This object is created internally by an HTTP server, not by the user. It is passed as the second parameter to the 'request'
event.
class
http.ServerResponse
class ServerResponse<Request extends IncomingMessage = IncomingMessage>
- statusCode: number
When using implicit headers (not calling
response.writeHead()
explicitly), this property controls the status code that will be sent to the client when the headers get flushed.response.statusCode = 404;
After response header was sent to the client, this property indicates the status code which was sent out.
- statusMessage: string
When using implicit headers (not calling
response.writeHead()
explicitly), this property controls the status message that will be sent to the client when the headers get flushed. If this is left asundefined
then the standard message for the status code will be used.response.statusMessage = 'Not found';
After response header was sent to the client, this property indicates the status message which was sent out.
- strictContentLength: boolean
If set to
true
, Node.js will check whether theContent-Length
header value and the size of the body, in bytes, are equal. Mismatching theContent-Length
header value will result in anError
being thrown, identified bycode:``'ERR_HTTP_CONTENT_LENGTH_MISMATCH'
. - readonly writable: boolean
Is
true
if it is safe to callwritable.write()
, which means the stream has not been destroyed, errored, or ended. - readonly writableCorked: number
Number of times
writable.uncork()
needs to be called in order to fully uncork the stream. - readonly writableEnded: boolean
Is
true
afterwritable.end()
has been called. This property does not indicate whether the data has been flushed, for this usewritable.writableFinished
instead. - readonly writableHighWaterMark: number
Return the value of
highWaterMark
passed when creating thisWritable
. - readonly writableLength: number
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
. - readonly writableNeedDrain: boolean
Is
true
if the stream's buffer has been full and stream will emit'drain'
. - static captureRejections: boolean
Value: boolean
Change the default
captureRejections
option on all newEventEmitter
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 individualEventEmitter
instances using theemitter.setMaxListeners(n)
method. To change the default for allEventEmitter
instances, theevents.defaultMaxListeners
property can be used. If this value is not a positive number, aRangeError
is thrown.Take caution when setting the
events.defaultMaxListeners
because the change affects allEventEmitter
instances, including those created before the change is made. However, callingemitter.setMaxListeners(n)
still has precedence overevents.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 singleEventEmitter
, theemitter.getMaxListeners()
andemitter.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 additionalemitter
,type
, andcount
properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Itsname
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. - event: 'close',listener: () => void): this;
Event emitter The defined events on documents including:
- close
- drain
- error
- finish
- pipe
- unpipe
event: 'drain',listener: () => void): this;Event emitter The defined events on documents including:
- close
- drain
- error
- finish
- pipe
- unpipe
event: 'error',): this;Event emitter The defined events on documents including:
- close
- drain
- error
- finish
- pipe
- unpipe
event: 'finish',listener: () => void): this;Event emitter The defined events on documents including:
- close
- drain
- error
- finish
- pipe
- unpipe
event: 'pipe',): this;Event emitter The defined events on documents including:
- close
- drain
- error
- finish
- pipe
- unpipe
event: 'unpipe',): this;Event emitter The defined events on documents including:
- close
- drain
- error
- finish
- pipe
- unpipe
event: string | symbol,listener: (...args: any[]) => void): this;Event emitter The defined events on documents including:
- close
- drain
- error
- finish
- pipe
- unpipe
- ): void;
Adds HTTP trailers (headers but at the end of the message) to the message.
Trailers will only be emitted if the message is chunked encoded. If not, the trailers will be silently discarded.
HTTP requires the
Trailer
header to be sent to emit trailers, with a list of header field names in its value, e.g.message.writeHead(200, { 'Content-Type': 'text/plain', 'Trailer': 'Content-MD5' }); message.write(fileData); message.addTrailers({ 'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667' }); message.end();
Attempting to set a header field name or value that contains invalid characters will result in a
TypeError
being thrown. - name: string,value: string | readonly string[]): this;
Append a single header value to the header object.
If the value is an array, this is equivalent to calling this method multiple times.
If there were no previous values for the header, this is equivalent to calling
outgoingMessage.setHeader(name, value)
.Depending of the value of
options.uniqueHeaders
when the client request or the server were created, this will end up in the header being sent multiple times or a single time with values joined using;
.@param nameHeader name
@param valueHeader value
- stream: ComposeFnParam | T | Iterable<T, any, any> | AsyncIterable<T, any, any>,): T;
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 untilwritable.uncork()
is called, which will pass them all towritable._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 ofwritable.cork()
without implementingwritable._writev()
may have an adverse effect on throughput.See also:
writable.uncork()
,writable._writev()
.- ): this;
Destroy the stream. Optionally emit an
'error'
event, and emit a'close'
event (unlessemitClose
is set tofalse
). After this call, the writable stream has ended and subsequent calls towrite()
orend()
will result in anERR_STREAM_DESTROYED
error. This is a destructive and immediate way to destroy a stream. Previous calls towrite()
may not have drained, and may trigger anERR_STREAM_DESTROYED
error. Useend()
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()
.@param errorOptional, an error to emit with
'error'
event. - emit(event: 'close'): 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
- end(cb?: () => void): this;
Calling the
writable.end()
method signals that no more data will be written to theWritable
. The optionalchunk
andencoding
arguments allow one final additional chunk of data to be written immediately before closing the stream.Calling the write method after calling end will raise an error.
// Write 'hello, ' and then end with 'world!'. import fs from 'node:fs'; const file = fs.createWriteStream('example.txt'); file.write('hello, '); file.end('world!'); // Writing more now is not allowed!
end(chunk: any,cb?: () => void): this;Calling the
writable.end()
method signals that no more data will be written to theWritable
. The optionalchunk
andencoding
arguments allow one final additional chunk of data to be written immediately before closing the stream.Calling the write method after calling end will raise an error.
// Write 'hello, ' and then end with 'world!'. import fs from 'node:fs'; const file = fs.createWriteStream('example.txt'); file.write('hello, '); file.end('world!'); // Writing more now is not allowed!
@param chunkOptional data to write. For streams not operating in object mode,
chunk
must be a {string}, {Buffer}, {TypedArray} or {DataView}. For object mode streams,chunk
may be any JavaScript value other thannull
.end(chunk: any,encoding: BufferEncoding,cb?: () => void): this;Calling the
writable.end()
method signals that no more data will be written to theWritable
. The optionalchunk
andencoding
arguments allow one final additional chunk of data to be written immediately before closing the stream.Calling the write method after calling end will raise an error.
// Write 'hello, ' and then end with 'world!'. import fs from 'node:fs'; const file = fs.createWriteStream('example.txt'); file.write('hello, '); file.end('world!'); // Writing more now is not allowed!
@param chunkOptional data to write. For streams not operating in object mode,
chunk
must be a {string}, {Buffer}, {TypedArray} or {DataView}. For object mode streams,chunk
may be any JavaScript value other thannull
.@param encodingThe encoding if
chunk
is a string Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or
Symbol
s.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) ]
Flushes the message headers.
For efficiency reason, Node.js normally buffers the message headers until
outgoingMessage.end()
is called or the first chunk of message data is written. It then tries to pack the headers and data into a single TCP packet.It is usually desired (it saves a TCP round-trip), but not when the first data is not sent until possibly much later.
outgoingMessage.flushHeaders()
bypasses the optimization and kickstarts the message.- name: string): undefined | string | number | string[];
Gets the value of the HTTP header with the given name. If that header is not set, the returned value will be
undefined
.@param nameName of header
Returns an array containing the unique names of the current outgoing headers. All names are lowercase.
Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related HTTP module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.
The object returned by the
outgoingMessage.getHeaders()
method does not prototypically inherit from the JavaScriptObject
. This means that typicalObject
methods such asobj.toString()
,obj.hasOwnProperty()
, and others are not defined and will not work.outgoingMessage.setHeader('Foo', 'bar'); outgoingMessage.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']); const headers = outgoingMessage.getHeaders(); // headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }
Returns the current max listener value for the
EventEmitter
which is either set byemitter.setMaxListeners(n)
or defaults to EventEmitter.defaultMaxListeners.- name: string): boolean;
Returns
true
if the header identified byname
is currently set in the outgoing headers. The header name is case-insensitive.const hasContentType = outgoingMessage.hasHeader('content-type');
- eventName: string | symbol,listener?: Function): number;
Returns the number of listeners listening for the event named
eventName
. Iflistener
is provided, it will return how many times the listener is found in the list of the listeners of the event.@param eventNameThe name of the event being listened for
@param listenerThe event handler function
- 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] ]
- eventName: string | symbol,listener: (...args: any[]) => void): this;
Alias for
emitter.removeListener()
. - on(event: 'close',listener: () => void): this;
Adds the
listener
function to the end of the listeners array for the event namedeventName
. No checks are made to see if thelistener
has already been added. Multiple calls passing the same combination ofeventName
andlistener
will result in thelistener
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 listenerThe callback function
- once(event: 'close',listener: () => void): this;
Adds a one-time
listener
function for the event namedeventName
. The next timeeventName
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 listenerThe callback function
- event: 'close',listener: () => void): this;
Adds the
listener
function to the beginning of the listeners array for the event namedeventName
. No checks are made to see if thelistener
has already been added. Multiple calls passing the same combination ofeventName
andlistener
will result in thelistener
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 listenerThe callback function
- event: 'close',listener: () => void): this;
Adds a one-time
listener
function for the event namedeventName
to the beginning of the listeners array. The next timeeventName
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 listenerThe callback function
- 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');
- 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. - name: string): void;
Removes a header that is queued for implicit sending.
outgoingMessage.removeHeader('Content-Encoding');
@param nameHeader name
- event: 'close',listener: () => void): this;
Removes 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 specifiedeventName
, thenremoveListener()
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()
orremoveAllListeners()
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 theonce('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. - encoding: BufferEncoding): this;
The
writable.setDefaultEncoding()
method sets the defaultencoding
for aWritable
stream.@param encodingThe new default encoding
- name: string,value: string | number | readonly string[]): this;
Sets a single header value. If the header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings to send multiple headers with the same name.
@param nameHeader name
@param valueHeader value
- ): this;
Sets multiple header values for implicit headers. headers must be an instance of
Headers
orMap
, if a header already exists in the to-be-sent headers, its value will be replaced.const headers = new Headers({ foo: 'bar' }); outgoingMessage.setHeaders(headers);
or
const headers = new Map([['foo', 'bar']]); outgoingMessage.setHeaders(headers);
When headers have been set with
outgoingMessage.setHeaders()
, they will be merged with any headers passed toresponse.writeHead()
, with the headers passed toresponse.writeHead()
given precedence.// Returns content-type = text/plain const server = http.createServer((req, res) => { const headers = new Headers({ 'Content-Type': 'text/html' }); res.setHeaders(headers); res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('ok'); });
- n: number): this;
By default
EventEmitter
s will print a warning if more than10
listeners are added for a particular event. This is a useful default that helps finding memory leaks. Theemitter.setMaxListeners()
method allows the limit to be modified for this specificEventEmitter
instance. The value can be set toInfinity
(or0
) to indicate an unlimited number of listeners.Returns a reference to the
EventEmitter
, so that calls can be chained. - msecs: number,callback?: () => void): this;
Once a socket is associated with the message and is connected,
socket.setTimeout()
will be called withmsecs
as the first parameter.@param callbackOptional function to be called when a timeout occurs. Same as binding to the
timeout
event. The
writable.uncork()
method flushes all data buffered since cork was called.When using
writable.cork()
andwritable.uncork()
to manage the buffering of writes to a stream, defer calls towritable.uncork()
usingprocess.nextTick()
. Doing so allows batching of allwritable.write()
calls that occur within a given Node.js event loop phase.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 towritable.uncork()
must be called to flush the buffered data.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()
.- chunk: any,): boolean;
The
writable.write()
method writes some data to the stream, and calls the suppliedcallback
once the data has been fully handled. If an error occurs, thecallback
will be called with the error as its first argument. Thecallback
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 admittingchunk
. Iffalse
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 bufferchunk
, and return false. Once all currently buffered chunks are drained (accepted for delivery by the operating system), the'drain'
event will be emitted. Oncewrite()
returns false, do not write more chunks until the'drain'
event is emitted. While callingwrite()
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 theTransform
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 callingwrite()
is preferred, it is possible to respect backpressure and avoid memory issues using the'drain'
event: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 theencoding
argument.@param chunkOptional data to write. For streams not operating in object mode,
chunk
must be a {string}, {Buffer}, {TypedArray} or {DataView}. For object mode streams,chunk
may be any JavaScript value other thannull
.@param callbackCallback for when this chunk of data is flushed.
@returnsfalse
if the stream wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.chunk: any,encoding: BufferEncoding,): boolean;The
writable.write()
method writes some data to the stream, and calls the suppliedcallback
once the data has been fully handled. If an error occurs, thecallback
will be called with the error as its first argument. Thecallback
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 admittingchunk
. Iffalse
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 bufferchunk
, and return false. Once all currently buffered chunks are drained (accepted for delivery by the operating system), the'drain'
event will be emitted. Oncewrite()
returns false, do not write more chunks until the'drain'
event is emitted. While callingwrite()
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 theTransform
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 callingwrite()
is preferred, it is possible to respect backpressure and avoid memory issues using the'drain'
event: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 theencoding
argument.@param chunkOptional data to write. For streams not operating in object mode,
chunk
must be a {string}, {Buffer}, {TypedArray} or {DataView}. For object mode streams,chunk
may be any JavaScript value other thannull
.@param encodingThe encoding, if
chunk
is a string.@param callbackCallback for when this chunk of data is flushed.
@returnsfalse
if the stream wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
. - callback?: () => void): void;
Sends an HTTP/1.1 100 Continue message to the client, indicating that the request body should be sent. See the
'checkContinue'
event onServer
. - hints: Record<string, string | string[]>,callback?: () => void): void;
Sends an HTTP/1.1 103 Early Hints message to the client with a Link header, indicating that the user agent can preload/preconnect the linked resources. The
hints
is an object containing the values of headers to be sent with early hints message. The optionalcallback
argument will be called when the response message has been written.Example
const earlyHintsLink = '</styles.css>; rel=preload; as=style'; response.writeEarlyHints({ 'link': earlyHintsLink, }); const earlyHintsLinks = [ '</styles.css>; rel=preload; as=style', '</scripts.js>; rel=preload; as=script', ]; response.writeEarlyHints({ 'link': earlyHintsLinks, 'x-trace-id': 'id for diagnostics', }); const earlyHintsCallback = () => console.log('early hints message sent'); response.writeEarlyHints({ 'link': earlyHintsLinks, }, earlyHintsCallback);
@param hintsAn object containing the values of headers
@param callbackWill be called when the response message has been written
- statusCode: number,statusMessage?: string,): this;
Sends a response header to the request. The status code is a 3-digit HTTP status code, like
404
. The last argument,headers
, are the response headers. Optionally one can give a human-readablestatusMessage
as the second argument.headers
may be anArray
where the keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values. The array is in the same format asrequest.rawHeaders
.Returns a reference to the
ServerResponse
, so that calls can be chained.const body = 'hello world'; response .writeHead(200, { 'Content-Length': Buffer.byteLength(body), 'Content-Type': 'text/plain', }) .end(body);
This method must only be called once on a message and it must be called before
response.end()
is called.If
response.write()
orresponse.end()
are called before calling this, the implicit/mutable headers will be calculated and call this function.When headers have been set with
response.setHeader()
, they will be merged with any headers passed toresponse.writeHead()
, with the headers passed toresponse.writeHead()
given precedence.If this method is called and
response.setHeader()
has not been called, it will directly write the supplied header values onto the network channel without caching internally, and theresponse.getHeader()
on the header will not yield the expected result. If progressive population of headers is desired with potential future retrieval and modification, useresponse.setHeader()
instead.// Returns content-type = text/plain const server = http.createServer((req, res) => { res.setHeader('Content-Type', 'text/html'); res.setHeader('X-Foo', 'bar'); res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('ok'); });
Content-Length
is read in bytes, not characters. UseBuffer.byteLength()
to determine the length of the body in bytes. Node.js will check whetherContent-Length
and the length of the body which has been transmitted are equal or not.Attempting to set a header field name or value that contains invalid characters will result in a [
Error
][] being thrown.statusCode: number,): this;Sends a response header to the request. The status code is a 3-digit HTTP status code, like
404
. The last argument,headers
, are the response headers. Optionally one can give a human-readablestatusMessage
as the second argument.headers
may be anArray
where the keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values. The array is in the same format asrequest.rawHeaders
.Returns a reference to the
ServerResponse
, so that calls can be chained.const body = 'hello world'; response .writeHead(200, { 'Content-Length': Buffer.byteLength(body), 'Content-Type': 'text/plain', }) .end(body);
This method must only be called once on a message and it must be called before
response.end()
is called.If
response.write()
orresponse.end()
are called before calling this, the implicit/mutable headers will be calculated and call this function.When headers have been set with
response.setHeader()
, they will be merged with any headers passed toresponse.writeHead()
, with the headers passed toresponse.writeHead()
given precedence.If this method is called and
response.setHeader()
has not been called, it will directly write the supplied header values onto the network channel without caching internally, and theresponse.getHeader()
on the header will not yield the expected result. If progressive population of headers is desired with potential future retrieval and modification, useresponse.setHeader()
instead.// Returns content-type = text/plain const server = http.createServer((req, res) => { res.setHeader('Content-Type', 'text/html'); res.setHeader('X-Foo', 'bar'); res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('ok'); });
Content-Length
is read in bytes, not characters. UseBuffer.byteLength()
to determine the length of the body in bytes. Node.js will check whetherContent-Length
and the length of the body which has been transmitted are equal or not.Attempting to set a header field name or value that contains invalid characters will result in a [
Error
][] being thrown. Sends a HTTP/1.1 102 Processing message to the client, indicating that the request body should be sent.
- ): Disposable;
Listens once to the
abort
event on the providedsignal
.Listening to the
abort
event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can calle.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
AbortSignal
s in Node.js APIs by solving these two issues by listening to the event such thatstopImmediatePropagation
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](); } }
@returnsDisposable that removes the
abort
listener. - options?: Pick<WritableOptions<Writable>, 'signal' | 'decodeStrings' | 'highWaterMark' | 'objectMode'>
A utility method for creating a
Writable
from a webWritableStream
. - name: string | symbol): Function[];
Returns a copy of the array of listeners for the event named
eventName
.For
EventEmitter
s this behaves exactly the same as calling.listeners
on the emitter.For
EventTarget
s 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] ] }
- ): number;
Returns the currently set max amount of listeners.
For
EventEmitter
s this behaves exactly the same as calling.getMaxListeners
on the emitter.For
EventTarget
s 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 }
- 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 iterateseventName
events. It will throw if theEventEmitter
emits'error'
. It removes all listeners when exiting the loop. Thevalue
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'
@returnsAn
AsyncIterator
that iterateseventName
events emitted by theemitter
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 iterateseventName
events. It will throw if theEventEmitter
emits'error'
. It removes all listeners when exiting the loop. Thevalue
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'
@returnsAn
AsyncIterator
that iterateseventName
events emitted by theemitter
- emitter: EventEmitter,eventName: string | symbol,options?: StaticEventEmitterOptions): Promise<any[]>;
Creates a
Promise
that is fulfilled when theEventEmitter
emits the given event or that is rejected if theEventEmitter
emits'error'
while waiting. ThePromise
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 whenevents.once()
is used to wait for another event. Ifevents.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!
eventName: string,options?: StaticEventEmitterOptions): Promise<any[]>;Creates a
Promise
that is fulfilled when theEventEmitter
emits the given event or that is rejected if theEventEmitter
emits'error'
while waiting. ThePromise
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 whenevents.once()
is used to wait for another event. Ifevents.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!
- n?: number,): void;
import { setMaxListeners, EventEmitter } from 'node:events'; const target = new EventTarget(); const emitter = new EventEmitter(); setMaxListeners(5, target, emitter);
@param nA non-negative number. The maximum number of listeners per
EventTarget
event.@param eventTargetsZero 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. A utility method for creating a web
WritableStream
from aWritable
.