Module

std/node/stream.ts

Deno standard library
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class Writable
implements WritableStream
extends Stream
import { Writable } from "https://deno.land/std@0.145.0/node/stream.ts";

Constructors

new
Writable(opts?: WritableOptions)

Properties

destroyed: boolean

Is true after writable.destroy() has been called.

readonly
writable: boolean

Is true if it is safe to call writable.write(), which means the stream has not been destroyed, errored or ended.

readonly
optional
writableBuffer: Buffered[]
readonly
writableCorked: number

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

readonly
writableEnded: boolean

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

readonly
writableFinished: boolean

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

readonly
writableHighWaterMark: number

Return the value of highWaterMark passed when creating this Writable.

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
writableObjectMode: boolean

Getter for the property objectMode of a given Writable stream.

Methods

optional
_construct(callback: (error?: Error | null) => void): void
_destroy(error: Error | null, callback: (error?: Error | null) => void): void
_final(callback: (error?: Error | null) => void): void
_write(
chunk: any,
encoding: BufferEncoding,
callback: (error?: Error | null) => void,
): void
optional
_writev(chunks: Array<{ chunk: any; encoding: BufferEncoding; }>, callback: (error?: Error | null) => void): void
addListener(event: "close", listener: () => void): this

Event emitter The defined events on documents including:

  1. close
  2. drain
  3. error
  4. finish
  5. pipe
  6. unpipe
addListener(event: "drain", listener: () => void): this
addListener(event: "error", listener: (err: Error) => void): this
addListener(event: "finish", listener: () => void): this
addListener(event: "pipe", listener: (src: Readable) => void): this
addListener(event: "unpipe", listener: (src: Readable) => void): this
addListener(event: string | symbol, listener: (...args: any[]) => void): this
cork(): void

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

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

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

destroy(error?: Error): void

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

Once destroy() has been called any further calls will be a no-op and no further errors except from _destroy() may be emitted as 'error'.

Implementors should not override this method, but instead implement writable._destroy().

emit(event: "close"): boolean
emit(event: "drain"): boolean
emit(event: "error", err: Error): boolean
emit(event: "finish"): boolean
emit(event: "pipe", src: Readable): boolean
emit(event: "unpipe", src: Readable): boolean
emit(event: string | symbol, ...args: any[]): boolean
end(cb?: () => void): void

Calling the writable.end() method signals that no more data will be written to the Writable. The optional chunk and encoding arguments allow one final additional chunk of data to be written immediately before closing the stream.

Calling the {@link write} method after calling {@link end} will raise an error.

// Write 'hello, ' and then end with 'world!'.
const fs = require('fs');
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// Writing more now is not allowed!
end(chunk: any, cb?: () => void): void
end(
chunk: any,
encoding: BufferEncoding,
cb?: () => void,
): void
on(event: "close", listener: () => void): this
on(event: "drain", listener: () => void): this
on(event: "error", listener: (err: Error) => void): this
on(event: "finish", listener: () => void): this
on(event: "pipe", listener: (src: Readable) => void): this
on(event: "unpipe", listener: (src: Readable) => void): this
on(event: string | symbol, listener: (...args: any[]) => void): this
once(event: "close", listener: () => void): this
once(event: "drain", listener: () => void): this
once(event: "error", listener: (err: Error) => void): this
once(event: "finish", listener: () => void): this
once(event: "pipe", listener: (src: Readable) => void): this
once(event: "unpipe", listener: (src: Readable) => void): this
once(event: string | symbol, listener: (...args: any[]) => void): this
prependListener(event: "close", listener: () => void): this
prependListener(event: "drain", listener: () => void): this
prependListener(event: "error", listener: (err: Error) => void): this
prependListener(event: "finish", listener: () => void): this
prependListener(event: "pipe", listener: (src: Readable) => void): this
prependListener(event: "unpipe", listener: (src: Readable) => void): this
prependListener(event: string | symbol, listener: (...args: any[]) => void): this
prependOnceListener(event: "close", listener: () => void): this
prependOnceListener(event: "drain", listener: () => void): this
prependOnceListener(event: "error", listener: (err: Error) => void): this
prependOnceListener(event: "finish", listener: () => void): this
prependOnceListener(event: "pipe", listener: (src: Readable) => void): this
prependOnceListener(event: "unpipe", listener: (src: Readable) => void): this
prependOnceListener(event: string | symbol, listener: (...args: any[]) => void): this
removeListener(event: "close", listener: () => void): this
removeListener(event: "drain", listener: () => void): this
removeListener(event: "error", listener: (err: Error) => void): this
removeListener(event: "finish", listener: () => void): this
removeListener(event: "pipe", listener: (src: Readable) => void): this
removeListener(event: "unpipe", listener: (src: Readable) => void): this
removeListener(event: string | symbol, listener: (...args: any[]) => void): this
setDefaultEncoding(encoding: BufferEncoding): this

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

uncork(): void

The writable.uncork() method flushes all data buffered since {@link cork} was called.

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

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

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

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().

write(chunk: any, callback?: (error: Error | null | undefined) => void): boolean

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

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

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

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

If the data to be written can be generated or fetched on demand, it is recommended to encapsulate the logic into a Readable and use {@link pipe}. However, if calling write() 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 the encoding argument.

write(
chunk: any,
encoding: BufferEncoding,
callback?: (error: Error | null | undefined) => void,
): boolean