class

net.Server

class Server

This class is used to create a TCP or IPC server.

connections: number
readonly listening: boolean
Indicates whether or not the server is listening for connections.
maxConnections: number
Set this property to reject connections when the server's connection count gets high.
It is not recommended to use this option once a socket has been sent to a child with child_process.fork().
static captureRejections: boolean
Value: boolean
Change the default captureRejections option on all new EventEmitter objects.
readonly static captureRejectionSymbol: typeof captureRejectionSymbol
Value: Symbol.for('nodejs.rejection')
See how to write a custom rejection handler.
static defaultMaxListeners: number
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 allEventEmitter instances, the events.defaultMaxListeners property can be used. If this value is not a positive number, a RangeError is thrown.
Take caution when setting the events.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over events.defaultMaxListeners.
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:
import { EventEmitter } from 'node:events'; const emitter = new EventEmitter(); emitter.setMaxListeners(emitter.getMaxListeners() + 1); emitter.once('event', () => { // do stuff emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0)); });
The --trace-warnings command-line flag can be used to display the stack trace for such warnings.
The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type, and count properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.
readonly static errorMonitor: typeof errorMonitor
This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.
Installing a listener using this symbol does not change the behavior once an 'error' event is emitted. Therefore, the process will still crash if no regular 'error' listener is installed.
[Symbol.asyncDispose](): Promise<void>;
Calls () and returns a promise that fulfills when the server has closed.
[events.captureRejectionSymbol]<K>(
error: Error,
event: string | symbol,
...args: AnyRest
): void;
addListener(
event: string,
listener: (...args: any[]) => void
): this;
events.EventEmitter
close
connection
error
listening
drop
addListener(
event: 'close',
listener: () => void
): this;
events.EventEmitter
close
connection
error
listening
drop
addListener(
event: 'connection',
listener: (socket: Socket) => void
): this;
events.EventEmitter
close
connection
error
listening
drop
addListener(
event: 'error',
listener: (err: Error) => void
): this;
events.EventEmitter
close
connection
error
listening
drop
addListener(
event: 'listening',
listener: () => void
): this;
events.EventEmitter
close
connection
error
listening
drop
addListener(
event: 'drop',
listener: (data?: DropArgument) => void
): this;
events.EventEmitter
close
connection
error
listening
drop
address(): null | string | AddressInfo;
Returns the bound address, the address family name, and port of the server as reported by the operating system if listening on an IP socket (useful to find which port was assigned when getting an OS-assigned address):{ port: 12346, family: 'IPv4', address: '127.0.0.1' }.
For a server listening on a pipe or Unix domain socket, the name is returned as a string.
const server = net.createServer((socket) => { socket.end('goodbye\n'); }).on('error', (err) => { // Handle errors here. throw err; }); // Grab an arbitrary unused port. server.listen(() => { console.log('opened server on', server.address()); });
server.address() returns null before the 'listening' event has been emitted or after calling server.close().
close(
callback?: (err?: Error) => void
): this;
Stops the server from accepting new connections and keeps existing connections. This function is asynchronous, the server is finally closed when all connections are ended and the server emits a 'close' event. The optional callback will be called once the 'close' event occurs. Unlike that event, it will be called with an Error as its only argument if the server was not open when it was closed.
@param callback
Called when the server is closed.
emit(
event: string | symbol,
...args: any[]
): boolean;
Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.
Returns true if the event had listeners, false otherwise.
import { EventEmitter } from 'node: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
emit(
event: 'close'
): boolean;
emit(
event: 'connection',
socket: Socket
): boolean;
emit(
event: 'error',
err: Error
): boolean;
emit(
event: 'listening'
): boolean;
emit(
event: 'drop',
data?: DropArgument
): boolean;
eventNames(): string | symbol[];
Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.
import { EventEmitter } from 'node: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) ]
getConnections(
cb: (error: null | Error, count: number) => void
): this;
Asynchronously get the number of concurrent connections on the server. Works when sockets were sent to forks.
Callback should take two arguments err and count.
getMaxListeners(): number;
Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to EventEmitter.defaultMaxListeners.
listen(
port?: number,
hostname?: string,
backlog?: number,
listeningListener?: () => void
): this;
Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback]) for IPC servers
server.listen([port[, host[, backlog]]][, callback]) for TCP servers
This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.
All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).
All Socket are set to SO_REUSEADDR (see socket(7) for details).
The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.
One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:
server.on('error', (e) => { if (e.code === 'EADDRINUSE') { console.error('Address in use, retrying...'); setTimeout(() => { server.close(); server.listen(PORT, HOST); }, 1000); } });
listen(
port?: number,
hostname?: string,
listeningListener?: () => void
): this;
Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback]) for IPC servers
server.listen([port[, host[, backlog]]][, callback]) for TCP servers
This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.
All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).
All Socket are set to SO_REUSEADDR (see socket(7) for details).
The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.
One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:
server.on('error', (e) => { if (e.code === 'EADDRINUSE') { console.error('Address in use, retrying...'); setTimeout(() => { server.close(); server.listen(PORT, HOST); }, 1000); } });
listen(
port?: number,
backlog?: number,
listeningListener?: () => void
): this;
Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback]) for IPC servers
server.listen([port[, host[, backlog]]][, callback]) for TCP servers
This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.
All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).
All Socket are set to SO_REUSEADDR (see socket(7) for details).
The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.
One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:
server.on('error', (e) => { if (e.code === 'EADDRINUSE') { console.error('Address in use, retrying...'); setTimeout(() => { server.close(); server.listen(PORT, HOST); }, 1000); } });
listen(
port?: number,
listeningListener?: () => void
): this;
Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback]) for IPC servers
server.listen([port[, host[, backlog]]][, callback]) for TCP servers
This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.
All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).
All Socket are set to SO_REUSEADDR (see socket(7) for details).
The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.
One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:
server.on('error', (e) => { if (e.code === 'EADDRINUSE') { console.error('Address in use, retrying...'); setTimeout(() => { server.close(); server.listen(PORT, HOST); }, 1000); } });
listen(
path: string,
backlog?: number,
listeningListener?: () => void
): this;
Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback]) for IPC servers
server.listen([port[, host[, backlog]]][, callback]) for TCP servers
This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.
All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).
All Socket are set to SO_REUSEADDR (see socket(7) for details).
The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.
One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:
server.on('error', (e) => { if (e.code === 'EADDRINUSE') { console.error('Address in use, retrying...'); setTimeout(() => { server.close(); server.listen(PORT, HOST); }, 1000); } });
listen(
path: string,
listeningListener?: () => void
): this;
Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback]) for IPC servers
server.listen([port[, host[, backlog]]][, callback]) for TCP servers
This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.
All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).
All Socket are set to SO_REUSEADDR (see socket(7) for details).
The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.
One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:
server.on('error', (e) => { if (e.code === 'EADDRINUSE') { console.error('Address in use, retrying...'); setTimeout(() => { server.close(); server.listen(PORT, HOST); }, 1000); } });
listen(
options: ListenOptions,
listeningListener?: () => void
): this;
Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback]) for IPC servers
server.listen([port[, host[, backlog]]][, callback]) for TCP servers
This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.
All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).
All Socket are set to SO_REUSEADDR (see socket(7) for details).
The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.
One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:
server.on('error', (e) => { if (e.code === 'EADDRINUSE') { console.error('Address in use, retrying...'); setTimeout(() => { server.close(); server.listen(PORT, HOST); }, 1000); } });
listen(
handle: any,
backlog?: number,
listeningListener?: () => void
): this;
Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback]) for IPC servers
server.listen([port[, host[, backlog]]][, callback]) for TCP servers
This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.
All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).
All Socket are set to SO_REUSEADDR (see socket(7) for details).
The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.
One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:
server.on('error', (e) => { if (e.code === 'EADDRINUSE') { console.error('Address in use, retrying...'); setTimeout(() => { server.close(); server.listen(PORT, HOST); }, 1000); } });
listen(
handle: any,
listeningListener?: () => void
): this;
Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback]) for IPC servers
server.listen([port[, host[, backlog]]][, callback]) for TCP servers
This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.
All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).
All Socket are set to SO_REUSEADDR (see socket(7) for details).
The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.
One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:
server.on('error', (e) => { if (e.code === 'EADDRINUSE') { console.error('Address in use, retrying...'); setTimeout(() => { server.close(); server.listen(PORT, HOST); }, 1000); } });
listenerCount<K>(
eventName: string | symbol,
listener?: Function
): number;
Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.
@param eventName
The name of the event being listened for
@param listener
The event handler function
listeners<K>(
eventName: string | symbol
): Function[];
Returns a copy of the array of listeners for the event named eventName.
server.on('connection', (stream) => { console.log('someone connected!'); }); console.log(util.inspect(server.listeners('connection'))); // Prints: [ [Function] ]
off<K>(
eventName: string | symbol,
listener: (...args: any[]) => void
): this;
Alias for emitter.removeListener().
on(
event: string,
listener: (...args: any[]) => void
): this;
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 eventName 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 that calls can be chained.
By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
@param listener
The callback function
on(
event: 'close',
listener: () => void
): this;
on(
event: 'connection',
listener: (socket: Socket) => void
): this;
on(
event: 'error',
listener: (err: Error) => void
): this;
on(
event: 'listening',
listener: () => void
): this;
on(
event: 'drop',
listener: (data?: DropArgument) => void
): this;
once(
event: string,
listener: (...args: any[]) => void
): this;
Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
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. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
@param listener
The callback function
once(
event: 'close',
listener: () => void
): this;
once(
event: 'connection',
listener: (socket: Socket) => void
): this;
once(
event: 'error',
listener: (err: Error) => void
): this;
once(
event: 'listening',
listener: () => void
): this;
once(
event: 'drop',
listener: (data?: DropArgument) => void
): this;
prependListener(
event: string,
listener: (...args: any[]) => void
): this;
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 eventName and listener will result in the listener being added, and called, multiple times.
server.prependListener('connection', (stream) => { console.log('someone connected!'); });
Returns a reference to the EventEmitter, so that calls can be chained.
@param listener
The callback function
prependListener(
event: 'close',
listener: () => void
): this;
prependListener(
event: 'connection',
listener: (socket: Socket) => void
): this;
prependListener(
event: 'error',
listener: (err: Error) => void
): this;
prependListener(
event: 'listening',
listener: () => void
): this;
prependListener(
event: 'drop',
listener: (data?: DropArgument) => void
): this;
prependOnceListener(
event: string,
listener: (...args: any[]) => void
): this;
Adds a one-timelistener 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.
server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });
Returns a reference to the EventEmitter, so that calls can be chained.
@param listener
The callback function
prependOnceListener(
event: 'close',
listener: () => void
): this;
prependOnceListener(
event: 'connection',
listener: (socket: Socket) => void
): this;
prependOnceListener(
event: 'error',
listener: (err: Error) => void
): this;
prependOnceListener(
event: 'listening',
listener: () => void
): this;
prependOnceListener(
event: 'drop',
listener: (data?: DropArgument) => void
): this;
rawListeners<K>(
eventName: string | symbol
): Function[];
Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).
import { EventEmitter } from 'node:events'; 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');
ref(): this;
Opposite of unref(), calling ref() on a previously unrefed server will not let the program exit if it's the only server left (the default behavior). If the server is refed calling ref() again will have no effect.
removeAllListeners(
eventName?: string | symbol
): this;
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.
removeListener<K>(
eventName: string | symbol,
listener: (...args: any[]) => void
): this;
Removes the specified listener from the listener array for the event named eventName.
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 any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.
import { EventEmitter } from 'node:events'; class MyEmitter extends EventEmitter {} 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:
import { EventEmitter } from 'node:events'; 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.
setMaxListeners(
n: number
): this;
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 to Infinity (or 0) to indicate an unlimited number of listeners.
Returns a reference to the EventEmitter, so that calls can be chained.
unref(): this;
Calling unref() on a server will allow the program to exit if this is the only active server in the event system. If the server is already unrefed callingunref() again will have no effect.
static addAbortListener(
signal: AbortSignal,
resource: (event: Event) => void
): Disposable;
Listens once to the abort event on the provided signal.
Listening to the abort event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can call e.stopImmediatePropagation(). Unfortunately Node.js cannot change this since it would violate the web standard. Additionally, the original API makes it easy to forget to remove listeners.
This API allows safely using AbortSignals in Node.js APIs by solving these two issues by listening to the event such that stopImmediatePropagation does not prevent the listener from running.
Returns a disposable so that it may be unsubscribed from more easily.
import { addAbortListener } from 'node:events'; function example(signal) { let disposable; try { signal.addEventListener('abort', (e) => e.stopImmediatePropagation()); disposable = addAbortListener(signal, (e) => { // Do something when signal is aborted. }); } finally { disposable?.[Symbol.dispose](); } }
@returns
Disposable that removes the abort listener.
static getEventListeners(
emitter: EventEmitter<DefaultEventMap> | EventTarget,
name: string | symbol
): Function[];
Returns a copy of the array of listeners for the event named eventName.
For EventEmitters this behaves exactly the same as calling .listeners on the emitter.
For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.
import { getEventListeners, EventEmitter } from 'node:events'; { const ee = new EventEmitter(); const listener = () => console.log('Events are fun'); ee.on('foo', listener); console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ] } { const et = new EventTarget(); const listener = () => console.log('Events are fun'); et.addEventListener('foo', listener); console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ] }
static getMaxListeners(
emitter: EventEmitter<DefaultEventMap> | EventTarget
): number;
Returns the currently set max amount of listeners.
For EventEmitters this behaves exactly the same as calling .getMaxListeners on the emitter.
For EventTargets this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.
import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events'; { const ee = new EventEmitter(); console.log(getMaxListeners(ee)); // 10 setMaxListeners(11, ee); console.log(getMaxListeners(ee)); // 11 } { const et = new EventTarget(); console.log(getMaxListeners(et)); // 10 setMaxListeners(11, et); console.log(getMaxListeners(et)); // 11 }
static on(
emitter: EventEmitter,
eventName: string | symbol,
options?: StaticEventEmitterIteratorOptions
): AsyncIterator<any[]>;
import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); }); for await (const event of on(ee, 'foo')) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here
Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.
An AbortSignal can be used to cancel waiting on events:
import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ac = new AbortController(); (async () => { const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); }); for await (const event of on(ee, 'foo', { signal: ac.signal })) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here })(); process.nextTick(() => ac.abort());
Use the close option to specify an array of event names that will end the iteration:
import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); ee.emit('close'); }); for await (const event of on(ee, 'foo', { close: ['close'] })) { console.log(event); // prints ['bar'] [42] } // the loop will exit after 'close' is emitted console.log('done'); // prints 'done'
@returns
An AsyncIterator that iterates eventName events emitted by the emitter
static on(
emitter: EventTarget,
eventName: string,
options?: StaticEventEmitterIteratorOptions
): AsyncIterator<any[]>;
import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); }); for await (const event of on(ee, 'foo')) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here
Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.
An AbortSignal can be used to cancel waiting on events:
import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ac = new AbortController(); (async () => { const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); }); for await (const event of on(ee, 'foo', { signal: ac.signal })) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here })(); process.nextTick(() => ac.abort());
Use the close option to specify an array of event names that will end the iteration:
import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); ee.emit('close'); }); for await (const event of on(ee, 'foo', { close: ['close'] })) { console.log(event); // prints ['bar'] [42] } // the loop will exit after 'close' is emitted console.log('done'); // prints 'done'
@returns
An AsyncIterator that iterates eventName events emitted by the emitter
static once(
emitter: EventEmitter,
eventName: string | symbol,
options?: StaticEventEmitterOptions
): Promise<any[]>;
Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.
This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.
import { once, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); process.nextTick(() => { ee.emit('myevent', 42); }); const [value] = await once(ee, 'myevent'); console.log(value); const err = new Error('kaboom'); process.nextTick(() => { ee.emit('error', err); }); try { await once(ee, 'myevent'); } catch (err) { console.error('error happened', err); }
The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:
import { EventEmitter, once } from 'node:events'; const ee = new EventEmitter(); once(ee, 'error') .then(([err]) => console.log('ok', err.message)) .catch((err) => console.error('error', err.message)); ee.emit('error', new Error('boom')); // Prints: ok boom
An AbortSignal can be used to cancel waiting for the event:
import { EventEmitter, once } from 'node:events'; const ee = new EventEmitter(); const ac = new AbortController(); async function foo(emitter, event, signal) { try { await once(emitter, event, { signal }); console.log('event emitted!'); } catch (error) { if (error.name === 'AbortError') { console.error('Waiting for the event was canceled!'); } else { console.error('There was an error', error.message); } } } foo(ee, 'foo', ac.signal); ac.abort(); // Abort waiting for the event ee.emit('foo'); // Prints: Waiting for the event was canceled!
static once(
emitter: EventTarget,
eventName: string,
options?: StaticEventEmitterOptions
): Promise<any[]>;
Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.
This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.
import { once, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); process.nextTick(() => { ee.emit('myevent', 42); }); const [value] = await once(ee, 'myevent'); console.log(value); const err = new Error('kaboom'); process.nextTick(() => { ee.emit('error', err); }); try { await once(ee, 'myevent'); } catch (err) { console.error('error happened', err); }
The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:
import { EventEmitter, once } from 'node:events'; const ee = new EventEmitter(); once(ee, 'error') .then(([err]) => console.log('ok', err.message)) .catch((err) => console.error('error', err.message)); ee.emit('error', new Error('boom')); // Prints: ok boom
An AbortSignal can be used to cancel waiting for the event:
import { EventEmitter, once } from 'node:events'; const ee = new EventEmitter(); const ac = new AbortController(); async function foo(emitter, event, signal) { try { await once(emitter, event, { signal }); console.log('event emitted!'); } catch (error) { if (error.name === 'AbortError') { console.error('Waiting for the event was canceled!'); } else { console.error('There was an error', error.message); } } } foo(ee, 'foo', ac.signal); ac.abort(); // Abort waiting for the event ee.emit('foo'); // Prints: Waiting for the event was canceled!
static setMaxListeners(
n?: number,
...eventTargets: EventEmitter<DefaultEventMap> | EventTarget[]
): void;
import { setMaxListeners, EventEmitter } from 'node:events'; const target = new EventTarget(); const emitter = new EventEmitter(); setMaxListeners(5, target, emitter);
@param n
A non-negative number. The maximum number of listeners per EventTarget event.
@param eventTargets
Zero or more {EventTarget} or {EventEmitter} instances. If none are specified, n is set as the default max for all newly created {EventTarget} and {EventEmitter} objects.