Alias for emitter.on(eventName, listener).

Event emitter The defined events on documents including: 1. close 2. data 3. end 4. error 5. pause 6. readable 7. resume

Destroy the stream. Optionally emit an 'error' event, and emit a 'close'event (unless emitClose is set to false). After this call, the readable stream will release any internal resources and subsequent calls to push()will be ignored.

Once destroy() has been called any further calls will be a no-op and no further errors except from _destroy() may be emitted as 'error'.

Implementors should not override this method, but instead implement readable._destroy().

Is true after readable.destroy() has been called.

Synchronously calls each of the listeners registered for the event namedeventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

js const EventEmitter = require('events'); const myEmitter = new EventEmitter();

// First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(event with parameters ${arg1}, ${arg2} in second listener); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(event with parameters ${parameters} in third listener); });

console.log(myEmitter.listeners('event'));

myEmitter.emit('event', 1, 2, 3, 4, 5);

// Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener

Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.

js const EventEmitter = require('events'); const myEE = new EventEmitter(); myEE.on('foo', () => {}); myEE.on('bar', () => {});

const sym = Symbol('symbol'); myEE.on(sym, () => {});

console.log(myEE.eventNames()); // Prints: [ 'foo', 'bar', Symbol(symbol) ]

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to 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.

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

Alias for emitter.removeListener().

Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventNameand listener will result in the listener being added, and called, multiple times.

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

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. Theemitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

js const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a

Adds a **one-time**listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

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

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. Theemitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

js const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a

Adds the listener function to the _beginning_ of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventNameand listener will result in the listener being added, and called, multiple times.

js server.prependListener('connection', (stream) => { console.log('someone connected!'); });

Returns a reference to the EventEmitter, so that calls can be chained.

Adds a **one-time**listener function for the event named eventName to the_beginning_ of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

js server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });

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, including any wrappers (such as those created by .once()).

js const emitter = new EventEmitter(); emitter.once('log', () => console.log('log once'));

// Returns a new Array with a function onceWrapper which has a property // listener which contains the original listener bound above const listeners = emitter.rawListeners('log'); const logFnWrapper = listeners[0];

// Logs "log once" to the console and does not unbind the once event logFnWrapper.listener();

// Logs "log once" to the console and removes the listener logFnWrapper();

emitter.on('log', () => console.log('log persistently')); // Will return a new Array with a single function bound by .on() above const newListeners = emitter.rawListeners('log');

// Logs "log persistently" twice newListeners[0](); emitter.emit('log');

Returns whether the stream was destroyed or errored before emitting 'end'.

Returns whether 'data' has been emitted.

Getter for the property encoding of a given Readable stream. The encodingproperty can be set using the readable.setEncoding() method.

Becomes true when 'end' event is emitted.

This property reflects the current state of a Readable stream as described in the Three states section.

Returns the value of highWaterMark passed when creating this Readable.

This property contains the number of bytes (or objects) in the queue ready to be read. The value provides introspection data regarding the status of the highWaterMark.

Getter for the property objectMode of a given Readable stream.

Removes all listeners, or those of the specified eventName.

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 that calls can be chained.

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

js const callback = (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 eventName, then removeListener() must be called multiple times to remove each instance.

Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that anyremoveListener() or removeAllListeners() calls _after_ emitting and_before_ the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

js const myEmitter = new MyEmitter();

const callbackA = () => { console.log('A'); myEmitter.removeListener('event', callbackB); };

const callbackB = () => { console.log('B'); };

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

// callbackA removes listener callbackB but it will still be called. // Internal listener array at time of emit [callbackA, callbackB] myEmitter.emit('event'); // Prints: // A // B

// callbackB is now removed. // Internal listener array [callbackA] myEmitter.emit('event'); // Prints: // A

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 means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping')listener is removed:

js const ee = new EventEmitter();

function pong() { console.log('pong'); }

ee.on('ping', pong); ee.once('ping', pong); ee.removeListener('ping', pong);

ee.emit('ping'); ee.emit('ping');

Returns a reference to the EventEmitter, so that calls can be chained.

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. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set toInfinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.

Passing chunk as null signals the end of the stream (EOF) and behaves the same as readable.push(null), after which no more data can be written. The EOF signal is put at the end of the buffer and any buffered data will still be flushed.

The readable.unshift() method pushes a chunk of data back into the internal buffer. This is useful in certain situations where a stream is being consumed by code that needs to "un-consume" some amount of data that it has optimistically pulled out of the source, so that the data can be passed on to some other party.

The stream.unshift(chunk) method cannot be called after the 'end' event has been emitted or a runtime error will be thrown.

Developers using stream.unshift() often should consider switching to use of a Transform stream instead. See the API for stream implementers section for more information.

js // Pull off a header delimited by \n\n. // Use unshift() if we get too much. // Call the callback with (error, header, stream). const { StringDecoder } = require('string_decoder'); function parseHeader(stream, callback) { stream.on('error', callback); stream.on('readable', onReadable); const decoder = new StringDecoder('utf8'); let header = ; function onReadable() { let chunk; while (null !== (chunk = stream.read())) { const str = decoder.write(chunk); if (str.match(/\n\n/)) { // Found the header boundary. const split = str.split(/\n\n/); header += split.shift(); const remaining = split.join('\n\n'); const buf = Buffer.from(remaining, 'utf8'); stream.removeListener('error', callback); // Remove the 'readable' listener before unshifting. stream.removeListener('readable', onReadable); if (buf.length) stream.unshift(buf); // Now the body of the message can be read from the stream. callback(null, header, stream); } else { // Still reading the header. header += str; } } } }

Unlike push, stream.unshift(chunk) will not end the reading process by resetting the internal reading state of the stream. This can cause unexpected results if readable.unshift() is called during a read (i.e. from within a _read implementation on a custom stream). Following the call to readable.unshift() with an immediate push will reset the reading state appropriately, however it is best to simply avoid calling readable.unshift() while in the process of performing a read.