc"><ul> 123<li><span class="stability_0"><a href="#domain">Domain</a></span> 124<ul> 125<li><a href="#warning-dont-ignore-errors">Warning: Don't ignore errors!</a></li> 126<li><a href="#additions-to-error-objects">Additions to <code>Error</code> objects</a></li> 127<li><a href="#implicit-binding">Implicit binding</a></li> 128<li><a href="#explicit-binding">Explicit binding</a></li> 129<li><a href="#domaincreate"><code>domain.create()</code></a></li> 130<li><a href="#class-domain">Class: <code>Domain</code></a> 131<ul> 132<li><a href="#domainmembers"><code>domain.members</code></a></li> 133<li><a href="#domainaddemitter"><code>domain.add(emitter)</code></a></li> 134<li><a href="#domainbindcallback"><code>domain.bind(callback)</code></a></li> 135<li><a href="#domainenter"><code>domain.enter()</code></a></li> 136<li><a href="#domainexit"><code>domain.exit()</code></a></li> 137<li><a href="#domaininterceptcallback"><code>domain.intercept(callback)</code></a></li> 138<li><a href="#domainremoveemitter"><code>domain.remove(emitter)</code></a></li> 139<li><a href="#domainrunfn-args"><code>domain.run(fn[, ...args])</code></a></li> 140</ul> 141</li> 142<li><a href="#domains-and-promises">Domains and promises</a></li> 143</ul> 144</li> 145</ul></div></div> 146 </li> 147 148 149 <li class="picker-header"> 150 <a href="#"> 151 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 152 Index 153 </a> 154 155 <div class="picker"><ul> 156<li><a href="documentation.html" class="nav-documentation">About this documentation</a></li> 157<li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li> 158 159 <li> 160 <a href="index.html">Index</a> 161 </li> 162 </ul> 163 164<hr class="line"> 165<ul> 166<li><a 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href="https://nodejs.org/docs/latest-v18.x/api/domain.html">18.x <b>LTS</b></a></li> 243<li><a href="https://nodejs.org/docs/latest-v17.x/api/domain.html">17.x</a></li> 244<li><a href="https://nodejs.org/docs/latest-v16.x/api/domain.html">16.x</a></li> 245<li><a href="https://nodejs.org/docs/latest-v15.x/api/domain.html">15.x</a></li> 246<li><a href="https://nodejs.org/docs/latest-v14.x/api/domain.html">14.x</a></li> 247<li><a href="https://nodejs.org/docs/latest-v13.x/api/domain.html">13.x</a></li> 248<li><a href="https://nodejs.org/docs/latest-v12.x/api/domain.html">12.x</a></li> 249<li><a href="https://nodejs.org/docs/latest-v11.x/api/domain.html">11.x</a></li> 250<li><a href="https://nodejs.org/docs/latest-v10.x/api/domain.html">10.x</a></li> 251<li><a href="https://nodejs.org/docs/latest-v9.x/api/domain.html">9.x</a></li> 252<li><a href="https://nodejs.org/docs/latest-v8.x/api/domain.html">8.x</a></li> 253<li><a href="https://nodejs.org/docs/latest-v7.x/api/domain.html">7.x</a></li> 254<li><a href="https://nodejs.org/docs/latest-v6.x/api/domain.html">6.x</a></li> 255<li><a href="https://nodejs.org/docs/latest-v5.x/api/domain.html">5.x</a></li> 256<li><a href="https://nodejs.org/docs/latest-v4.x/api/domain.html">4.x</a></li> 257<li><a href="https://nodejs.org/docs/latest-v0.12.x/api/domain.html">0.12.x</a></li> 258<li><a href="https://nodejs.org/docs/latest-v0.10.x/api/domain.html">0.10.x</a></li></ol></div> 259 </li> 260 261 <li class="picker-header"> 262 <a href="#"> 263 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 264 Options 265 </a> 266 267 <div class="picker"> 268 <ul> 269 <li> 270 <a href="all.html">View on single page</a> 271 </li> 272 <li> 273 <a href="domain.json">View as JSON</a> 274 </li> 275 <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/main/doc/api/domain.md">Edit on GitHub</a></li> 276 </ul> 277 </div> 278 </li> 279 </ul> 280 </div> 281 <hr> 282 </header> 283 284 <details id="toc" open><summary>Table of contents</summary><ul> 285<li><span class="stability_0"><a href="#domain">Domain</a></span> 286<ul> 287<li><a href="#warning-dont-ignore-errors">Warning: Don't ignore errors!</a></li> 288<li><a href="#additions-to-error-objects">Additions to <code>Error</code> objects</a></li> 289<li><a href="#implicit-binding">Implicit binding</a></li> 290<li><a href="#explicit-binding">Explicit binding</a></li> 291<li><a href="#domaincreate"><code>domain.create()</code></a></li> 292<li><a href="#class-domain">Class: <code>Domain</code></a> 293<ul> 294<li><a href="#domainmembers"><code>domain.members</code></a></li> 295<li><a href="#domainaddemitter"><code>domain.add(emitter)</code></a></li> 296<li><a href="#domainbindcallback"><code>domain.bind(callback)</code></a></li> 297<li><a href="#domainenter"><code>domain.enter()</code></a></li> 298<li><a href="#domainexit"><code>domain.exit()</code></a></li> 299<li><a href="#domaininterceptcallback"><code>domain.intercept(callback)</code></a></li> 300<li><a href="#domainremoveemitter"><code>domain.remove(emitter)</code></a></li> 301<li><a href="#domainrunfn-args"><code>domain.run(fn[, ...args])</code></a></li> 302</ul> 303</li> 304<li><a href="#domains-and-promises">Domains and promises</a></li> 305</ul> 306</li> 307</ul></details> 308 309 <div id="apicontent"> 310 <h2>Domain<span><a class="mark" href="#domain" id="domain">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain"></a></h2> 311<div class="api_metadata"> 312<details class="changelog"><summary>History</summary> 313<table> 314<tbody><tr><th>Version</th><th>Changes</th></tr> 315<tr><td>v8.8.0</td> 316<td><p>Any <code>Promise</code>s created in VM contexts no longer have a <code>.domain</code> property. Their handlers are still executed in the proper domain, however, and <code>Promise</code>s created in the main context still possess a <code>.domain</code> property.</p></td></tr> 317<tr><td>v8.0.0</td> 318<td><p>Handlers for <code>Promise</code>s are now invoked in the domain in which the first promise of a chain was created.</p></td></tr> 319<tr><td>v1.4.2</td> 320<td><p><span>Deprecated since: v1.4.2</span></p></td></tr> 321</tbody></table> 322</details> 323</div> 324 325<p></p><div class="api_stability api_stability_0"><a href="documentation.html#stability-index">Stability: 0</a> - Deprecated</div><p></p> 326<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v18.20.1/lib/domain.js">lib/domain.js</a></p> 327<p><strong>This module is pending deprecation.</strong> Once a replacement API has been 328finalized, this module will be fully deprecated. Most developers should 329<strong>not</strong> have cause to use this module. Users who absolutely must have 330the functionality that domains provide may rely on it for the time being 331but should expect to have to migrate to a different solution 332in the future.</p> 333<p>Domains provide a way to handle multiple different IO operations as a 334single group. If any of the event emitters or callbacks registered to a 335domain emit an <code>'error'</code> event, or throw an error, then the domain object 336will be notified, rather than losing the context of the error in the 337<code>process.on('uncaughtException')</code> handler, or causing the program to 338exit immediately with an error code.</p> 339<section><h3>Warning: Don't ignore errors!<span><a class="mark" href="#warning-dont-ignore-errors" id="warning-dont-ignore-errors">#</a></span><a aria-hidden="true" class="legacy" id="domain_warning_don_t_ignore_errors"></a></h3> 340 341<p>Domain error handlers are not a substitute for closing down a 342process when an error occurs.</p> 343<p>By the very nature of how <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/throw"><code>throw</code></a> works in JavaScript, there is almost 344never any way to safely "pick up where it left off", without leaking 345references, or creating some other sort of undefined brittle state.</p> 346<p>The safest way to respond to a thrown error is to shut down the 347process. Of course, in a normal web server, there may be many 348open connections, and it is not reasonable to abruptly shut those down 349because an error was triggered by someone else.</p> 350<p>The better approach is to send an error response to the request that 351triggered the error, while letting the others finish in their normal 352time, and stop listening for new requests in that worker.</p> 353<p>In this way, <code>domain</code> usage goes hand-in-hand with the cluster module, 354since the primary process can fork a new worker when a worker 355encounters an error. For Node.js programs that scale to multiple 356machines, the terminating proxy or service registry can take note of 357the failure, and react accordingly.</p> 358<p>For example, this is not a good idea:</p> 359<pre><code class="language-js"><span class="hljs-comment">// XXX WARNING! BAD IDEA!</span> 360 361<span class="hljs-keyword">const</span> d = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:domain'</span>).<span class="hljs-title function_">create</span>(); 362d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 363 <span class="hljs-comment">// The error won't crash the process, but what it does is worse!</span> 364 <span class="hljs-comment">// Though we've prevented abrupt process restarting, we are leaking</span> 365 <span class="hljs-comment">// a lot of resources if this ever happens.</span> 366 <span class="hljs-comment">// This is no better than process.on('uncaughtException')!</span> 367 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`error, but oh well <span class="hljs-subst">${er.message}</span>`</span>); 368}); 369d.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 370 <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:http'</span>).<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { 371 <span class="hljs-title function_">handleRequest</span>(req, res); 372 }).<span class="hljs-title function_">listen</span>(<span class="hljs-variable constant_">PORT</span>); 373});</code> <button class="copy-button">copy</button></pre> 374<p>By using the context of a domain, and the resilience of separating our 375program into multiple worker processes, we can react more 376appropriately, and handle errors with much greater safety.</p> 377<pre><code class="language-js"><span class="hljs-comment">// Much better!</span> 378 379<span class="hljs-keyword">const</span> cluster = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:cluster'</span>); 380<span class="hljs-keyword">const</span> <span class="hljs-variable constant_">PORT</span> = +process.<span class="hljs-property">env</span>.<span class="hljs-property">PORT</span> || <span class="hljs-number">1337</span>; 381 382<span class="hljs-keyword">if</span> (cluster.<span class="hljs-property">isPrimary</span>) { 383 <span class="hljs-comment">// A more realistic scenario would have more than 2 workers,</span> 384 <span class="hljs-comment">// and perhaps not put the primary and worker in the same file.</span> 385 <span class="hljs-comment">//</span> 386 <span class="hljs-comment">// It is also possible to get a bit fancier about logging, and</span> 387 <span class="hljs-comment">// implement whatever custom logic is needed to prevent DoS</span> 388 <span class="hljs-comment">// attacks and other bad behavior.</span> 389 <span class="hljs-comment">//</span> 390 <span class="hljs-comment">// See the options in the cluster documentation.</span> 391 <span class="hljs-comment">//</span> 392 <span class="hljs-comment">// The important thing is that the primary does very little,</span> 393 <span class="hljs-comment">// increasing our resilience to unexpected errors.</span> 394 395 cluster.<span class="hljs-title function_">fork</span>(); 396 cluster.<span class="hljs-title function_">fork</span>(); 397 398 cluster.<span class="hljs-title function_">on</span>(<span class="hljs-string">'disconnect'</span>, <span class="hljs-function">(<span class="hljs-params">worker</span>) =></span> { 399 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'disconnect!'</span>); 400 cluster.<span class="hljs-title function_">fork</span>(); 401 }); 402 403} <span class="hljs-keyword">else</span> { 404 <span class="hljs-comment">// the worker</span> 405 <span class="hljs-comment">//</span> 406 <span class="hljs-comment">// This is where we put our bugs!</span> 407 408 <span class="hljs-keyword">const</span> domain = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:domain'</span>); 409 410 <span class="hljs-comment">// See the cluster documentation for more details about using</span> 411 <span class="hljs-comment">// worker processes to serve requests. How it works, caveats, etc.</span> 412 413 <span class="hljs-keyword">const</span> server = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:http'</span>).<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { 414 <span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 415 d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 416 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">`error <span class="hljs-subst">${er.stack}</span>`</span>); 417 418 <span class="hljs-comment">// We're in dangerous territory!</span> 419 <span class="hljs-comment">// By definition, something unexpected occurred,</span> 420 <span class="hljs-comment">// which we probably didn't want.</span> 421 <span class="hljs-comment">// Anything can happen now! Be very careful!</span> 422 423 <span class="hljs-keyword">try</span> { 424 <span class="hljs-comment">// Make sure we close down within 30 seconds</span> 425 <span class="hljs-keyword">const</span> killtimer = <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 426 process.<span class="hljs-title function_">exit</span>(<span class="hljs-number">1</span>); 427 }, <span class="hljs-number">30000</span>); 428 <span class="hljs-comment">// But don't keep the process open just for that!</span> 429 killtimer.<span class="hljs-title function_">unref</span>(); 430 431 <span class="hljs-comment">// Stop taking new requests.</span> 432 server.<span class="hljs-title function_">close</span>(); 433 434 <span class="hljs-comment">// Let the primary know we're dead. This will trigger a</span> 435 <span class="hljs-comment">// 'disconnect' in the cluster primary, and then it will fork</span> 436 <span class="hljs-comment">// a new worker.</span> 437 cluster.<span class="hljs-property">worker</span>.<span class="hljs-title function_">disconnect</span>(); 438 439 <span class="hljs-comment">// Try to send an error to the request that triggered the problem</span> 440 res.<span class="hljs-property">statusCode</span> = <span class="hljs-number">500</span>; 441 res.<span class="hljs-title function_">setHeader</span>(<span class="hljs-string">'content-type'</span>, <span class="hljs-string">'text/plain'</span>); 442 res.<span class="hljs-title function_">end</span>(<span class="hljs-string">'Oops, there was a problem!\n'</span>); 443 } <span class="hljs-keyword">catch</span> (er2) { 444 <span class="hljs-comment">// Oh well, not much we can do at this point.</span> 445 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">`Error sending 500! <span class="hljs-subst">${er2.stack}</span>`</span>); 446 } 447 }); 448 449 <span class="hljs-comment">// Because req and res were created before this domain existed,</span> 450 <span class="hljs-comment">// we need to explicitly add them.</span> 451 <span class="hljs-comment">// See the explanation of implicit vs explicit binding below.</span> 452 d.<span class="hljs-title function_">add</span>(req); 453 d.<span class="hljs-title function_">add</span>(res); 454 455 <span class="hljs-comment">// Now run the handler function in the domain.</span> 456 d.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 457 <span class="hljs-title function_">handleRequest</span>(req, res); 458 }); 459 }); 460 server.<span class="hljs-title function_">listen</span>(<span class="hljs-variable constant_">PORT</span>); 461} 462 463<span class="hljs-comment">// This part is not important. Just an example routing thing.</span> 464<span class="hljs-comment">// Put fancy application logic here.</span> 465<span class="hljs-keyword">function</span> <span class="hljs-title function_">handleRequest</span>(<span class="hljs-params">req, res</span>) { 466 <span class="hljs-keyword">switch</span> (req.<span class="hljs-property">url</span>) { 467 <span class="hljs-keyword">case</span> <span class="hljs-string">'/error'</span>: 468 <span class="hljs-comment">// We do some async stuff, and then...</span> 469 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 470 <span class="hljs-comment">// Whoops!</span> 471 flerb.<span class="hljs-title function_">bark</span>(); 472 }, timeout); 473 <span class="hljs-keyword">break</span>; 474 <span class="hljs-attr">default</span>: 475 res.<span class="hljs-title function_">end</span>(<span class="hljs-string">'ok'</span>); 476 } 477}</code> <button class="copy-button">copy</button></pre> 478</section><section><h3>Additions to <code>Error</code> objects<span><a class="mark" href="#additions-to-error-objects" id="additions-to-error-objects">#</a></span><a aria-hidden="true" class="legacy" id="domain_additions_to_error_objects"></a></h3> 479 480<p>Any time an <code>Error</code> object is routed through a domain, a few extra fields 481are added to it.</p> 482<ul> 483<li><code>error.domain</code> The domain that first handled the error.</li> 484<li><code>error.domainEmitter</code> The event emitter that emitted an <code>'error'</code> event 485with the error object.</li> 486<li><code>error.domainBound</code> The callback function which was bound to the 487domain, and passed an error as its first argument.</li> 488<li><code>error.domainThrown</code> A boolean indicating whether the error was 489thrown, emitted, or passed to a bound callback function.</li> 490</ul> 491</section><section><h3>Implicit binding<span><a class="mark" href="#implicit-binding" id="implicit-binding">#</a></span><a aria-hidden="true" class="legacy" id="domain_implicit_binding"></a></h3> 492 493<p>If domains are in use, then all <strong>new</strong> <code>EventEmitter</code> objects (including 494Stream objects, requests, responses, etc.) will be implicitly bound to 495the active domain at the time of their creation.</p> 496<p>Additionally, callbacks passed to low-level event loop requests (such as 497to <code>fs.open()</code>, or other callback-taking methods) will automatically be 498bound to the active domain. If they throw, then the domain will catch 499the error.</p> 500<p>In order to prevent excessive memory usage, <code>Domain</code> objects themselves 501are not implicitly added as children of the active domain. If they 502were, then it would be too easy to prevent request and response objects 503from being properly garbage collected.</p> 504<p>To nest <code>Domain</code> objects as children of a parent <code>Domain</code> they must be 505explicitly added.</p> 506<p>Implicit binding routes thrown errors and <code>'error'</code> events to the 507<code>Domain</code>'s <code>'error'</code> event, but does not register the <code>EventEmitter</code> on the 508<code>Domain</code>. 509Implicit binding only takes care of thrown errors and <code>'error'</code> events.</p> 510</section><section><h3>Explicit binding<span><a class="mark" href="#explicit-binding" id="explicit-binding">#</a></span><a aria-hidden="true" class="legacy" id="domain_explicit_binding"></a></h3> 511 512<p>Sometimes, the domain in use is not the one that ought to be used for a 513specific event emitter. Or, the event emitter could have been created 514in the context of one domain, but ought to instead be bound to some 515other domain.</p> 516<p>For example, there could be one domain in use for an HTTP server, but 517perhaps we would like to have a separate domain to use for each request.</p> 518<p>That is possible via explicit binding.</p> 519<pre><code class="language-js"><span class="hljs-comment">// Create a top-level domain for the server</span> 520<span class="hljs-keyword">const</span> domain = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:domain'</span>); 521<span class="hljs-keyword">const</span> http = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:http'</span>); 522<span class="hljs-keyword">const</span> serverDomain = domain.<span class="hljs-title function_">create</span>(); 523 524serverDomain.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 525 <span class="hljs-comment">// Server is created in the scope of serverDomain</span> 526 http.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { 527 <span class="hljs-comment">// Req and res are also created in the scope of serverDomain</span> 528 <span class="hljs-comment">// however, we'd prefer to have a separate domain for each request.</span> 529 <span class="hljs-comment">// create it first thing, and add req and res to it.</span> 530 <span class="hljs-keyword">const</span> reqd = domain.<span class="hljs-title function_">create</span>(); 531 reqd.<span class="hljs-title function_">add</span>(req); 532 reqd.<span class="hljs-title function_">add</span>(res); 533 reqd.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 534 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'Error'</span>, er, req.<span class="hljs-property">url</span>); 535 <span class="hljs-keyword">try</span> { 536 res.<span class="hljs-title function_">writeHead</span>(<span class="hljs-number">500</span>); 537 res.<span class="hljs-title function_">end</span>(<span class="hljs-string">'Error occurred, sorry.'</span>); 538 } <span class="hljs-keyword">catch</span> (er2) { 539 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'Error sending 500'</span>, er2, req.<span class="hljs-property">url</span>); 540 } 541 }); 542 }).<span class="hljs-title function_">listen</span>(<span class="hljs-number">1337</span>); 543});</code> <button class="copy-button">copy</button></pre> 544</section><section><h3><code>domain.create()</code><span><a class="mark" href="#domaincreate" id="domaincreate">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain_create"></a></h3> 545<ul> 546<li>Returns: <a href="domain.html#class-domain" class="type"><Domain></a></li> 547</ul> 548</section><section><h3>Class: <code>Domain</code><span><a class="mark" href="#class-domain" id="class-domain">#</a></span><a aria-hidden="true" class="legacy" id="domain_class_domain"></a></h3> 549<ul> 550<li>Extends: <a href="events.html#class-eventemitter" class="type"><EventEmitter></a></li> 551</ul> 552<p>The <code>Domain</code> class encapsulates the functionality of routing errors and 553uncaught exceptions to the active <code>Domain</code> object.</p> 554<p>To handle the errors that it catches, listen to its <code>'error'</code> event.</p> 555<h4><code>domain.members</code><span><a class="mark" href="#domainmembers" id="domainmembers">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain_members"></a></h4> 556<ul> 557<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array" class="type"><Array></a></li> 558</ul> 559<p>An array of timers and event emitters that have been explicitly added 560to the domain.</p> 561<h4><code>domain.add(emitter)</code><span><a class="mark" href="#domainaddemitter" id="domainaddemitter">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain_add_emitter"></a></h4> 562<ul> 563<li><code>emitter</code> <a href="events.html#class-eventemitter" class="type"><EventEmitter></a> | <a href="timers.html#timers" class="type"><Timer></a> emitter or timer to be added to the domain</li> 564</ul> 565<p>Explicitly adds an emitter to the domain. If any event handlers called by 566the emitter throw an error, or if the emitter emits an <code>'error'</code> event, it 567will be routed to the domain's <code>'error'</code> event, just like with implicit 568binding.</p> 569<p>This also works with timers that are returned from <a href="timers.html#setintervalcallback-delay-args"><code>setInterval()</code></a> and 570<a href="timers.html#settimeoutcallback-delay-args"><code>setTimeout()</code></a>. If their callback function throws, it will be caught by 571the domain <code>'error'</code> handler.</p> 572<p>If the Timer or <code>EventEmitter</code> was already bound to a domain, it is removed 573from that one, and bound to this one instead.</p> 574<h4><code>domain.bind(callback)</code><span><a class="mark" href="#domainbindcallback" id="domainbindcallback">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain_bind_callback"></a></h4> 575<ul> 576<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The callback function</li> 577<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The bound function</li> 578</ul> 579<p>The returned function will be a wrapper around the supplied callback 580function. When the returned function is called, any errors that are 581thrown will be routed to the domain's <code>'error'</code> event.</p> 582<pre><code class="language-js"><span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 583 584<span class="hljs-keyword">function</span> <span class="hljs-title function_">readSomeFile</span>(<span class="hljs-params">filename, cb</span>) { 585 fs.<span class="hljs-title function_">readFile</span>(filename, <span class="hljs-string">'utf8'</span>, d.<span class="hljs-title function_">bind</span>(<span class="hljs-function">(<span class="hljs-params">er, data</span>) =></span> { 586 <span class="hljs-comment">// If this throws, it will also be passed to the domain.</span> 587 <span class="hljs-keyword">return</span> <span class="hljs-title function_">cb</span>(er, data ? <span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">parse</span>(data) : <span class="hljs-literal">null</span>); 588 })); 589} 590 591d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 592 <span class="hljs-comment">// An error occurred somewhere. If we throw it now, it will crash the program</span> 593 <span class="hljs-comment">// with the normal line number and stack message.</span> 594});</code> <button class="copy-button">copy</button></pre> 595<h4><code>domain.enter()</code><span><a class="mark" href="#domainenter" id="domainenter">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain_enter"></a></h4> 596<p>The <code>enter()</code> method is plumbing used by the <code>run()</code>, <code>bind()</code>, and 597<code>intercept()</code> methods to set the active domain. It sets <code>domain.active</code> and 598<code>process.domain</code> to the domain, and implicitly pushes the domain onto the domain 599stack managed by the domain module (see <a href="#domainexit"><code>domain.exit()</code></a> for details on the 600domain stack). The call to <code>enter()</code> delimits the beginning of a chain of 601asynchronous calls and I/O operations bound to a domain.</p> 602<p>Calling <code>enter()</code> changes only the active domain, and does not alter the domain 603itself. <code>enter()</code> and <code>exit()</code> can be called an arbitrary number of times on a 604single domain.</p> 605<h4><code>domain.exit()</code><span><a class="mark" href="#domainexit" id="domainexit">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain_exit"></a></h4> 606<p>The <code>exit()</code> method exits the current domain, popping it off the domain stack. 607Any time execution is going to switch to the context of a different chain of 608asynchronous calls, it's important to ensure that the current domain is exited. 609The call to <code>exit()</code> delimits either the end of or an interruption to the chain 610of asynchronous calls and I/O operations bound to a domain.</p> 611<p>If there are multiple, nested domains bound to the current execution context, 612<code>exit()</code> will exit any domains nested within this domain.</p> 613<p>Calling <code>exit()</code> changes only the active domain, and does not alter the domain 614itself. <code>enter()</code> and <code>exit()</code> can be called an arbitrary number of times on a 615single domain.</p> 616<h4><code>domain.intercept(callback)</code><span><a class="mark" href="#domaininterceptcallback" id="domaininterceptcallback">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain_intercept_callback"></a></h4> 617<ul> 618<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The callback function</li> 619<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The intercepted function</li> 620</ul> 621<p>This method is almost identical to <a href="#domainbindcallback"><code>domain.bind(callback)</code></a>. However, in 622addition to catching thrown errors, it will also intercept <a href="errors.html#class-error"><code>Error</code></a> 623objects sent as the first argument to the function.</p> 624<p>In this way, the common <code>if (err) return callback(err);</code> pattern can be replaced 625with a single error handler in a single place.</p> 626<pre><code class="language-js"><span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 627 628<span class="hljs-keyword">function</span> <span class="hljs-title function_">readSomeFile</span>(<span class="hljs-params">filename, cb</span>) { 629 fs.<span class="hljs-title function_">readFile</span>(filename, <span class="hljs-string">'utf8'</span>, d.<span class="hljs-title function_">intercept</span>(<span class="hljs-function">(<span class="hljs-params">data</span>) =></span> { 630 <span class="hljs-comment">// Note, the first argument is never passed to the</span> 631 <span class="hljs-comment">// callback since it is assumed to be the 'Error' argument</span> 632 <span class="hljs-comment">// and thus intercepted by the domain.</span> 633 634 <span class="hljs-comment">// If this throws, it will also be passed to the domain</span> 635 <span class="hljs-comment">// so the error-handling logic can be moved to the 'error'</span> 636 <span class="hljs-comment">// event on the domain instead of being repeated throughout</span> 637 <span class="hljs-comment">// the program.</span> 638 <span class="hljs-keyword">return</span> <span class="hljs-title function_">cb</span>(<span class="hljs-literal">null</span>, <span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">parse</span>(data)); 639 })); 640} 641 642d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 643 <span class="hljs-comment">// An error occurred somewhere. If we throw it now, it will crash the program</span> 644 <span class="hljs-comment">// with the normal line number and stack message.</span> 645});</code> <button class="copy-button">copy</button></pre> 646<h4><code>domain.remove(emitter)</code><span><a class="mark" href="#domainremoveemitter" id="domainremoveemitter">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain_remove_emitter"></a></h4> 647<ul> 648<li><code>emitter</code> <a href="events.html#class-eventemitter" class="type"><EventEmitter></a> | <a href="timers.html#timers" class="type"><Timer></a> emitter or timer to be removed from the domain</li> 649</ul> 650<p>The opposite of <a href="#domainaddemitter"><code>domain.add(emitter)</code></a>. Removes domain handling from the 651specified emitter.</p> 652<h4><code>domain.run(fn[, ...args])</code><span><a class="mark" href="#domainrunfn-args" id="domainrunfn-args">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain_run_fn_args"></a></h4> 653<ul> 654<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> 655<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 656</ul> 657<p>Run the supplied function in the context of the domain, implicitly 658binding all event emitters, timers, and low-level requests that are 659created in that context. Optionally, arguments can be passed to 660the function.</p> 661<p>This is the most basic way to use a domain.</p> 662<pre><code class="language-js"><span class="hljs-keyword">const</span> domain = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:domain'</span>); 663<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 664<span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 665d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 666 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'Caught error!'</span>, er); 667}); 668d.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 669 process.<span class="hljs-title function_">nextTick</span>(<span class="hljs-function">() =></span> { 670 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { <span class="hljs-comment">// Simulating some various async stuff</span> 671 fs.<span class="hljs-title function_">open</span>(<span class="hljs-string">'non-existent file'</span>, <span class="hljs-string">'r'</span>, <span class="hljs-function">(<span class="hljs-params">er, fd</span>) =></span> { 672 <span class="hljs-keyword">if</span> (er) <span class="hljs-keyword">throw</span> er; 673 <span class="hljs-comment">// proceed...</span> 674 }); 675 }, <span class="hljs-number">100</span>); 676 }); 677});</code> <button class="copy-button">copy</button></pre> 678<p>In this example, the <code>d.on('error')</code> handler will be triggered, rather 679than crashing the program.</p> 680</section><section><h3>Domains and promises<span><a class="mark" href="#domains-and-promises" id="domains-and-promises">#</a></span><a aria-hidden="true" class="legacy" id="domain_domains_and_promises"></a></h3> 681<p>As of Node.js 8.0.0, the handlers of promises are run inside the domain in 682which the call to <code>.then()</code> or <code>.catch()</code> itself was made:</p> 683<pre><code class="language-js"><span class="hljs-keyword">const</span> d1 = domain.<span class="hljs-title function_">create</span>(); 684<span class="hljs-keyword">const</span> d2 = domain.<span class="hljs-title function_">create</span>(); 685 686<span class="hljs-keyword">let</span> p; 687d1.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 688 p = <span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">42</span>); 689}); 690 691d2.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 692 p.<span class="hljs-title function_">then</span>(<span class="hljs-function">(<span class="hljs-params">v</span>) =></span> { 693 <span class="hljs-comment">// running in d2</span> 694 }); 695});</code> <button class="copy-button">copy</button></pre> 696<p>A callback may be bound to a specific domain using <a href="#domainbindcallback"><code>domain.bind(callback)</code></a>:</p> 697<pre><code class="language-js"><span class="hljs-keyword">const</span> d1 = domain.<span class="hljs-title function_">create</span>(); 698<span class="hljs-keyword">const</span> d2 = domain.<span class="hljs-title function_">create</span>(); 699 700<span class="hljs-keyword">let</span> p; 701d1.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 702 p = <span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">42</span>); 703}); 704 705d2.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 706 p.<span class="hljs-title function_">then</span>(p.<span class="hljs-property">domain</span>.<span class="hljs-title function_">bind</span>(<span class="hljs-function">(<span class="hljs-params">v</span>) =></span> { 707 <span class="hljs-comment">// running in d1</span> 708 })); 709});</code> <button class="copy-button">copy</button></pre> 710<p>Domains will not interfere with the error handling mechanisms for 711promises. In other words, no <code>'error'</code> event will be emitted for unhandled 712<code>Promise</code> rejections.</p></section> 713 <!-- API END --> 714 </div> 715 </div> 716 </div> 717</body> 718</html> 719