nodejs/doc/api/events.markdown

Events

Stability: 2 - Stable

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");

Typically, event names are represented by a camel-cased string, however, there aren't any strict restrictions on that, as any string will be accepted.

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.

Class: events.EventEmitter

Use require('events') to access the EventEmitter class.

var EventEmitter = require('events');

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.

All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when a listener is removed.

emitter.addListener(event, listener)

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.

server.on('connection', function (stream) {
  console.log('someone connected!');
});

Returns emitter, 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.

server.once('connection', function (stream) {
  console.log('Ah, we have our first user!');
});

Returns emitter, 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.

var callback = function(stream) {
  console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);

removeListener will remove, at most, one instance of a listener from the 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.

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).

Returns emitter, 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.

Returns emitter, so calls can be chained.

emitter.getMaxListeners()

Returns the current max listener value for the emitter 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', function () {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

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.

Note that emitter.setMaxListeners(n) still has precedence over EventEmitter.defaultMaxListeners.

emitter.listeners(event)

Returns a copy of the array of listeners for the specified event.

server.on('connection', function (stream) {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection'))); // [ [Function] ]

emitter.emit(event[, arg1][, arg2][, ...])

Calls each of the listeners in order with the supplied arguments.

Returns true if event had listeners, false otherwise.

emitter.listenerCount(type)

• type {Value} The type of event

Returns the number of listeners listening to the type of event.

Class Method: EventEmitter.listenerCount(emitter, event)

Stability: 0 - Deprecated: Use [emitter.listenerCount][] instead.

Returns the number of listeners for a given event.

Event: 'newListener'

• event {String} 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.

Event: 'removeListener'

• event {String} 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.

Inheriting from 'EventEmitter'

Inheriting from EventEmitter is no different from inheriting from any other constructor function. For example:

'use strict';
const util = require('util');
const EventEmitter = require('events');

function MyEventEmitter() {
  // Initialize necessary properties from `EventEmitter` in this instance
  EventEmitter.call(this);
}

// Inherit functions from `EventEmitter`'s prototype
util.inherits(MyEventEmitter, EventEmitter);