Alias for emitter.on(eventName, listener).

Event emitter The defined events on documents including: 1. close 2. drain 3. error 4. finish 5. pipe 6. unpipe

The writable.cork() method forces all written data to be buffered in memory. The buffered data will be flushed when either the uncork or end methods are called.

The primary intent of writable.cork() is to accommodate a situation in which several small chunks are written to the stream in rapid succession. Instead of immediately forwarding them to the underlying destination, writable.cork()buffers all the chunks until writable.uncork() is called, which will pass them all to writable._writev(), if present. This prevents a head-of-line blocking situation where data is being buffered while waiting for the first small chunk to be processed. However, use of writable.cork() without implementingwritable._writev() may have an adverse effect on throughput.

See also: writable.uncork(), writable._writev().

Destroy the stream. Optionally emit an 'error' event, and emit a 'close'event (unless emitClose is set to false). After this call, the writable stream has ended and subsequent calls to write() or end() will result in an ERR_STREAM_DESTROYED error. This is a destructive and immediate way to destroy a stream. Previous calls towrite() may not have drained, and may trigger an ERR_STREAM_DESTROYED error. Use end() instead of destroy if data should flush before close, or wait for the 'drain' event before destroying the stream.

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 writable._destroy().

Is true after writable.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');

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.

The writable.setDefaultEncoding() method sets the default encoding for a Writable stream.

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.

The writable.uncork() method flushes all data buffered since cork was called.

When using writable.cork() and writable.uncork() to manage the buffering of writes to a stream, it is recommended that calls to writable.uncork() be deferred using process.nextTick(). Doing so allows batching of allwritable.write() calls that occur within a given Node.js event loop phase.

js stream.cork(); stream.write('some '); stream.write('data '); process.nextTick(() => stream.uncork());

If the writable.cork() method is called multiple times on a stream, the same number of calls to writable.uncork() must be called to flush the buffered data.

js stream.cork(); stream.write('some '); stream.cork(); stream.write('data '); process.nextTick(() => { stream.uncork(); // The data will not be flushed until uncork() is called a second time. stream.uncork(); });

See also: writable.cork().

Number of times writable.uncork() needs to be called in order to fully uncork the stream.

Is true after writable.end() has been called. This property does not indicate whether the data has been flushed, for this use writable.writableFinished instead.

Is set to true immediately before the 'finish' event is emitted.

Return the value of highWaterMark passed when creating this Writable.

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

Getter for the property objectMode of a given Writable stream.

The writable.write() method writes some data to the stream, and calls the supplied callback once the data has been fully handled. If an error occurs, the callback will be called with the error as its first argument. The callback is called asynchronously and before 'error' is emitted.

The return value is true if the internal buffer is less than thehighWaterMark configured when the stream was created after admitting chunk. If false is returned, further attempts to write data to the stream should stop until the 'drain' event is emitted.

While a stream is not draining, calls to write() will buffer chunk, and return false. Once all currently buffered chunks are drained (accepted for delivery by the operating system), the 'drain' event will be emitted. It is recommended that once write() returns false, no more chunks be written until the 'drain' event is emitted. While calling write() on a stream that is not draining is allowed, Node.js will buffer all written chunks until maximum memory usage occurs, at which point it will abort unconditionally. Even before it aborts, high memory usage will cause poor garbage collector performance and high RSS (which is not typically released back to the system, even after the memory is no longer required). Since TCP sockets may never drain if the remote peer does not read the data, writing a socket that is not draining may lead to a remotely exploitable vulnerability.

Writing data while the stream is not draining is particularly problematic for a Transform, because the Transform streams are paused by default until they are piped or a 'data' or 'readable' event handler is added.

If the data to be written can be generated or fetched on demand, it is recommended to encapsulate the logic into a Readable and use pipe. However, if calling write() is preferred, it is possible to respect backpressure and avoid memory issues using the 'drain' event:

js function write(data, cb) { if (!stream.write(data)) { stream.once('drain', cb); } else { process.nextTick(cb); } }

// Wait for cb to be called before doing any other write. write('hello', () => { console.log('Write completed, do more writes now.'); });

A Writable stream in object mode will always ignore the encoding argument.