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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-v17.x/api/domain.html">17.x</a></li> 243<li><a href="https://nodejs.org/docs/latest-v16.x/api/domain.html">16.x</a></li> 244<li><a href="https://nodejs.org/docs/latest-v15.x/api/domain.html">15.x</a></li> 245<li><a href="https://nodejs.org/docs/latest-v14.x/api/domain.html">14.x</a></li> 246<li><a href="https://nodejs.org/docs/latest-v13.x/api/domain.html">13.x</a></li> 247<li><a href="https://nodejs.org/docs/latest-v12.x/api/domain.html">12.x</a></li> 248<li><a href="https://nodejs.org/docs/latest-v11.x/api/domain.html">11.x</a></li> 249<li><a href="https://nodejs.org/docs/latest-v10.x/api/domain.html">10.x</a></li> 250<li><a href="https://nodejs.org/docs/latest-v9.x/api/domain.html">9.x</a></li> 251<li><a href="https://nodejs.org/docs/latest-v8.x/api/domain.html">8.x</a></li> 252<li><a href="https://nodejs.org/docs/latest-v7.x/api/domain.html">7.x</a></li> 253<li><a href="https://nodejs.org/docs/latest-v6.x/api/domain.html">6.x</a></li> 254<li><a href="https://nodejs.org/docs/latest-v5.x/api/domain.html">5.x</a></li> 255<li><a href="https://nodejs.org/docs/latest-v4.x/api/domain.html">4.x</a></li> 256<li><a href="https://nodejs.org/docs/latest-v0.12.x/api/domain.html">0.12.x</a></li> 257<li><a href="https://nodejs.org/docs/latest-v0.10.x/api/domain.html">0.10.x</a></li></ol></div> 258 </li> 259 260 <li class="picker-header"> 261 <a href="#"> 262 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 263 Options 264 </a> 265 266 <div class="picker"> 267 <ul> 268 <li> 269 <a href="all.html">View on single page</a> 270 </li> 271 <li> 272 <a href="domain.json">View as JSON</a> 273 </li> 274 <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/main/doc/api/domain.md">Edit on GitHub</a></li> 275 </ul> 276 </div> 277 </li> 278 </ul> 279 </div> 280 <hr> 281 </header> 282 283 <details id="toc" open><summary>Table of contents</summary><ul> 284<li><span class="stability_0"><a href="#domain">Domain</a></span> 285<ul> 286<li><a href="#warning-dont-ignore-errors">Warning: Don't ignore errors!</a></li> 287<li><a href="#additions-to-error-objects">Additions to <code>Error</code> objects</a></li> 288<li><a href="#implicit-binding">Implicit binding</a></li> 289<li><a href="#explicit-binding">Explicit binding</a></li> 290<li><a href="#domaincreate"><code>domain.create()</code></a></li> 291<li><a href="#class-domain">Class: <code>Domain</code></a> 292<ul> 293<li><a href="#domainmembers"><code>domain.members</code></a></li> 294<li><a href="#domainaddemitter"><code>domain.add(emitter)</code></a></li> 295<li><a href="#domainbindcallback"><code>domain.bind(callback)</code></a></li> 296<li><a href="#domainenter"><code>domain.enter()</code></a></li> 297<li><a href="#domainexit"><code>domain.exit()</code></a></li> 298<li><a href="#domaininterceptcallback"><code>domain.intercept(callback)</code></a></li> 299<li><a href="#domainremoveemitter"><code>domain.remove(emitter)</code></a></li> 300<li><a href="#domainrunfn-args"><code>domain.run(fn[, ...args])</code></a></li> 301</ul> 302</li> 303<li><a href="#domains-and-promises">Domains and promises</a></li> 304</ul> 305</li> 306</ul></details> 307 308 <div id="apicontent"> 309 <h2>Domain<span><a class="mark" href="#domain" id="domain">#</a></span><a aria-hidden="true" class="legacy" id="domain_domain"></a></h2> 310<div class="api_metadata"> 311<details class="changelog"><summary>History</summary> 312<table> 313<tbody><tr><th>Version</th><th>Changes</th></tr> 314<tr><td>v8.8.0</td> 315<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> 316<tr><td>v8.0.0</td> 317<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> 318<tr><td>v1.4.2</td> 319<td><p><span>Deprecated since: v1.4.2</span></p></td></tr> 320</tbody></table> 321</details> 322</div> 323 324<p></p><div class="api_stability api_stability_0"><a href="documentation.html#stability-index">Stability: 0</a> - Deprecated</div><p></p> 325<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v18.18.2/lib/domain.js">lib/domain.js</a></p> 326<p><strong>This module is pending deprecation.</strong> Once a replacement API has been 327finalized, this module will be fully deprecated. Most developers should 328<strong>not</strong> have cause to use this module. Users who absolutely must have 329the functionality that domains provide may rely on it for the time being 330but should expect to have to migrate to a different solution 331in the future.</p> 332<p>Domains provide a way to handle multiple different IO operations as a 333single group. If any of the event emitters or callbacks registered to a 334domain emit an <code>'error'</code> event, or throw an error, then the domain object 335will be notified, rather than losing the context of the error in the 336<code>process.on('uncaughtException')</code> handler, or causing the program to 337exit immediately with an error code.</p> 338<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> 339 340<p>Domain error handlers are not a substitute for closing down a 341process when an error occurs.</p> 342<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 343never any way to safely "pick up where it left off", without leaking 344references, or creating some other sort of undefined brittle state.</p> 345<p>The safest way to respond to a thrown error is to shut down the 346process. Of course, in a normal web server, there may be many 347open connections, and it is not reasonable to abruptly shut those down 348because an error was triggered by someone else.</p> 349<p>The better approach is to send an error response to the request that 350triggered the error, while letting the others finish in their normal 351time, and stop listening for new requests in that worker.</p> 352<p>In this way, <code>domain</code> usage goes hand-in-hand with the cluster module, 353since the primary process can fork a new worker when a worker 354encounters an error. For Node.js programs that scale to multiple 355machines, the terminating proxy or service registry can take note of 356the failure, and react accordingly.</p> 357<p>For example, this is not a good idea:</p> 358<pre><code class="language-js"><span class="hljs-comment">// XXX WARNING! BAD IDEA!</span> 359 360<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>(); 361d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 362 <span class="hljs-comment">// The error won't crash the process, but what it does is worse!</span> 363 <span class="hljs-comment">// Though we've prevented abrupt process restarting, we are leaking</span> 364 <span class="hljs-comment">// a lot of resources if this ever happens.</span> 365 <span class="hljs-comment">// This is no better than process.on('uncaughtException')!</span> 366 <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>); 367}); 368d.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 369 <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> { 370 <span class="hljs-title function_">handleRequest</span>(req, res); 371 }).<span class="hljs-title function_">listen</span>(<span class="hljs-variable constant_">PORT</span>); 372});</code> <button class="copy-button">copy</button></pre> 373<p>By using the context of a domain, and the resilience of separating our 374program into multiple worker processes, we can react more 375appropriately, and handle errors with much greater safety.</p> 376<pre><code class="language-js"><span class="hljs-comment">// Much better!</span> 377 378<span class="hljs-keyword">const</span> cluster = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:cluster'</span>); 379<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>; 380 381<span class="hljs-keyword">if</span> (cluster.<span class="hljs-property">isPrimary</span>) { 382 <span class="hljs-comment">// A more realistic scenario would have more than 2 workers,</span> 383 <span class="hljs-comment">// and perhaps not put the primary and worker in the same file.</span> 384 <span class="hljs-comment">//</span> 385 <span class="hljs-comment">// It is also possible to get a bit fancier about logging, and</span> 386 <span class="hljs-comment">// implement whatever custom logic is needed to prevent DoS</span> 387 <span class="hljs-comment">// attacks and other bad behavior.</span> 388 <span class="hljs-comment">//</span> 389 <span class="hljs-comment">// See the options in the cluster documentation.</span> 390 <span class="hljs-comment">//</span> 391 <span class="hljs-comment">// The important thing is that the primary does very little,</span> 392 <span class="hljs-comment">// increasing our resilience to unexpected errors.</span> 393 394 cluster.<span class="hljs-title function_">fork</span>(); 395 cluster.<span class="hljs-title function_">fork</span>(); 396 397 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> { 398 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'disconnect!'</span>); 399 cluster.<span class="hljs-title function_">fork</span>(); 400 }); 401 402} <span class="hljs-keyword">else</span> { 403 <span class="hljs-comment">// the worker</span> 404 <span class="hljs-comment">//</span> 405 <span class="hljs-comment">// This is where we put our bugs!</span> 406 407 <span class="hljs-keyword">const</span> domain = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:domain'</span>); 408 409 <span class="hljs-comment">// See the cluster documentation for more details about using</span> 410 <span class="hljs-comment">// worker processes to serve requests. How it works, caveats, etc.</span> 411 412 <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> { 413 <span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 414 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> { 415 <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>); 416 417 <span class="hljs-comment">// We're in dangerous territory!</span> 418 <span class="hljs-comment">// By definition, something unexpected occurred,</span> 419 <span class="hljs-comment">// which we probably didn't want.</span> 420 <span class="hljs-comment">// Anything can happen now! Be very careful!</span> 421 422 <span class="hljs-keyword">try</span> { 423 <span class="hljs-comment">// Make sure we close down within 30 seconds</span> 424 <span class="hljs-keyword">const</span> killtimer = <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 425 process.<span class="hljs-title function_">exit</span>(<span class="hljs-number">1</span>); 426 }, <span class="hljs-number">30000</span>); 427 <span class="hljs-comment">// But don't keep the process open just for that!</span> 428 killtimer.<span class="hljs-title function_">unref</span>(); 429 430 <span class="hljs-comment">// Stop taking new requests.</span> 431 server.<span class="hljs-title function_">close</span>(); 432 433 <span class="hljs-comment">// Let the primary know we're dead. This will trigger a</span> 434 <span class="hljs-comment">// 'disconnect' in the cluster primary, and then it will fork</span> 435 <span class="hljs-comment">// a new worker.</span> 436 cluster.<span class="hljs-property">worker</span>.<span class="hljs-title function_">disconnect</span>(); 437 438 <span class="hljs-comment">// Try to send an error to the request that triggered the problem</span> 439 res.<span class="hljs-property">statusCode</span> = <span class="hljs-number">500</span>; 440 res.<span class="hljs-title function_">setHeader</span>(<span class="hljs-string">'content-type'</span>, <span class="hljs-string">'text/plain'</span>); 441 res.<span class="hljs-title function_">end</span>(<span class="hljs-string">'Oops, there was a problem!\n'</span>); 442 } <span class="hljs-keyword">catch</span> (er2) { 443 <span class="hljs-comment">// Oh well, not much we can do at this point.</span> 444 <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>); 445 } 446 }); 447 448 <span class="hljs-comment">// Because req and res were created before this domain existed,</span> 449 <span class="hljs-comment">// we need to explicitly add them.</span> 450 <span class="hljs-comment">// See the explanation of implicit vs explicit binding below.</span> 451 d.<span class="hljs-title function_">add</span>(req); 452 d.<span class="hljs-title function_">add</span>(res); 453 454 <span class="hljs-comment">// Now run the handler function in the domain.</span> 455 d.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 456 <span class="hljs-title function_">handleRequest</span>(req, res); 457 }); 458 }); 459 server.<span class="hljs-title function_">listen</span>(<span class="hljs-variable constant_">PORT</span>); 460} 461 462<span class="hljs-comment">// This part is not important. Just an example routing thing.</span> 463<span class="hljs-comment">// Put fancy application logic here.</span> 464<span class="hljs-keyword">function</span> <span class="hljs-title function_">handleRequest</span>(<span class="hljs-params">req, res</span>) { 465 <span class="hljs-keyword">switch</span> (req.<span class="hljs-property">url</span>) { 466 <span class="hljs-keyword">case</span> <span class="hljs-string">'/error'</span>: 467 <span class="hljs-comment">// We do some async stuff, and then...</span> 468 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 469 <span class="hljs-comment">// Whoops!</span> 470 flerb.<span class="hljs-title function_">bark</span>(); 471 }, timeout); 472 <span class="hljs-keyword">break</span>; 473 <span class="hljs-attr">default</span>: 474 res.<span class="hljs-title function_">end</span>(<span class="hljs-string">'ok'</span>); 475 } 476}</code> <button class="copy-button">copy</button></pre> 477</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> 478 479<p>Any time an <code>Error</code> object is routed through a domain, a few extra fields 480are added to it.</p> 481<ul> 482<li><code>error.domain</code> The domain that first handled the error.</li> 483<li><code>error.domainEmitter</code> The event emitter that emitted an <code>'error'</code> event 484with the error object.</li> 485<li><code>error.domainBound</code> The callback function which was bound to the 486domain, and passed an error as its first argument.</li> 487<li><code>error.domainThrown</code> A boolean indicating whether the error was 488thrown, emitted, or passed to a bound callback function.</li> 489</ul> 490</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> 491 492<p>If domains are in use, then all <strong>new</strong> <code>EventEmitter</code> objects (including 493Stream objects, requests, responses, etc.) will be implicitly bound to 494the active domain at the time of their creation.</p> 495<p>Additionally, callbacks passed to low-level event loop requests (such as 496to <code>fs.open()</code>, or other callback-taking methods) will automatically be 497bound to the active domain. If they throw, then the domain will catch 498the error.</p> 499<p>In order to prevent excessive memory usage, <code>Domain</code> objects themselves 500are not implicitly added as children of the active domain. If they 501were, then it would be too easy to prevent request and response objects 502from being properly garbage collected.</p> 503<p>To nest <code>Domain</code> objects as children of a parent <code>Domain</code> they must be 504explicitly added.</p> 505<p>Implicit binding routes thrown errors and <code>'error'</code> events to the 506<code>Domain</code>'s <code>'error'</code> event, but does not register the <code>EventEmitter</code> on the 507<code>Domain</code>. 508Implicit binding only takes care of thrown errors and <code>'error'</code> events.</p> 509</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> 510 511<p>Sometimes, the domain in use is not the one that ought to be used for a 512specific event emitter. Or, the event emitter could have been created 513in the context of one domain, but ought to instead be bound to some 514other domain.</p> 515<p>For example, there could be one domain in use for an HTTP server, but 516perhaps we would like to have a separate domain to use for each request.</p> 517<p>That is possible via explicit binding.</p> 518<pre><code class="language-js"><span class="hljs-comment">// Create a top-level domain for the server</span> 519<span class="hljs-keyword">const</span> domain = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:domain'</span>); 520<span class="hljs-keyword">const</span> http = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:http'</span>); 521<span class="hljs-keyword">const</span> serverDomain = domain.<span class="hljs-title function_">create</span>(); 522 523serverDomain.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 524 <span class="hljs-comment">// Server is created in the scope of serverDomain</span> 525 http.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { 526 <span class="hljs-comment">// Req and res are also created in the scope of serverDomain</span> 527 <span class="hljs-comment">// however, we'd prefer to have a separate domain for each request.</span> 528 <span class="hljs-comment">// create it first thing, and add req and res to it.</span> 529 <span class="hljs-keyword">const</span> reqd = domain.<span class="hljs-title function_">create</span>(); 530 reqd.<span class="hljs-title function_">add</span>(req); 531 reqd.<span class="hljs-title function_">add</span>(res); 532 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> { 533 <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>); 534 <span class="hljs-keyword">try</span> { 535 res.<span class="hljs-title function_">writeHead</span>(<span class="hljs-number">500</span>); 536 res.<span class="hljs-title function_">end</span>(<span class="hljs-string">'Error occurred, sorry.'</span>); 537 } <span class="hljs-keyword">catch</span> (er2) { 538 <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>); 539 } 540 }); 541 }).<span class="hljs-title function_">listen</span>(<span class="hljs-number">1337</span>); 542});</code> <button class="copy-button">copy</button></pre> 543</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> 544<ul> 545<li>Returns: <a href="domain.html#class-domain" class="type"><Domain></a></li> 546</ul> 547</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> 548<ul> 549<li>Extends: <a href="events.html#class-eventemitter" class="type"><EventEmitter></a></li> 550</ul> 551<p>The <code>Domain</code> class encapsulates the functionality of routing errors and 552uncaught exceptions to the active <code>Domain</code> object.</p> 553<p>To handle the errors that it catches, listen to its <code>'error'</code> event.</p> 554<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> 555<ul> 556<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array" class="type"><Array></a></li> 557</ul> 558<p>An array of timers and event emitters that have been explicitly added 559to the domain.</p> 560<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> 561<ul> 562<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> 563</ul> 564<p>Explicitly adds an emitter to the domain. If any event handlers called by 565the emitter throw an error, or if the emitter emits an <code>'error'</code> event, it 566will be routed to the domain's <code>'error'</code> event, just like with implicit 567binding.</p> 568<p>This also works with timers that are returned from <a href="timers.html#setintervalcallback-delay-args"><code>setInterval()</code></a> and 569<a href="timers.html#settimeoutcallback-delay-args"><code>setTimeout()</code></a>. If their callback function throws, it will be caught by 570the domain <code>'error'</code> handler.</p> 571<p>If the Timer or <code>EventEmitter</code> was already bound to a domain, it is removed 572from that one, and bound to this one instead.</p> 573<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> 574<ul> 575<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> 576<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> 577</ul> 578<p>The returned function will be a wrapper around the supplied callback 579function. When the returned function is called, any errors that are 580thrown will be routed to the domain's <code>'error'</code> event.</p> 581<pre><code class="language-js"><span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 582 583<span class="hljs-keyword">function</span> <span class="hljs-title function_">readSomeFile</span>(<span class="hljs-params">filename, cb</span>) { 584 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> { 585 <span class="hljs-comment">// If this throws, it will also be passed to the domain.</span> 586 <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>); 587 })); 588} 589 590d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 591 <span class="hljs-comment">// An error occurred somewhere. If we throw it now, it will crash the program</span> 592 <span class="hljs-comment">// with the normal line number and stack message.</span> 593});</code> <button class="copy-button">copy</button></pre> 594<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> 595<p>The <code>enter()</code> method is plumbing used by the <code>run()</code>, <code>bind()</code>, and 596<code>intercept()</code> methods to set the active domain. It sets <code>domain.active</code> and 597<code>process.domain</code> to the domain, and implicitly pushes the domain onto the domain 598stack managed by the domain module (see <a href="#domainexit"><code>domain.exit()</code></a> for details on the 599domain stack). The call to <code>enter()</code> delimits the beginning of a chain of 600asynchronous calls and I/O operations bound to a domain.</p> 601<p>Calling <code>enter()</code> changes only the active domain, and does not alter the domain 602itself. <code>enter()</code> and <code>exit()</code> can be called an arbitrary number of times on a 603single domain.</p> 604<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> 605<p>The <code>exit()</code> method exits the current domain, popping it off the domain stack. 606Any time execution is going to switch to the context of a different chain of 607asynchronous calls, it's important to ensure that the current domain is exited. 608The call to <code>exit()</code> delimits either the end of or an interruption to the chain 609of asynchronous calls and I/O operations bound to a domain.</p> 610<p>If there are multiple, nested domains bound to the current execution context, 611<code>exit()</code> will exit any domains nested within this domain.</p> 612<p>Calling <code>exit()</code> changes only the active domain, and does not alter the domain 613itself. <code>enter()</code> and <code>exit()</code> can be called an arbitrary number of times on a 614single domain.</p> 615<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> 616<ul> 617<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> 618<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> 619</ul> 620<p>This method is almost identical to <a href="#domainbindcallback"><code>domain.bind(callback)</code></a>. However, in 621addition to catching thrown errors, it will also intercept <a href="errors.html#class-error"><code>Error</code></a> 622objects sent as the first argument to the function.</p> 623<p>In this way, the common <code>if (err) return callback(err);</code> pattern can be replaced 624with a single error handler in a single place.</p> 625<pre><code class="language-js"><span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 626 627<span class="hljs-keyword">function</span> <span class="hljs-title function_">readSomeFile</span>(<span class="hljs-params">filename, cb</span>) { 628 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> { 629 <span class="hljs-comment">// Note, the first argument is never passed to the</span> 630 <span class="hljs-comment">// callback since it is assumed to be the 'Error' argument</span> 631 <span class="hljs-comment">// and thus intercepted by the domain.</span> 632 633 <span class="hljs-comment">// If this throws, it will also be passed to the domain</span> 634 <span class="hljs-comment">// so the error-handling logic can be moved to the 'error'</span> 635 <span class="hljs-comment">// event on the domain instead of being repeated throughout</span> 636 <span class="hljs-comment">// the program.</span> 637 <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)); 638 })); 639} 640 641d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 642 <span class="hljs-comment">// An error occurred somewhere. If we throw it now, it will crash the program</span> 643 <span class="hljs-comment">// with the normal line number and stack message.</span> 644});</code> <button class="copy-button">copy</button></pre> 645<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> 646<ul> 647<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> 648</ul> 649<p>The opposite of <a href="#domainaddemitter"><code>domain.add(emitter)</code></a>. Removes domain handling from the 650specified emitter.</p> 651<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> 652<ul> 653<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> 654<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 655</ul> 656<p>Run the supplied function in the context of the domain, implicitly 657binding all event emitters, timers, and low-level requests that are 658created in that context. Optionally, arguments can be passed to 659the function.</p> 660<p>This is the most basic way to use a domain.</p> 661<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>); 662<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 663<span class="hljs-keyword">const</span> d = domain.<span class="hljs-title function_">create</span>(); 664d.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">er</span>) =></span> { 665 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(<span class="hljs-string">'Caught error!'</span>, er); 666}); 667d.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 668 process.<span class="hljs-title function_">nextTick</span>(<span class="hljs-function">() =></span> { 669 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { <span class="hljs-comment">// Simulating some various async stuff</span> 670 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> { 671 <span class="hljs-keyword">if</span> (er) <span class="hljs-keyword">throw</span> er; 672 <span class="hljs-comment">// proceed...</span> 673 }); 674 }, <span class="hljs-number">100</span>); 675 }); 676});</code> <button class="copy-button">copy</button></pre> 677<p>In this example, the <code>d.on('error')</code> handler will be triggered, rather 678than crashing the program.</p> 679</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> 680<p>As of Node.js 8.0.0, the handlers of promises are run inside the domain in 681which the call to <code>.then()</code> or <code>.catch()</code> itself was made:</p> 682<pre><code class="language-js"><span class="hljs-keyword">const</span> d1 = domain.<span class="hljs-title function_">create</span>(); 683<span class="hljs-keyword">const</span> d2 = domain.<span class="hljs-title function_">create</span>(); 684 685<span class="hljs-keyword">let</span> p; 686d1.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 687 p = <span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">42</span>); 688}); 689 690d2.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 691 p.<span class="hljs-title function_">then</span>(<span class="hljs-function">(<span class="hljs-params">v</span>) =></span> { 692 <span class="hljs-comment">// running in d2</span> 693 }); 694});</code> <button class="copy-button">copy</button></pre> 695<p>A callback may be bound to a specific domain using <a href="#domainbindcallback"><code>domain.bind(callback)</code></a>:</p> 696<pre><code class="language-js"><span class="hljs-keyword">const</span> d1 = domain.<span class="hljs-title function_">create</span>(); 697<span class="hljs-keyword">const</span> d2 = domain.<span class="hljs-title function_">create</span>(); 698 699<span class="hljs-keyword">let</span> p; 700d1.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 701 p = <span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">42</span>); 702}); 703 704d2.<span class="hljs-title function_">run</span>(<span class="hljs-function">() =></span> { 705 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> { 706 <span class="hljs-comment">// running in d1</span> 707 })); 708});</code> <button class="copy-button">copy</button></pre> 709<p>Domains will not interfere with the error handling mechanisms for 710promises. In other words, no <code>'error'</code> event will be emitted for unhandled 711<code>Promise</code> rejections.</p></section> 712 <!-- API END --> 713 </div> 714 </div> 715 </div> 716</body> 717</html> 718