diff --git a/doc/api/events.markdown b/doc/api/events.markdown index 02617b10e8dc5c..0fdfc078749046 100644 --- a/doc/api/events.markdown +++ b/doc/api/events.markdown @@ -4,88 +4,257 @@ -Many objects in Node.js emit events: a [`net.Server`][] emits an event each -time a peer connects to it, a [`fs.ReadStream`][] emits an event when the file -is opened. All objects which emit events are instances of `events.EventEmitter`. -You can access this module by doing: `require("events");` +Much of the Node.js core API is built around an idiomatic asynchronous +event-driven architecture in which certain kinds of objects (called "emitters") +periodically emit named events that cause Function objects ("listeners") to be +called. -Typically, event names are represented by a camel-cased string, however, -there aren't any strict restrictions on that, as any valid property key will be -accepted. +For instance: a [`net.Server`][] object emits an event each time a peer +connects to it; a [`fs.ReadStream`][] emits an event when the file is opened; +a [stream][] emits an event whenever data is available to be read. -Functions can then be attached to objects, to be executed when an event -is emitted. These functions are called _listeners_. Inside a listener -function, `this` refers to the `EventEmitter` that the listener was -attached to. +All objects that emit events are instances of the `EventEmitter` class. These +objects expose an `eventEmitter.on()` function that allows one or more +Functions to be attached to named events emitted by the object. Typically, +event names are camel-cased strings but any valid JavaScript property key +can be used. +When the `EventEmitter` object emits an event, all of the Functions attached +to that specific event are called _synchronously_. Any values returned by the +called listeners are _ignored_ and will be discarded. -## Class: events.EventEmitter +The following example shows a simple `EventEmitter` instance with a single +listener. The `eventEmitter.on()` method is used to register listeners, while +the `eventEmitter.emit()` method is used to trigger the event. -Use `require('events')` to access the EventEmitter class. + const EventEmitter = require('events'); + const util = require('util'); + + function MyEmitter() { + EventEmitter.call(this); + } + util.inherits(MyEmitter, EventEmitter); -```javascript -const EventEmitter = require('events'); -``` + const myEmitter = new MyEmitter(); + myEmitter.on('event', function() { + console.log('an event occurred!'); + }); + myEmitter.emit('event'); -When an `EventEmitter` instance experiences an error, the typical action is -to emit an `'error'` event. Error events are treated as a special case in -Node.js. If there is no listener for it, then the default action is to print -a stack trace and exit the program. +Any object can become an `EventEmitter` through inheritance. The example above +uses the traditional Node.js style prototypical inheritance using +the `util.inherits()` method. It is, however, possible to use ES6 classes as +well: -All EventEmitters emit the event `'newListener'` when new listeners are -added and `'removeListener'` when a listener is removed. + const EventEmitter = require('events'); -### Inheriting from 'EventEmitter' + class MyEmitter extends EventEmitter {} -Inheriting from `EventEmitter` is no different from inheriting from any other -constructor function. For example: + const myEmitter = new MyEmitter(); + myEmitter.on('event', function() { + console.log('an event occurred!'); + }); + myEmitter.emit('event'); + +## Passing arguments and `this` to listeners + +The `eventEmitter.emit()` method allows an arbitrary set of arguments to be +passed to the listener functions. It is important to keep in mind that when an +ordinary listener function is called by the `EventEmitter`, the standard `this` +keyword is intentionally set to reference the `EventEmitter` to which the +listener is attached. + + const myEmitter = new MyEmitter(); + myEmitter.on('event', function(a, b) { + console.log(a, b, this); + // Prints: + // a b MyEmitter { + // domain: null, + // _events: { event: [Function] }, + // _eventsCount: 1, + // _maxListeners: undefined } + }); + myEmitter.emit('event', 'a', 'b'); - 'use strict'; - const util = require('util'); - const EventEmitter = require('events'); +It is possible to use ES6 Arrow Functions as listeners, however, when doing so, +the `this` keyword will no longer reference the `EventEmitter` instance: - function MyEventEmitter() { - // Initialize necessary properties from `EventEmitter` in this instance - EventEmitter.call(this); - } + const myEmitter = new MyEmitter(); + myEmitter.on('event', (a, b) => { + console.log(a, b, this); + // Prints: a b {} + }); + myEmitter.emit('event', 'a', 'b'); + +## Asynchronous vs. Synchronous + +The `EventListener` calls all listeners synchronously in the order in which +they were registered. This is important to ensure the proper sequencing of +events and to avoid race conditions or logic errors. When appropriate, +listener functions can switch to an asynchronous mode of operation using +the `setImmediate()` or `process.nextTick()` methods: + + const myEmitter = new MyEmitter(); + myEmitter.on('event', (a, b) => { + setImmediate(() => { + console.log('this happens asynchronously'); + }); + }); + myEmitter.emit('event', 'a', 'b'); + +## Handling events only once + +When a listener is registered using the `eventEmitter.on()` method, that +listener will be invoked _every time_ the named event is emitted. + + const myEmitter = new MyEmitter(); + var m = 0; + myEmitter.on('event', () => { + console.log(++m); + }); + myEmitter.emit('event'); + // Prints: 1 + myEmitter.emit('event'); + // Prints: 2 + +Using the `eventEmitter.once()` method, it is possible to register a listener +that is immediately unregistered after it is called. + + const myEmitter = new MyEmitter(); + var m = 0; + myEmitter.once('event', () => { + console.log(++m); + }); + myEmitter.emit('event'); + // Prints: 1 + myEmitter.emit('event'); + // Ignored + +## Error events + +When an error occurs within an `EventEmitter` instance, the typical action is +for an `'error'` event to be emitted. These are treated as a special case +within Node.js. + +If an `EventEmitter` does _not_ have at least one listener registered for the +`'error'` event, and an `'error'` event is emitted, the error is thrown, a +stack trace is printed, and the Node.js process exits. + + const myEmitter = new MyEmitter(); + myEmitter.emit('error', new Error('whoops!')); + // Throws and crashes Node.js + +To guard against crashing the Node.js process, developers can either register +a listener for the `process.on('uncaughtException')` event or use the +[`domain`][] module (_Note, however, that the `domain` module has been +deprecated_). + + const myEmitter = new MyEmitter(); + + process.on('uncaughtException', (err) => { + console.log('whoops! there was an error'); + }); + + myEmitter.emit('error', new Error('whoops!')); + // Prints: whoops! there was an error + +As a best practice, developers should always register listeners for the +`'error'` event: + + const myEmitter = new MyEmitter(); + myEmitter.on('error', (err) => { + console.log('whoops! there was an error'); + }); + myEmitter.emit('error', new Error('whoops!')); + // Prints: whoops! there was an error - // Inherit functions from `EventEmitter`'s prototype - util.inherits(MyEventEmitter, EventEmitter); +## Class: EventEmitter -### Class Method: EventEmitter.listenerCount(emitter, event) +The `EventEmitter` class is defined and exposed by the `events` module: - Stability: 0 - Deprecated: Use [emitter.listenerCount][] instead. + const EventEmitter = require('events'); -Returns the number of listeners for a given event. +All EventEmitters emit the event `'newListener'` when new listeners are +added and `'removeListener'` when a listener is removed. ### Event: 'newListener' * `event` {String|Symbol} The event name * `listener` {Function} The event handler function -This event is emitted *before* a listener is added. When this event is -triggered, the listener has not been added to the array of listeners for the -`event`. Any listeners added to the event `name` in the newListener event -callback will be added *before* the listener that is in the process of being -added. +The `EventEmitter` instance will emit it's own `'newListener'` event *before* +a listener is added to it's internal array of listeners. + +Listeners registered for the `'newListener'` event will be passed the event +name and a reference to the listener being added. + +The fact that the event is triggered before adding the listener has a subtle +but important side effect: any *additional* listeners registered to the same +`name` *within* the `'newListener'` callback will be inserted *before* the +listener that is in the process of being added. + + const myEmitter = new MyEmitter(); + // Only do this once so we don't loop forever + myEmitter.once('newListener', (event, listener) => { + if (event === 'event') { + // Insert a new listener in front + myEmitter.on('event', () => { + console.log('B'); + }); + } + }); + myEmitter.on('event', () => { + console.log('A'); + }); + myEmitter.emit('event'); + // Prints: + // B + // A ### Event: 'removeListener' * `event` {String|Symbol} The event name * `listener` {Function} The event handler function -This event is emitted *after* a listener is removed. When this event is -triggered, the listener has been removed from the array of listeners for the -`event`. +The `'removeListener'` event is emitted *after* a listener is removed. + +### EventEmitter.listenerCount(emitter, event) + + Stability: 0 - Deprecated: Use [`emitter.listenerCount()`][] instead. + +A class method that returns the number of listeners for the given `event` +registered on the given `emitter`. + + const myEmitter = new MyEmitter(); + myEmitter.on('event', () => {}); + myEmitter.on('event', () => {}); + console.log(EventEmitter.listenerCount(myEmitter, 'event')); + // Prints: 2 ### EventEmitter.defaultMaxListeners -[`emitter.setMaxListeners(n)`][] sets the maximum on a per-instance basis. -This class property lets you set it for *all* `EventEmitter` instances, -current and future, effective immediately. Use with care. +By default, a maximum of `10` listeners can be registered for any single +event. This limit can be changed for individual `EventEmitter` instances +using the [`emitter.setMaxListeners(n)`][] method. To change the default +for *all* `EventEmitter` instances, the `EventEmitter.defaultMaxListeners` +property can be used. + +Take caution when setting the `EventEmitter.defaultMaxListeners` because the +change effects *all* `EventEmitter` instances, including those created before +the change is made. However, calling [`emitter.setMaxListeners(n)`][] still has +precedence over `EventEmitter.defaultMaxListeners`. -Note that [`emitter.setMaxListeners(n)`][] still has precedence over -`EventEmitter.defaultMaxListeners`. +Note that this is not a hard limit. The `EventEmitter` instance will allow +more listeners to be added but will output a trace warning to stderr indicating +that a `possible EventEmitter memory leak` has been detected. For any single +`EventEmitter`, the `emitter.getMaxListeners()` and `emitter.setMaxListeners()` +methods can be used to temporarily avoid this warning: + + emitter.setMaxListeners(emitter.getMaxListeners() + 1); + emitter.once('event', () => { + // do stuff + emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0)); + }); ### emitter.addListener(event, listener) @@ -93,77 +262,72 @@ Alias for `emitter.on(event, listener)`. ### emitter.emit(event[, arg1][, arg2][, ...]) -Calls each of the listeners in order with the supplied arguments. +Synchronously calls each of the listeners registered for `event`, in the order +they were registered, passing the supplied arguments to each. Returns `true` if event had listeners, `false` otherwise. ### emitter.getMaxListeners() -Returns the current max listener value for the emitter which is either set by -[`emitter.setMaxListeners(n)`][] or defaults to +Returns the current max listener value for the `EventEmitter` which is either +set by [`emitter.setMaxListeners(n)`][] or defaults to [`EventEmitter.defaultMaxListeners`][]. -This can be useful to increment/decrement max listeners to avoid the warning -while not being irresponsible and setting a too big number. - - emitter.setMaxListeners(emitter.getMaxListeners() + 1); - emitter.once('event', () => { - // do stuff - emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0)); - }); - -### emitter.listenerCount(type) +### emitter.listenerCount(event) -* `type` {Value} The type of event +* `event` {Value} The type of event -Returns the number of listeners listening to the `type` of event. +Returns the number of listeners listening to the `event` type. ### emitter.listeners(event) -Returns a copy of the array of listeners for the specified event. +Returns a copy of the array of listeners for the specified `event`. server.on('connection', (stream) => { console.log('someone connected!'); }); - console.log(util.inspect(server.listeners('connection'))); // [ [Function] ] + console.log(util.inspect(server.listeners('connection'))); + // Prints: [ [Function] ] ### emitter.on(event, listener) -Adds a listener to the end of the listeners array for the specified `event`. -No checks are made to see if the `listener` has already been added. Multiple -calls passing the same combination of `event` and `listener` will result in the -`listener` being added multiple times. +Adds the `listener` function to the end of the listeners array for the +specified `event`. No checks are made to see if the `listener` has already +been added. Multiple calls passing the same combination of `event` and +`listener` will result in the `listener` being added, and called, multiple +times. server.on('connection', (stream) => { console.log('someone connected!'); }); -Returns emitter, so calls can be chained. +Returns a reference to the `EventEmitter` so calls can be chained. ### emitter.once(event, listener) -Adds a **one time** listener for the event. This listener is -invoked only the next time the event is fired, after which -it is removed. +Adds a **one time** `listener` function for the `event`. This listener is +invoked only the next time `event` is triggered, after which it is removed. server.once('connection', (stream) => { console.log('Ah, we have our first user!'); }); -Returns emitter, so calls can be chained. +Returns a reference to the `EventEmitter` so calls can be chained. ### emitter.removeAllListeners([event]) -Removes all listeners, or those of the specified event. It's not a good idea to -remove listeners that were added elsewhere in the code, especially when it's on -an emitter that you didn't create (e.g. sockets or file streams). +Removes all listeners, or those of the specified `event`. + +Note that it is bad practice to remove listeners added elsewhere in the code, +particularly when the `EventEmitter` instance was created by some other +component or module (e.g. sockets or file streams). -Returns emitter, so calls can be chained. +Returns a reference to the `EventEmitter` so calls can be chained. ### emitter.removeListener(event, listener) -Removes a listener from the listener array for the specified event. -**Caution**: changes array indices in the listener array behind the listener. +Removes the specified `listener` from the listener array for the specified +`event`. var callback = function(stream) { console.log('someone connected!'); @@ -177,19 +341,29 @@ listener array. If any single listener has been added multiple times to the listener array for the specified `event`, then `removeListener` must be called multiple times to remove each instance. -Returns emitter, so calls can be chained. +Because listeners are managed using an internal array, calling this will +change the position indices of any listener registered *after* the listener +being removed. This will not impact the order in which listeners are called, +but it will means that any copies of the listener array as returned by +the `emitter.listeners()` method will need to be recreated. + +Returns a reference to the `EventEmitter` so calls can be chained. ### emitter.setMaxListeners(n) -By default EventEmitters will print a warning if more than 10 listeners are -added for a particular event. This is a useful default which helps finding -memory leaks. Obviously not all Emitters should be limited to 10. This function -allows that to be increased. Set to `Infinity` (or `0`) for unlimited. +By default EventEmitters will print a warning if more than `10` listeners are +added for a particular event. This is a useful default that helps finding +memory leaks. Obviously, not all events should be limited to just 10 listeners. +The `emitter.setMaxListeners()` method allows the limit to be modified for this +specific `EventEmitter` instance. The value can be set to `Infinity` (or `0`) +for to indicate an unlimited number of listeners. -Returns emitter, so calls can be chained. +Returns a reference to the `EventEmitter` so calls can be chained. [`net.Server`]: net.html#net_class_net_server [`fs.ReadStream`]: fs.html#fs_class_fs_readstream [`emitter.setMaxListeners(n)`]: #events_emitter_setmaxlisteners_n [`EventEmitter.defaultMaxListeners`]: #events_eventemitter_defaultmaxlisteners -[emitter.listenerCount]: #events_emitter_listenercount_type +[`emitter.listenerCount()`]: #events_emitter_listenercount_event +[`domain`]: domain.html +[stream]: stream.html