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 Function objects (\"listeners\") to be called.
For instance: a net.Server object emits an event each time a peer\nconnects to it; a fs.ReadStream emits an event when the file is opened;\na stream emits an event whenever data is available to be read.
All objects that emit events are instances of the EventEmitter class. These\nobjects expose an eventEmitter.on() 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.
When the EventEmitter object emits an event, all of the functions attached\nto that specific event are called synchronously. Any values returned by the\ncalled listeners are ignored and will be discarded.
The following example shows a simple EventEmitter instance with a single\nlistener. The eventEmitter.on() method is used to register listeners, while\nthe eventEmitter.emit() method is used to trigger the event.
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');\nThe eventEmitter.emit() 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 this keyword\nis intentionally set to reference the EventEmitter instance to which the\nlistener is attached.
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');\nIt is possible to use ES6 Arrow Functions as listeners, however, when doing so,\nthe this keyword will no longer reference the EventEmitter instance:
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');\nThe EventEmitter 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 setImmediate() or process.nextTick() methods:
const myEmitter = new MyEmitter();\nmyEmitter.on('event', (a, b) => {\n setImmediate(() => {\n console.log('this happens asynchronously');\n });\n});\nmyEmitter.emit('event', 'a', 'b');\nWhen a listener is registered using the eventEmitter.on() method, that\nlistener will be invoked every time the named event is emitted.
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\nUsing the eventEmitter.once() 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 then called.
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\nWhen an error occurs within an EventEmitter instance, the typical action is\nfor an 'error' event to be emitted. These are treated as special cases\nwithin Node.js.
If an EventEmitter does not have at least one listener registered for the\n'error' event, and an 'error' event is emitted, the error is thrown, a\nstack trace is printed, and the Node.js process exits.
const myEmitter = new MyEmitter();\nmyEmitter.emit('error', new Error('whoops!'));\n// Throws and crashes Node.js\nTo guard against crashing the Node.js process the domain module can be\nused. (Note, however, that the domain module is deprecated.)
As a best practice, listeners should always be added for the 'error' events.
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\nThe EventEmitter class is defined and exposed by the events module:
const EventEmitter = require('events');\nAll EventEmitters emit the event 'newListener' when new listeners are\nadded and 'removeListener' when existing listeners are removed.
The EventEmitter instance will emit its own 'newListener' event before\na listener is added to its internal array of listeners.
Listeners registered for the 'newListener' event will be passed the event\nname and a reference to the listener being added.
The fact that the event is triggered before adding the listener has a subtle\nbut important side effect: any additional listeners registered to the same\nname within the 'newListener' callback will be inserted before the\nlistener that is in the process of being added.
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\nThe 'removeListener' event is emitted after the listener is removed.
A class method that returns the number of listeners for the given eventName\nregistered on the given emitter.
const myEmitter = new MyEmitter();\nmyEmitter.on('event', () => {});\nmyEmitter.on('event', () => {});\nconsole.log(EventEmitter.listenerCount(myEmitter, 'event'));\n// Prints: 2\nAlias for emitter.on(eventName, listener).
...args <any>Synchronously calls each of the listeners registered for the event named\neventName, in the order they were registered, passing the supplied arguments\nto each.
Returns true if the event had listeners, false otherwise.
Returns an array listing the events for which the emitter has registered\nlisteners. The values in the array will be strings or Symbols.
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) ]\nReturns the current max listener value for the EventEmitter which is either\nset by emitter.setMaxListeners(n) or defaults to\nEventEmitter.defaultMaxListeners.
Returns the number of listeners listening to the event named eventName.
Returns a copy of the array of listeners for the event named eventName.
server.on('connection', (stream) => {\n console.log('someone connected!');\n});\nconsole.log(util.inspect(server.listeners('connection')));\n// Prints: [ [Function] ]\nAlias for emitter.removeListener().
Adds the listener function to the end of the listeners array for the\nevent named eventName. No checks are made to see if the listener has\nalready been added. Multiple calls passing the same combination of eventName\nand listener will result in the listener being added, and called, multiple\ntimes.
server.on('connection', (stream) => {\n console.log('someone connected!');\n});\nReturns a reference to the EventEmitter, so that calls can be chained.
By default, event listeners are invoked in the order they are added. The\nemitter.prependListener() method can be used as an alternative to add the\nevent listener to the beginning of the listeners array.
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\nAdds a one-time listener function for the event named eventName. The\nnext time eventName is triggered, this listener is removed and then invoked.
server.once('connection', (stream) => {\n console.log('Ah, we have our first user!');\n});\nReturns a reference to the EventEmitter, so that calls can be chained.
By default, event listeners are invoked in the order they are added. The\nemitter.prependOnceListener() method can be used as an alternative to add the\nevent listener to the beginning of the listeners array.
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\nAdds the listener function to the beginning of the listeners array for the\nevent named eventName. No checks are made to see if the listener has\nalready been added. Multiple calls passing the same combination of eventName\nand listener will result in the listener being added, and called, multiple\ntimes.
server.prependListener('connection', (stream) => {\n console.log('someone connected!');\n});\nReturns a reference to the EventEmitter, so that calls can be chained.
Adds a one-time listener function for the event named eventName to the\nbeginning of the listeners array. The next time eventName is triggered, this\nlistener is removed, and then invoked.
server.prependOnceListener('connection', (stream) => {\n console.log('Ah, we have our first user!');\n});\nReturns a reference to the EventEmitter, so that calls can be chained.
Removes all listeners, or those of the specified eventName.
Note that it is bad practice to remove listeners added elsewhere in the code,\nparticularly when the EventEmitter instance was created by some other\ncomponent or module (e.g. sockets or file streams).
Returns a reference to the EventEmitter, so that calls can be chained.
Removes the specified listener from the listener array for the event named\neventName.
const callback = (stream) => {\n console.log('someone connected!');\n};\nserver.on('connection', callback);\n// ...\nserver.removeListener('connection', callback);\nremoveListener() 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 eventName, then removeListener() must be\ncalled multiple times to remove each instance.
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\nremoveListener() or removeAllListeners() calls after emitting and\nbefore the last listener finishes execution will not remove them from\nemit() in progress. Subsequent events will behave as expected.
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\nBecause listeners are managed using an internal array, calling this will\nchange the position indices of any listener registered after 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 emitter.listeners() method will need to be recreated.
When a single function has been added as a handler multiple times for a single\nevent (as in the example below), removeListener() will remove the most\nrecently added instance. In the example the once('ping')\nlistener is removed:
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');\nReturns a reference to the EventEmitter, so that calls can be chained.
By default EventEmitters will print a warning if more than 10 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 emitter.setMaxListeners() method allows the limit to be modified for this\nspecific EventEmitter instance. The value can be set to Infinity (or 0)\nto indicate an unlimited number of listeners.
Returns a reference to the EventEmitter, so that calls can be chained.
Returns a copy of the array of listeners for the event named eventName,\nincluding any wrappers (such as those created by .once()).
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');\nBy default, a maximum of 10 listeners can be registered for any single\nevent. This limit can be changed for individual EventEmitter instances\nusing the emitter.setMaxListeners(n) method. To change the default\nfor all EventEmitter instances, the EventEmitter.defaultMaxListeners\nproperty can be used. If this value is not a positive number, a TypeError\nwill be thrown.
Take caution when setting the EventEmitter.defaultMaxListeners because the\nchange affects all EventEmitter instances, including those created before\nthe change is made. However, calling emitter.setMaxListeners(n) still has\nprecedence over EventEmitter.defaultMaxListeners.
Note that this is not a hard limit. The EventEmitter 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\nEventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners()\nmethods can be used to temporarily avoid this warning:
emitter.setMaxListeners(emitter.getMaxListeners() + 1);\nemitter.once('event', () => {\n // do stuff\n emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));\n});\nThe --trace-warnings command line flag can be used to display the\nstack trace for such warnings.
The emitted warning can be inspected with process.on('warning') and will\nhave the additional emitter, type and count properties, referring to\nthe event emitter instance, the event’s name and the number of attached\nlisteners, respectively.\nIts name property is set to 'MaxListenersExceededWarning'.
Creates a Promise that is resolved when the EventEmitter emits the given\nevent or that is rejected when the EventEmitter emits 'error'.\nThe Promise will resolve with an array of all the arguments emitted to the\ngiven event.
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