{ "type": "module", "source": "doc/api/events.md", "modules": [ { "textRaw": "Events", "name": "Events", "introduced_in": "v0.10.0", "stability": 2, "stabilityText": "Stable", "type": "module", "desc": "<p>Much of the Node.js core API is built around an idiomatic asynchronous\nevent-driven architecture in which certain kinds of objects (called \"emitters\")\nemit named events that cause <code>Function</code> objects (\"listeners\") to be called.</p>\n<p>For instance: a <a href=\"net.html#net_class_net_server\"><code>net.Server</code></a> object emits an event each time a peer\nconnects to it; a <a href=\"fs.html#fs_class_fs_readstream\"><code>fs.ReadStream</code></a> emits an event when the file is opened;\na <a href=\"stream.html\">stream</a> emits an event whenever data is available to be read.</p>\n<p>All objects that emit events are instances of the <code>EventEmitter</code> class. These\nobjects expose an <code>eventEmitter.on()</code> function that allows one or more\nfunctions to be attached to named events emitted by the object. Typically,\nevent names are camel-cased strings but any valid JavaScript property key\ncan be used.</p>\n<p>When the <code>EventEmitter</code> object emits an event, all of the functions attached\nto that specific event are called <em>synchronously</em>. Any values returned by the\ncalled listeners are <em>ignored</em> and will be discarded.</p>\n<p>The following example shows a simple <code>EventEmitter</code> instance with a single\nlistener. The <code>eventEmitter.on()</code> method is used to register listeners, while\nthe <code>eventEmitter.emit()</code> method is used to trigger the event.</p>\n<pre><code class=\"language-js\">const EventEmitter = require('events');\n\nclass MyEmitter extends EventEmitter {}\n\nconst myEmitter = new MyEmitter();\nmyEmitter.on('event', () => {\n console.log('an event occurred!');\n});\nmyEmitter.emit('event');\n</code></pre>", "modules": [ { "textRaw": "Passing arguments and `this` to listeners", "name": "passing_arguments_and_`this`_to_listeners", "desc": "<p>The <code>eventEmitter.emit()</code> method allows an arbitrary set of arguments to be\npassed to the listener functions. It is important to keep in mind that when\nan ordinary listener function is called, the standard <code>this</code> keyword\nis intentionally set to reference the <code>EventEmitter</code> instance to which the\nlistener is attached.</p>\n<pre><code class=\"language-js\">const myEmitter = new MyEmitter();\nmyEmitter.on('event', function(a, b) {\n console.log(a, b, this, this === myEmitter);\n // Prints:\n // a b MyEmitter {\n // domain: null,\n // _events: { event: [Function] },\n // _eventsCount: 1,\n // _maxListeners: undefined } true\n});\nmyEmitter.emit('event', 'a', 'b');\n</code></pre>\n<p>It is possible to use ES6 Arrow Functions as listeners, however, when doing so,\nthe <code>this</code> keyword will no longer reference the <code>EventEmitter</code> instance:</p>\n<pre><code class=\"language-js\">const myEmitter = new MyEmitter();\nmyEmitter.on('event', (a, b) => {\n console.log(a, b, this);\n // Prints: a b {}\n});\nmyEmitter.emit('event', 'a', 'b');\n</code></pre>", "type": "module", "displayName": "Passing arguments and `this` to listeners" }, { "textRaw": "Asynchronous vs. Synchronous", "name": "asynchronous_vs._synchronous", "desc": "<p>The <code>EventEmitter</code> calls all listeners synchronously in the order in which\nthey were registered. This is important to ensure the proper sequencing of\nevents and to avoid race conditions or logic errors. When appropriate,\nlistener functions can switch to an asynchronous mode of operation using\nthe <code>setImmediate()</code> or <code>process.nextTick()</code> methods:</p>\n<pre><code class=\"language-js\">const myEmitter = new MyEmitter();\nmyEmitter.on('event', (a, b) => {\n setImmediate(() => {\n console.log('this happens asynchronously');\n });\n});\nmyEmitter.emit('event', 'a', 'b');\n</code></pre>", "type": "module", "displayName": "Asynchronous vs. Synchronous" }, { "textRaw": "Handling events only once", "name": "handling_events_only_once", "desc": "<p>When a listener is registered using the <code>eventEmitter.on()</code> method, that\nlistener will be invoked <em>every time</em> the named event is emitted.</p>\n<pre><code class=\"language-js\">const myEmitter = new MyEmitter();\nlet m = 0;\nmyEmitter.on('event', () => {\n console.log(++m);\n});\nmyEmitter.emit('event');\n// Prints: 1\nmyEmitter.emit('event');\n// Prints: 2\n</code></pre>\n<p>Using the <code>eventEmitter.once()</code> method, it is possible to register a listener\nthat is called at most once for a particular event. Once the event is emitted,\nthe listener is unregistered and <em>then</em> called.</p>\n<pre><code class=\"language-js\">const myEmitter = new MyEmitter();\nlet m = 0;\nmyEmitter.once('event', () => {\n console.log(++m);\n});\nmyEmitter.emit('event');\n// Prints: 1\nmyEmitter.emit('event');\n// Ignored\n</code></pre>", "type": "module", "displayName": "Handling events only once" }, { "textRaw": "Error events", "name": "error_events", "desc": "<p>When an error occurs within an <code>EventEmitter</code> instance, the typical action is\nfor an <code>'error'</code> event to be emitted. These are treated as special cases\nwithin Node.js.</p>\n<p>If an <code>EventEmitter</code> does <em>not</em> have at least one listener registered for the\n<code>'error'</code> event, and an <code>'error'</code> event is emitted, the error is thrown, a\nstack trace is printed, and the Node.js process exits.</p>\n<pre><code class=\"language-js\">const myEmitter = new MyEmitter();\nmyEmitter.emit('error', new Error('whoops!'));\n// Throws and crashes Node.js\n</code></pre>\n<p>To guard against crashing the Node.js process the <a href=\"domain.html\"><code>domain</code></a> module can be\nused. (Note, however, that the <code>domain</code> module is deprecated.)</p>\n<p>As a best practice, listeners should always be added for the <code>'error'</code> events.</p>\n<pre><code class=\"language-js\">const myEmitter = new MyEmitter();\nmyEmitter.on('error', (err) => {\n console.error('whoops! there was an error');\n});\nmyEmitter.emit('error', new Error('whoops!'));\n// Prints: whoops! there was an error\n</code></pre>", "type": "module", "displayName": "Error events" } ], "classes": [ { "textRaw": "Class: EventEmitter", "type": "class", "name": "EventEmitter", "meta": { "added": [ "v0.1.26" ], "changes": [] }, "desc": "<p>The <code>EventEmitter</code> class is defined and exposed by the <code>events</code> module:</p>\n<pre><code class=\"language-js\">const EventEmitter = require('events');\n</code></pre>\n<p>All <code>EventEmitter</code>s emit the event <code>'newListener'</code> when new listeners are\nadded and <code>'removeListener'</code> when existing listeners are removed.</p>", "events": [ { "textRaw": "Event: 'newListener'", "type": "event", "name": "newListener", "meta": { "added": [ "v0.1.26" ], "changes": [] }, "params": [ { "textRaw": "`eventName` {string|symbol} The name of the event being listened for", "name": "eventName", "type": "string|symbol", "desc": "The name of the event being listened for" }, { "textRaw": "`listener` {Function} The event handler function", "name": "listener", "type": "Function", "desc": "The event handler function" } ], "desc": "<p>The <code>EventEmitter</code> instance will emit its own <code>'newListener'</code> event <em>before</em>\na listener is added to its internal array of listeners.</p>\n<p>Listeners registered for the <code>'newListener'</code> event will be passed the event\nname and a reference to the listener being added.</p>\n<p>The fact that the event is triggered before adding the listener has a subtle\nbut important side effect: any <em>additional</em> listeners registered to the same\n<code>name</code> <em>within</em> the <code>'newListener'</code> callback will be inserted <em>before</em> the\nlistener that is in the process of being added.</p>\n<pre><code class=\"language-js\">const myEmitter = new MyEmitter();\n// Only do this once so we don't loop forever\nmyEmitter.once('newListener', (event, listener) => {\n if (event === 'event') {\n // Insert a new listener in front\n myEmitter.on('event', () => {\n console.log('B');\n });\n }\n});\nmyEmitter.on('event', () => {\n console.log('A');\n});\nmyEmitter.emit('event');\n// Prints:\n// B\n// A\n</code></pre>" }, { "textRaw": "Event: 'removeListener'", "type": "event", "name": "removeListener", "meta": { "added": [ "v0.9.3" ], "changes": [ { "version": "v6.1.0, v4.7.0", "pr-url": "https://github.com/nodejs/node/pull/6394", "description": "For listeners attached using `.once()`, the `listener` argument now yields the original listener function." } ] }, "params": [ { "textRaw": "`eventName` {string|symbol} The event name", "name": "eventName", "type": "string|symbol", "desc": "The event name" }, { "textRaw": "`listener` {Function} The event handler function", "name": "listener", "type": "Function", "desc": "The event handler function" } ], "desc": "<p>The <code>'removeListener'</code> event is emitted <em>after</em> the <code>listener</code> is removed.</p>" } ], "methods": [ { "textRaw": "EventEmitter.listenerCount(emitter, eventName)", "type": "method", "name": "listenerCount", "meta": { "added": [ "v0.9.12" ], "deprecated": [ "v4.0.0" ], "changes": [] }, "stability": 0, "stabilityText": "Deprecated: Use [`emitter.listenerCount()`][] instead.", "signatures": [ { "params": [ { "textRaw": "`emitter` {EventEmitter} The emitter to query", "name": "emitter", "type": "EventEmitter", "desc": "The emitter to query" }, { "textRaw": "`eventName` {string|symbol} The event name", "name": "eventName", "type": "string|symbol", "desc": "The event name" } ] } ], "desc": "<p>A class method that returns the number of listeners for the given <code>eventName</code>\nregistered on the given <code>emitter</code>.</p>\n<pre><code class=\"language-js\">const myEmitter = new MyEmitter();\nmyEmitter.on('event', () => {});\nmyEmitter.on('event', () => {});\nconsole.log(EventEmitter.listenerCount(myEmitter, 'event'));\n// Prints: 2\n</code></pre>" }, { "textRaw": "emitter.addListener(eventName, listener)", "type": "method", "name": "addListener", "meta": { "added": [ "v0.1.26" ], "changes": [] }, "signatures": [ { "params": [ { "textRaw": "`eventName` {string|symbol}", "name": "eventName", "type": "string|symbol" }, { "textRaw": "`listener` {Function}", "name": "listener", "type": "Function" } ] } ], "desc": "<p>Alias for <code>emitter.on(eventName, listener)</code>.</p>" }, { "textRaw": "emitter.emit(eventName[, ...args])", "type": "method", "name": "emit", "meta": { "added": [ "v0.1.26" ], "changes": [] }, "signatures": [ { "params": [ { "textRaw": "`eventName` {string|symbol}", "name": "eventName", "type": "string|symbol" }, { "name": "...args", "optional": true } ] } ], "desc": "<ul>\n<li><code>...args</code> <a href=\"https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types\" class=\"type\">&lt;any&gt;</a></li>\n</ul>\n<ul>\n<li>Returns: <a href=\"https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type\" class=\"type\">&lt;boolean&gt;</a></li>\n</ul>\n<p>Synchronously calls each of the listeners registered for the event named\n<code>eventName</code>, in the order they were registered, passing the supplied arguments\nto each.</p>\n<p>Returns <code>true</code> if the event had listeners, <code>false</code> otherwise.</p>" }, { "textRaw": "emitter.eventNames()", "type": "method", "name": "eventNames", "meta": { "added": [ "v6.0.0" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {Array}", "name": "return", "type": "Array" }, "params": [] } ], "desc": "<p>Returns an array listing the events for which the emitter has registered\nlisteners. The values in the array will be strings or <code>Symbol</code>s.</p>\n<pre><code class=\"language-js\">const EventEmitter = require('events');\nconst myEE = new EventEmitter();\nmyEE.on('foo', () => {});\nmyEE.on('bar', () => {});\n\nconst sym = Symbol('symbol');\nmyEE.on(sym, () => {});\n\nconsole.log(myEE.eventNames());\n// Prints: [ 'foo', 'bar', Symbol(symbol) ]\n</code></pre>" }, { "textRaw": "emitter.getMaxListeners()", "type": "method", "name": "getMaxListeners", "meta": { "added": [ "v1.0.0" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {integer}", "name": "return", "type": "integer" }, "params": [] } ], "desc": "<p>Returns the current max listener value for the <code>EventEmitter</code> which is either\nset by <a href=\"#events_emitter_setmaxlisteners_n\"><code>emitter.setMaxListeners(n)</code></a> or defaults to\n<a href=\"#events_eventemitter_defaultmaxlisteners\"><code>EventEmitter.defaultMaxListeners</code></a>.</p>" }, { "textRaw": "emitter.listenerCount(eventName)", "type": "method", "name": "listenerCount", "meta": { "added": [ "v3.2.0" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {integer}", "name": "return", "type": "integer" }, "params": [ { "textRaw": "`eventName` {string|symbol} The name of the event being listened for", "name": "eventName", "type": "string|symbol", "desc": "The name of the event being listened for" } ] } ], "desc": "<p>Returns the number of listeners listening to the event named <code>eventName</code>.</p>" }, { "textRaw": "emitter.listeners(eventName)", "type": "method", "name": "listeners", "meta": { "added": [ "v0.1.26" ], "changes": [ { "version": "v7.0.0", "pr-url": "https://github.com/nodejs/node/pull/6881", "description": "For listeners attached using `.once()` this returns the original listeners instead of wrapper functions now." } ] }, "signatures": [ { "return": { "textRaw": "Returns: {Function[]}", "name": "return", "type": "Function[]" }, "params": [ { "textRaw": "`eventName` {string|symbol}", "name": "eventName", "type": "string|symbol" } ] } ], "desc": "<p>Returns a copy of the array of listeners for the event named <code>eventName</code>.</p>\n<pre><code class=\"language-js\">server.on('connection', (stream) => {\n console.log('someone connected!');\n});\nconsole.log(util.inspect(server.listeners('connection')));\n// Prints: [ [Function] ]\n</code></pre>" }, { "textRaw": "emitter.off(eventName, listener)", "type": "method", "name": "off", "meta": { "added": [ "v10.0.0" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {EventEmitter}", "name": "return", "type": "EventEmitter" }, "params": [ { "textRaw": "`eventName` {string|symbol}", "name": "eventName", "type": "string|symbol" }, { "textRaw": "`listener` {Function}", "name": "listener", "type": "Function" } ] } ], "desc": "<p>Alias for <a href=\"#events_emitter_removelistener_eventname_listener\"><code>emitter.removeListener()</code></a>.</p>" }, { "textRaw": "emitter.on(eventName, listener)", "type": "method", "name": "on", "meta": { "added": [ "v0.1.101" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {EventEmitter}", "name": "return", "type": "EventEmitter" }, "params": [ { "textRaw": "`eventName` {string|symbol} The name of the event.", "name": "eventName", "type": "string|symbol", "desc": "The name of the event." }, { "textRaw": "`listener` {Function} The callback function", "name": "listener", "type": "Function", "desc": "The callback function" } ] } ], "desc": "<p>Adds the <code>listener</code> function to the end of the listeners array for the\nevent named <code>eventName</code>. No checks are made to see if the <code>listener</code> has\nalready been added. Multiple calls passing the same combination of <code>eventName</code>\nand <code>listener</code> will result in the <code>listener</code> being added, and called, multiple\ntimes.</p>\n<pre><code class=\"language-js\">server.on('connection', (stream) => {\n console.log('someone connected!');\n});\n</code></pre>\n<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p>\n<p>By default, event listeners are invoked in the order they are added. The\n<code>emitter.prependListener()</code> method can be used as an alternative to add the\nevent listener to the beginning of the listeners array.</p>\n<pre><code class=\"language-js\">const myEE = new EventEmitter();\nmyEE.on('foo', () => console.log('a'));\nmyEE.prependListener('foo', () => console.log('b'));\nmyEE.emit('foo');\n// Prints:\n// b\n// a\n</code></pre>" }, { "textRaw": "emitter.once(eventName, listener)", "type": "method", "name": "once", "meta": { "added": [ "v0.3.0" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {EventEmitter}", "name": "return", "type": "EventEmitter" }, "params": [ { "textRaw": "`eventName` {string|symbol} The name of the event.", "name": "eventName", "type": "string|symbol", "desc": "The name of the event." }, { "textRaw": "`listener` {Function} The callback function", "name": "listener", "type": "Function", "desc": "The callback function" } ] } ], "desc": "<p>Adds a <strong>one-time</strong> <code>listener</code> function for the event named <code>eventName</code>. The\nnext time <code>eventName</code> is triggered, this listener is removed and then invoked.</p>\n<pre><code class=\"language-js\">server.once('connection', (stream) => {\n console.log('Ah, we have our first user!');\n});\n</code></pre>\n<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p>\n<p>By default, event listeners are invoked in the order they are added. The\n<code>emitter.prependOnceListener()</code> method can be used as an alternative to add the\nevent listener to the beginning of the listeners array.</p>\n<pre><code class=\"language-js\">const myEE = new EventEmitter();\nmyEE.once('foo', () => console.log('a'));\nmyEE.prependOnceListener('foo', () => console.log('b'));\nmyEE.emit('foo');\n// Prints:\n// b\n// a\n</code></pre>" }, { "textRaw": "emitter.prependListener(eventName, listener)", "type": "method", "name": "prependListener", "meta": { "added": [ "v6.0.0" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {EventEmitter}", "name": "return", "type": "EventEmitter" }, "params": [ { "textRaw": "`eventName` {string|symbol} The name of the event.", "name": "eventName", "type": "string|symbol", "desc": "The name of the event." }, { "textRaw": "`listener` {Function} The callback function", "name": "listener", "type": "Function", "desc": "The callback function" } ] } ], "desc": "<p>Adds the <code>listener</code> function to the <em>beginning</em> of the listeners array for the\nevent named <code>eventName</code>. No checks are made to see if the <code>listener</code> has\nalready been added. Multiple calls passing the same combination of <code>eventName</code>\nand <code>listener</code> will result in the <code>listener</code> being added, and called, multiple\ntimes.</p>\n<pre><code class=\"language-js\">server.prependListener('connection', (stream) => {\n console.log('someone connected!');\n});\n</code></pre>\n<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p>" }, { "textRaw": "emitter.prependOnceListener(eventName, listener)", "type": "method", "name": "prependOnceListener", "meta": { "added": [ "v6.0.0" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {EventEmitter}", "name": "return", "type": "EventEmitter" }, "params": [ { "textRaw": "`eventName` {string|symbol} The name of the event.", "name": "eventName", "type": "string|symbol", "desc": "The name of the event." }, { "textRaw": "`listener` {Function} The callback function", "name": "listener", "type": "Function", "desc": "The callback function" } ] } ], "desc": "<p>Adds a <strong>one-time</strong> <code>listener</code> function for the event named <code>eventName</code> to the\n<em>beginning</em> of the listeners array. The next time <code>eventName</code> is triggered, this\nlistener is removed, and then invoked.</p>\n<pre><code class=\"language-js\">server.prependOnceListener('connection', (stream) => {\n console.log('Ah, we have our first user!');\n});\n</code></pre>\n<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p>" }, { "textRaw": "emitter.removeAllListeners([eventName])", "type": "method", "name": "removeAllListeners", "meta": { "added": [ "v0.1.26" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {EventEmitter}", "name": "return", "type": "EventEmitter" }, "params": [ { "textRaw": "`eventName` {string|symbol}", "name": "eventName", "type": "string|symbol", "optional": true } ] } ], "desc": "<p>Removes all listeners, or those of the specified <code>eventName</code>.</p>\n<p>Note that it is bad practice to remove listeners added elsewhere in the code,\nparticularly when the <code>EventEmitter</code> instance was created by some other\ncomponent or module (e.g. sockets or file streams).</p>\n<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p>" }, { "textRaw": "emitter.removeListener(eventName, listener)", "type": "method", "name": "removeListener", "meta": { "added": [ "v0.1.26" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {EventEmitter}", "name": "return", "type": "EventEmitter" }, "params": [ { "textRaw": "`eventName` {string|symbol}", "name": "eventName", "type": "string|symbol" }, { "textRaw": "`listener` {Function}", "name": "listener", "type": "Function" } ] } ], "desc": "<p>Removes the specified <code>listener</code> from the listener array for the event named\n<code>eventName</code>.</p>\n<pre><code class=\"language-js\">const callback = (stream) => {\n console.log('someone connected!');\n};\nserver.on('connection', callback);\n// ...\nserver.removeListener('connection', callback);\n</code></pre>\n<p><code>removeListener()</code> will remove, at most, one instance of a listener from the\nlistener array. If any single listener has been added multiple times to the\nlistener array for the specified <code>eventName</code>, then <code>removeListener()</code> must be\ncalled multiple times to remove each instance.</p>\n<p>Note that once an event has been emitted, all listeners attached to it at the\ntime of emitting will be called in order. This implies that any\n<code>removeListener()</code> or <code>removeAllListeners()</code> calls <em>after</em> emitting and\n<em>before</em> the last listener finishes execution will not remove them from\n<code>emit()</code> in progress. Subsequent events will behave as expected.</p>\n<pre><code class=\"language-js\">const myEmitter = new MyEmitter();\n\nconst callbackA = () => {\n console.log('A');\n myEmitter.removeListener('event', callbackB);\n};\n\nconst callbackB = () => {\n console.log('B');\n};\n\nmyEmitter.on('event', callbackA);\n\nmyEmitter.on('event', callbackB);\n\n// callbackA removes listener callbackB but it will still be called.\n// Internal listener array at time of emit [callbackA, callbackB]\nmyEmitter.emit('event');\n// Prints:\n// A\n// B\n\n// callbackB is now removed.\n// Internal listener array [callbackA]\nmyEmitter.emit('event');\n// Prints:\n// A\n</code></pre>\n<p>Because listeners are managed using an internal array, calling this will\nchange the position indices of any listener registered <em>after</em> the listener\nbeing removed. This will not impact the order in which listeners are called,\nbut it means that any copies of the listener array as returned by\nthe <code>emitter.listeners()</code> method will need to be recreated.</p>\n<p>When a single function has been added as a handler multiple times for a single\nevent (as in the example below), <code>removeListener()</code> will remove the most\nrecently added instance. In the example the <code>once('ping')</code>\nlistener is removed:</p>\n<pre><code class=\"language-js\">const ee = new EventEmitter();\n\nfunction pong() {\n console.log('pong');\n}\n\nee.on('ping', pong);\nee.once('ping', pong);\nee.removeListener('ping', pong);\n\nee.emit('ping');\nee.emit('ping');\n</code></pre>\n<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p>" }, { "textRaw": "emitter.setMaxListeners(n)", "type": "method", "name": "setMaxListeners", "meta": { "added": [ "v0.3.5" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {EventEmitter}", "name": "return", "type": "EventEmitter" }, "params": [ { "textRaw": "`n` {integer}", "name": "n", "type": "integer" } ] } ], "desc": "<p>By default <code>EventEmitter</code>s will print a warning if more than <code>10</code> listeners are\nadded for a particular event. This is a useful default that helps finding\nmemory leaks. Obviously, not all events should be limited to just 10 listeners.\nThe <code>emitter.setMaxListeners()</code> method allows the limit to be modified for this\nspecific <code>EventEmitter</code> instance. The value can be set to <code>Infinity</code> (or <code>0</code>)\nto indicate an unlimited number of listeners.</p>\n<p>Returns a reference to the <code>EventEmitter</code>, so that calls can be chained.</p>" }, { "textRaw": "emitter.rawListeners(eventName)", "type": "method", "name": "rawListeners", "meta": { "added": [ "v9.4.0" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {Function[]}", "name": "return", "type": "Function[]" }, "params": [ { "textRaw": "`eventName` {string|symbol}", "name": "eventName", "type": "string|symbol" } ] } ], "desc": "<p>Returns a copy of the array of listeners for the event named <code>eventName</code>,\nincluding any wrappers (such as those created by <code>.once()</code>).</p>\n<pre><code class=\"language-js\">const emitter = new EventEmitter();\nemitter.once('log', () => console.log('log once'));\n\n// Returns a new Array with a function `onceWrapper` which has a property\n// `listener` which contains the original listener bound above\nconst listeners = emitter.rawListeners('log');\nconst logFnWrapper = listeners[0];\n\n// logs \"log once\" to the console and does not unbind the `once` event\nlogFnWrapper.listener();\n\n// logs \"log once\" to the console and removes the listener\nlogFnWrapper();\n\nemitter.on('log', () => console.log('log persistently'));\n// will return a new Array with a single function bound by `.on()` above\nconst newListeners = emitter.rawListeners('log');\n\n// logs \"log persistently\" twice\nnewListeners[0]();\nemitter.emit('log');\n</code></pre>" } ], "properties": [ { "textRaw": "EventEmitter.defaultMaxListeners", "name": "defaultMaxListeners", "meta": { "added": [ "v0.11.2" ], "changes": [] }, "desc": "<p>By default, a maximum of <code>10</code> listeners can be registered for any single\nevent. This limit can be changed for individual <code>EventEmitter</code> instances\nusing the <a href=\"#events_emitter_setmaxlisteners_n\"><code>emitter.setMaxListeners(n)</code></a> method. To change the default\nfor <em>all</em> <code>EventEmitter</code> instances, the <code>EventEmitter.defaultMaxListeners</code>\nproperty can be used. If this value is not a positive number, a <code>TypeError</code>\nwill be thrown.</p>\n<p>Take caution when setting the <code>EventEmitter.defaultMaxListeners</code> because the\nchange affects <em>all</em> <code>EventEmitter</code> instances, including those created before\nthe change is made. However, calling <a href=\"#events_emitter_setmaxlisteners_n\"><code>emitter.setMaxListeners(n)</code></a> still has\nprecedence over <code>EventEmitter.defaultMaxListeners</code>.</p>\n<p>Note that this is not a hard limit. The <code>EventEmitter</code> instance will allow\nmore listeners to be added but will output a trace warning to stderr indicating\nthat a \"possible EventEmitter memory leak\" has been detected. For any single\n<code>EventEmitter</code>, the <code>emitter.getMaxListeners()</code> and <code>emitter.setMaxListeners()</code>\nmethods can be used to temporarily avoid this warning:</p>\n<pre><code class=\"language-js\">emitter.setMaxListeners(emitter.getMaxListeners() + 1);\nemitter.once('event', () => {\n // do stuff\n emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));\n});\n</code></pre>\n<p>The <a href=\"cli.html#cli_trace_warnings\"><code>--trace-warnings</code></a> command line flag can be used to display the\nstack trace for such warnings.</p>\n<p>The emitted warning can be inspected with <a href=\"process.html#process_event_warning\"><code>process.on('warning')</code></a> and will\nhave the additional <code>emitter</code>, <code>type</code> and <code>count</code> properties, referring to\nthe event emitter instance, the event’s name and the number of attached\nlisteners, respectively.\nIts <code>name</code> property is set to <code>'MaxListenersExceededWarning'</code>.</p>" } ] } ], "methods": [ { "textRaw": "events.once(emitter, name)", "type": "method", "name": "once", "meta": { "added": [ "v10.16.0" ], "changes": [] }, "signatures": [ { "return": { "textRaw": "Returns: {Promise}", "name": "return", "type": "Promise" }, "params": [ { "textRaw": "`emitter` {EventEmitter}", "name": "emitter", "type": "EventEmitter" }, { "textRaw": "`name` {string}", "name": "name", "type": "string" } ] } ], "desc": "<p>Creates a <code>Promise</code> that is resolved when the <code>EventEmitter</code> emits the given\nevent or that is rejected when the <code>EventEmitter</code> emits <code>'error'</code>.\nThe <code>Promise</code> will resolve with an array of all the arguments emitted to the\ngiven event.</p>\n<pre><code class=\"language-js\">const { once, EventEmitter } = require('events');\n\nasync function run() {\n const ee = new EventEmitter();\n\n process.nextTick(() => {\n ee.emit('myevent', 42);\n });\n\n const [value] = await once(ee, 'myevent');\n console.log(value);\n\n const err = new Error('kaboom');\n process.nextTick(() => {\n ee.emit('error', err);\n });\n\n try {\n await once(ee, 'myevent');\n } catch (err) {\n console.log('error happened', err);\n }\n}\n\nrun();\n</code></pre>" } ] } ] }