Node.js (1)

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# Events#

    Stability: 2 - Stable

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.

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.

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.

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.

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

    function MyEmitter() {
      EventEmitter.call(this);
    }
    util.inherits(MyEmitter, EventEmitter);

    const myEmitter = new MyEmitter();
    myEmitter.on('event', function() {
      console.log('an event occurred!');
    });
    myEmitter.emit('event');

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:

    const EventEmitter = require('events');

    class MyEmitter extends EventEmitter {}

    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');

It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the `this` keyword will no longer reference the `EventEmitter` instance:

    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

## Class: EventEmitter#

The `EventEmitter` class is defined and exposed by the `events` module:

    const EventEmitter = require('events');

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

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

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#

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 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)#

Alias for `emitter.on(event, listener)`.

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

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 `EventEmitter` which is either set by `emitter.setMaxListeners(n)` or defaults to `EventEmitter.defaultMaxListeners`.

### emitter.listenerCount(event)#

- `event` Value 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`.

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

### emitter.on(event, listener)#

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 a reference to the `EventEmitter` so calls can be chained.

### emitter.once(event, listener)#

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 a reference to the `EventEmitter` so calls can be chained.

### emitter.removeAllListeners([event])#

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 a reference to the `EventEmitter` so calls can be chained.

### emitter.removeListener(event, listener)#

Removes the specified `listener` from the listener array for the specified `event`.

    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.

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 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 a reference to the `EventEmitter` so calls can be chained.