Optionalopts: AgentOptionsReadonlyfreeAn object which contains arrays of sockets currently awaiting use by
the agent when keepAlive is enabled. Do not modify.
Sockets in the freeSockets list will be automatically destroyed and
removed from the array on 'timeout'.
By default set to 256. For agents with keepAlive enabled, this
sets the maximum number of sockets that will be left open in the free
state.
By default set to Infinity. Determines how many concurrent sockets the agent
can have open per origin. Origin is the returned value of agent.getName().
By default set to Infinity. Determines how many concurrent sockets the agent
can have open. Unlike maxSockets, this parameter applies across all origins.
ReadonlyrequestsAn object which contains queues of requests that have not yet been assigned to sockets. Do not modify.
ReadonlysocketsAn object which contains arrays of sockets currently in use by the agent. Do not modify.
Optional[captureThe Symbol.for('nodejs.rejection') method is called in case a
promise rejection happens when emitting an event and
captureRejections is enabled on the emitter.
It is possible to use events.captureRejectionSymbol in
place of Symbol.for('nodejs.rejection').
import { EventEmitter, captureRejectionSymbol } from 'node:events';
class MyClass extends EventEmitter {
constructor() {
super({ captureRejections: true });
}
[captureRejectionSymbol](err, event, ...args) {
console.log('rejection happened for', event, 'with', err, ...args);
this.destroy(err);
}
destroy(err) {
// Tear the resource down here.
}
}
Produces a socket/stream to be used for HTTP requests.
By default, this function is the same as net.createConnection(). However,
custom agents may override this method in case greater flexibility is desired.
A socket/stream can be supplied in one of two ways: by returning the
socket/stream from this function, or by passing the socket/stream to callback.
This method is guaranteed to return an instance of the net.Socket class,
a subclass of stream.Duplex, unless the user specifies a socket
type other than net.Socket.
callback has a signature of (err, stream).
Options containing connection details. Check createConnection for the format of the options
Optionalcallback: (err: Error | null, stream: Duplex) => voidCallback function that receives the created socket
Destroy any sockets that are currently in use by the agent.
It is usually not necessary to do this. However, if using an
agent with keepAlive enabled, then it is best to explicitly shut down
the agent when it is no longer needed. Otherwise,
sockets might stay open for quite a long time before the server
terminates them.
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
Returns an array listing the events for which the emitter has registered listeners.
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) ]
Get a unique name for a set of request options, to determine whether a
connection can be reused. For an HTTP agent, this returnshost:port:localAddress or host:port:localAddress:family. For an HTTPS agent,
the name includes the CA, cert, ciphers, and other HTTPS/TLS-specific options
that determine socket reusability.
Optionaloptions: ClientRequestArgsA set of options providing information for name generation
Called when socket is detached from a request and could be persisted by theAgent. Default behavior is to:
socket.setKeepAlive(true, this.keepAliveMsecs);
socket.unref();
return true;
This method can be overridden by a particular Agent subclass. If this
method returns a falsy value, the socket will be destroyed instead of persisting
it for use with the next request.
The socket argument can be an instance of net.Socket, a subclass of stream.Duplex.
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.
The name of the event being listened for
Optionallistener: (...args: any[]) => voidThe event handler 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] ]
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
The name of the event.
The callback function
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
The name of the event.
The callback function
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.
The name of the event.
The callback function
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.
server.prependOnceListener('connection', (stream) => {
console.log('Ah, we have our first user!');
});
Returns a reference to the EventEmitter, so that calls can be chained.
The name of the event.
The callback 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');
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.
OptionaleventName: string | symbolRemoves 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 from
emit() 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 indexes 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.
Called when socket is attached to request after being persisted because of
the keep-alive options. Default behavior is to:
socket.ref();
This method can be overridden by a particular Agent subclass.
The socket argument can be an instance of net.Socket, a subclass of stream.Duplex.
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.
An
Agentis responsible for managing connection persistence and reuse for HTTP clients. It maintains a queue of pending requests for a given host and port, reusing a single socket connection for each until the queue is empty, at which time the socket is either destroyed or put into a pool where it is kept to be used again for requests to the same host and port. Whether it is destroyed or pooled depends on thekeepAliveoption.Pooled connections have TCP Keep-Alive enabled for them, but servers may still close idle connections, in which case they will be removed from the pool and a new connection will be made when a new HTTP request is made for that host and port. Servers may also refuse to allow multiple requests over the same connection, in which case the connection will have to be remade for every request and cannot be pooled. The
Agentwill still make the requests to that server, but each one will occur over a new connection.When a connection is closed by the client or the server, it is removed from the pool. Any unused sockets in the pool will be unrefed so as not to keep the Node.js process running when there are no outstanding requests. (see
socket.unref()).It is good practice, to
destroy()anAgentinstance when it is no longer in use, because unused sockets consume OS resources.Sockets are removed from an agent when the socket emits either a
'close'event or an'agentRemove'event. When intending to keep one HTTP request open for a long time without keeping it in the agent, something like the following may be done:An agent may also be used for an individual request. By providing
{agent: false}as an option to thehttp.get()orhttp.request()functions, a one-time useAgentwith default options will be used for the client connection.agent:false:optionsinsocket.connect()are also supported.To configure any of them, a custom Agent instance must be created.
Since
v0.3.4