Optional options: EventEmitterOptionsStatic Readonly captureStatic captureSets or gets the default captureRejection value for all emitters.
Static defaultStatic Readonly errorThis 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.
events.EventEmitter
Rest ...args: any[]Tells the kernel to join a multicast group at the given multicastAddress andmulticastInterface using the IP_ADD_MEMBERSHIP socket option. If themulticastInterface argument is not
specified, the operating system will choose
one interface and will add membership to it. To add membership to every
available interface, call addMembership multiple times, once per interface.
When called on an unbound socket, this method will implicitly bind to a random port, listening on all interfaces.
When sharing a UDP socket across multiple cluster workers, thesocket.addMembership() function must be called only once or anEADDRINUSE error will occur:
import cluster from 'cluster';
import dgram from 'dgram';
if (cluster.isPrimary) {
cluster.fork(); // Works ok.
cluster.fork(); // Fails with EADDRINUSE.
} else {
const s = dgram.createSocket('udp4');
s.bind(1234, () => {
s.addMembership('224.0.0.114');
});
}
v0.6.9
Optional multicastInterface: stringTells the kernel to join a source-specific multicast channel at the givensourceAddress and groupAddress, using the multicastInterface with theIP_ADD_SOURCE_MEMBERSHIP socket
option. If the multicastInterface argument
is not specified, the operating system will choose one interface and will add
membership to it. To add membership to every available interface, callsocket.addSourceSpecificMembership() multiple times, once per interface.
When called on an unbound socket, this method will implicitly bind to a random port, listening on all interfaces.
v13.1.0, v12.16.0
Optional multicastInterface: stringReturns an object containing the address information for a socket.
For UDP sockets, this object will contain address, family and portproperties.
This method throws EBADF if called on an unbound socket.
v0.1.99
For UDP sockets, causes the dgram.Socket to listen for datagram
messages on a named port and optional address. If port is not
specified or is 0, the operating system will attempt to bind to a
random port. If address is not specified, the operating system will
attempt to listen on all addresses. Once binding is complete, a'listening' event is emitted and the optional callback function is
called.
Specifying both a 'listening' event listener and passing acallback to the socket.bind() method is not harmful but not very
useful.
A bound datagram socket keeps the Node.js process running to receive datagram messages.
If binding fails, an 'error' event is generated. In rare case (e.g.
attempting to bind with a closed socket), an Error may be thrown.
Example of a UDP server listening on port 41234:
import dgram from 'dgram';
const server = dgram.createSocket('udp4');
server.on('error', (err) => {
console.log(`server error:\n${err.stack}`);
server.close();
});
server.on('message', (msg, rinfo) => {
console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});
server.on('listening', () => {
const address = server.address();
console.log(`server listening ${address.address}:${address.port}`);
});
server.bind(41234);
// Prints: server listening 0.0.0.0:41234
v0.1.99
Optional port: numberOptional address: stringOptional callback: (() => void)with no parameters. Called when binding is complete.
Optional port: numberOptional callback: (() => void)Optional callback: (() => void)Optional callback: (() => void)Close the underlying socket and stop listening for data on it. If a callback is
provided, it is added as a listener for the 'close' event.
v0.1.99
Optional callback: (() => void)Called when the socket has been closed.
Associates the dgram.Socket to a remote address and port. Every
message sent by this handle is automatically sent to that destination. Also,
the socket will only receive messages from that remote peer.
Trying to call connect() on an already connected socket will result
in an ERR_SOCKET_DGRAM_IS_CONNECTED exception. If address is not
provided, '127.0.0.1' (for udp4 sockets) or '::1' (for udp6 sockets)
will be used by default. Once the connection is complete, a 'connect' event
is emitted and the optional callback function is called. In case of failure,
the callback is called or, failing this, an 'error' event is emitted.
v12.0.0
Optional address: stringOptional callback: (() => void)Called when the connection is completed or on error.
Instructs the kernel to leave a multicast group at multicastAddress using theIP_DROP_MEMBERSHIP socket option. This method is automatically called by the
kernel when the socket is closed or the process terminates, so most apps will
never have reason to call this.
If multicastInterface is not specified, the operating system will attempt to
drop membership on all valid interfaces.
v0.6.9
Optional multicastInterface: stringInstructs the kernel to leave a source-specific multicast channel at the givensourceAddress and groupAddress using the IP_DROP_SOURCE_MEMBERSHIPsocket option. This method is
automatically called by the kernel when the
socket is closed or the process terminates, so most apps will never have
reason to call this.
If multicastInterface is not specified, the operating system will attempt to
drop membership on all valid interfaces.
v13.1.0, v12.16.0
Optional multicastInterface: stringSynchronously 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.
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
v0.1.26
Rest ...args: any[]Returns an array listing the events for which the emitter has registered
listeners. The values in the array are strings or Symbols.
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) ]
v6.0.0
Returns the current max listener value for the EventEmitter which is either
set by emitter.setMaxListeners(n) or defaults to defaultMaxListeners.
v1.0.0
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] ]
v0.1.26
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.
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.
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
v0.1.101
The name of the event.
The callback function
Rest ...args: any[]Adds a one-timelistener 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. Theemitter.prependOnceListener() method can be used as an alternative to add the
event listener to the beginning of the listeners array.
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
v0.3.0
The name of the event.
The callback function
Rest ...args: any[]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.
server.prependListener('connection', (stream) => {
console.log('someone connected!');
});
Returns a reference to the EventEmitter, so that calls can be chained.
v6.0.0
The name of the event.
The callback function
Rest ...args: any[]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.
v6.0.0
The name of the event.
The callback function
Rest ...args: any[]Returns a copy of the array of listeners for the event named eventName,
including any wrappers (such as those created by .once()).
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');
v9.4.0
By default, binding a socket will cause it to block the Node.js process from
exiting as long as the socket is open. The socket.unref() method can be used
to exclude the socket from the reference counting that keeps the Node.js
process active. The socket.ref() method adds the socket back to the reference
counting and restores the default behavior.
Calling socket.ref() multiples times will have no additional effect.
The socket.ref() method returns a reference to the socket so calls can be
chained.
v0.9.1
Returns an object containing the address, family, and port of the remote
endpoint. This method throws an ERR_SOCKET_DGRAM_NOT_CONNECTED exception
if the socket is not connected.
v12.0.0
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.
v0.1.26
Optional event: string | symbolRemoves the specified listener from the listener array for the event namedeventName.
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.
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:
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.
v0.1.26
Rest ...args: any[]Broadcasts a datagram on the socket.
For connectionless sockets, the destination port and address must be
specified. Connected sockets, on the other hand, will use their associated
remote endpoint, so the port and address arguments must not be set.
The msg argument contains the message to be sent.
Depending on its type, different behavior can apply. If msg is a Buffer,
any TypedArray or a DataView,
the offset and length specify the offset within the Buffer where the
message begins and the number of bytes in the message, respectively.
If msg is a String, then it is automatically converted to a Bufferwith 'utf8' encoding. With messages that
contain multi-byte characters, offset and length will be calculated with
respect to byte length and not the character position.
If msg is an array, offset and length must not be specified.
The address argument is a string. If the value of address is a host name,
DNS will be used to resolve the address of the host. If address is not
provided or otherwise nullish, '127.0.0.1' (for udp4 sockets) or '::1'(for udp6 sockets) will be used by default.
If the socket has not been previously bound with a call to bind, the socket
is assigned a random port number and is bound to the "all interfaces" address
('0.0.0.0' for udp4 sockets, '::0' for udp6 sockets.)
An optional callback function may be specified to as a way of reporting
DNS errors or for determining when it is safe to reuse the buf object.
DNS lookups delay the time to send for at least one tick of the
Node.js event loop.
The only way to know for sure that the datagram has been sent is by using acallback. If an error occurs and a callback is given, the error will be
passed as the first argument to the callback. If a callback is not given,
the error is emitted as an 'error' event on the socket object.
Offset and length are optional but both must be set if either are used.
They are supported only when the first argument is a Buffer, a TypedArray,
or a DataView.
This method throws ERR_SOCKET_BAD_PORT if called on an unbound socket.
Example of sending a UDP packet to a port on localhost;
import dgram from 'dgram';
import { Buffer } from 'buffer';
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.send(message, 41234, 'localhost', (err) => {
client.close();
});
Example of sending a UDP packet composed of multiple buffers to a port on127.0.0.1;
import dgram from 'dgram';
import { Buffer } from 'buffer';
const buf1 = Buffer.from('Some ');
const buf2 = Buffer.from('bytes');
const client = dgram.createSocket('udp4');
client.send([buf1, buf2], 41234, (err) => {
client.close();
});
Sending multiple buffers might be faster or slower depending on the application and operating system. Run benchmarks to determine the optimal strategy on a case-by-case basis. Generally speaking, however, sending multiple buffers is faster.
Example of sending a UDP packet using a socket connected to a port onlocalhost:
import dgram from 'dgram';
import { Buffer } from 'buffer';
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.connect(41234, 'localhost', (err) => {
client.send(message, (err) => {
client.close();
});
});
v0.1.99
Message to be sent.
Optional port: numberDestination port.
Optional address: stringDestination host name or IP address.
Optional callback: ((error: null | Error, bytes: number) => void)Called when the message has been sent.
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.
v0.3.5
All references to scope in this section are referring to IPv6 Zone Indices, which are defined by RFC
4007. In string form, an IP
with a scope index is written as 'IP%scope' where scope is an interface name
or interface number.
Sets the default outgoing multicast interface of the socket to a chosen
interface or back to system interface selection. The multicastInterface must
be a valid string representation of an IP from the socket's family.
For IPv4 sockets, this should be the IP configured for the desired physical interface. All packets sent to multicast on the socket will be sent on the interface determined by the most recent successful use of this call.
For IPv6 sockets, multicastInterface should include a scope to indicate the
interface as in the examples that follow. In IPv6, individual send calls can
also use explicit scope in addresses, so only packets sent to a multicast
address without specifying an explicit scope are affected by the most recent
successful use of this call.
This method throws EBADF if called on an unbound socket.
On most systems, where scope format uses the interface name:
const socket = dgram.createSocket('udp6');
socket.bind(1234, () => {
socket.setMulticastInterface('::%eth1');
});
On Windows, where scope format uses an interface number:
const socket = dgram.createSocket('udp6');
socket.bind(1234, () => {
socket.setMulticastInterface('::%2');
});
All systems use an IP of the host on the desired physical interface:
const socket = dgram.createSocket('udp4');
socket.bind(1234, () => {
socket.setMulticastInterface('10.0.0.2');
});
v8.6.0
Sets the IP_MULTICAST_TTL socket option. While TTL generally stands for
"Time to Live", in this context it specifies the number of IP hops that a
packet is allowed to travel through, specifically for multicast traffic. Each
router or gateway that forwards a packet decrements the TTL. If the TTL is
decremented to 0 by a router, it will not be forwarded.
The ttl argument may be between 0 and 255. The default on most systems is 1.
This method throws EBADF if called on an unbound socket.
v0.3.8
Sets the IP_TTL socket option. While TTL generally stands for "Time to Live",
in this context it specifies the number of IP hops that a packet is allowed to
travel through. Each router or gateway that forwards a packet decrements the
TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.
Changing TTL values is typically done for network probes or when multicasting.
The ttl argument may be between 1 and 255. The default on most systems
is 64.
This method throws EBADF if called on an unbound socket.
v0.1.101
By default, binding a socket will cause it to block the Node.js process from
exiting as long as the socket is open. The socket.unref() method can be used
to exclude the socket from the reference counting that keeps the Node.js
process active, allowing the process to exit even if the socket is still
listening.
Calling socket.unref() multiple times will have no addition effect.
The socket.unref() method returns a reference to the socket so calls can be
chained.
v0.9.1
Static getReturns 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.
const { getEventListeners, EventEmitter } = require('events');
{
const ee = new EventEmitter();
const listener = () => console.log('Events are fun');
ee.on('foo', listener);
getEventListeners(ee, 'foo'); // [listener]
}
{
const et = new EventTarget();
const listener = () => console.log('Events are fun');
et.addEventListener('foo', listener);
getEventListeners(et, 'foo'); // [listener]
}
v15.2.0, v14.17.0
Static listenerA class method that returns the number of listeners for the given eventNameregistered on the given emitter.
const { EventEmitter, listenerCount } = require('events');
const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2
v0.9.12
Since v3.2.0 - Use listenerCount instead.
The emitter to query
The event name
Static onconst { on, EventEmitter } = require('events');
(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')) {
// 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:
const { on, EventEmitter } = require('events');
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());
v13.6.0, v12.16.0
that iterates eventName events emitted by the emitter
The name of the event being listened for
Optional options: StaticEventEmitterOptionsStatic onceCreates 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.
const { once, EventEmitter } = require('events');
async function run() {
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.log('error happened', err);
}
}
run();
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:
const { EventEmitter, once } = require('events');
const ee = new EventEmitter();
once(ee, 'error')
.then(([err]) => console.log('ok', err.message))
.catch((err) => console.log('error', err.message));
ee.emit('error', new Error('boom'));
// Prints: ok boom
An AbortSignal can be used to cancel waiting for the event:
const { EventEmitter, once } = require('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!
v11.13.0, v10.16.0
Optional options: StaticEventEmitterOptionsOptional options: StaticEventEmitterOptionsStatic setconst {
setMaxListeners,
EventEmitter
} = require('events');
const target = new EventTarget();
const emitter = new EventEmitter();
setMaxListeners(5, target, emitter);
v15.4.0
Optional n: numberA non-negative number. The maximum number of listeners per EventTarget event.
Rest ...eventTargets: (EventEmitter | _DOMEventTarget)[]Generated using TypeDoc
Encapsulates the datagram functionality.
New instances of
dgram.Socketare created using createSocket. Thenewkeyword is not to be used to createdgram.Socketinstances.Since
v0.1.99