Module

std/node/stream.ts

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

Constructors

new
Readable(opts?: ReadableOptions)

Properties

destroyed: boolean

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

readable: boolean

Is true if it is safe to call readable.read(), which means the stream has not been destroyed or emitted 'error' or 'end'.

readonly
readableAborted: boolean

Returns whether the stream was destroyed or errored before emitting 'end'.

readonly
readableDidRead: boolean

Returns whether 'data' has been emitted.

readonly
readableEncoding: BufferEncoding | null

Getter for the property encoding of a given Readable stream. The encodingproperty can be set using the readable.setEncoding() method.

readonly
readableEnded: boolean

Becomes true when 'end' event is emitted.

readonly
readableFlowing: boolean | null

This property reflects the current state of a Readable stream as described in the Three states section.

readonly
readableHighWaterMark: number

Returns the value of highWaterMark passed when creating this Readable.

readonly
readableLength: number

This property contains the number of bytes (or objects) in the queue ready to be read. The value provides introspection data regarding the status of the highWaterMark.

readonly
readableObjectMode: boolean

Getter for the property objectMode of a given Readable stream.

Methods

optional
_construct(callback: (error?: Error | null) => void): void
_destroy(error: Error | null, callback: (error?: Error | null) => void): void
_read(size: number): void
_undestroy(): void
addListener(event: "close", listener: () => void): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. end
  4. error
  5. pause
  6. readable
  7. resume
addListener(event: "data", listener: (chunk: any) => void): this
addListener(event: "end", listener: () => void): this
addListener(event: "error", listener: (err: Error) => void): this
addListener(event: "pause", listener: () => void): this
addListener(event: "readable", listener: () => void): this
addListener(event: "resume", listener: () => void): this
addListener(event: string | symbol, listener: (...args: any[]) => void): this
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 readable stream will release any internal resources and subsequent calls to push()will be ignored.

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 readable._destroy().

emit(event: "close"): boolean
emit(event: "data", chunk: any): boolean
emit(event: "end"): boolean
emit(event: "error", err: Error): boolean
emit(event: "pause"): boolean
emit(event: "readable"): boolean
emit(event: "resume"): boolean
emit(event: string | symbol, ...args: any[]): boolean
isPaused(): boolean

The readable.isPaused() method returns the current operating state of theReadable. This is used primarily by the mechanism that underlies thereadable.pipe() method. In most typical cases, there will be no reason to use this method directly.

const readable = new stream.Readable();

readable.isPaused(); // === false
readable.pause();
readable.isPaused(); // === true
readable.resume();
readable.isPaused(); // === false
on(event: "close", listener: () => void): this
on(event: "data", listener: (chunk: any) => void): this
on(event: "end", listener: () => void): this
on(event: "error", listener: (err: Error) => void): this
on(event: "pause", listener: () => void): this
on(event: "readable", listener: () => void): this
on(event: "resume", listener: () => void): this
on(event: string | symbol, listener: (...args: any[]) => void): this
once(event: "close", listener: () => void): this
once(event: "data", listener: (chunk: any) => void): this
once(event: "end", listener: () => void): this
once(event: "error", listener: (err: Error) => void): this
once(event: "pause", listener: () => void): this
once(event: "readable", listener: () => void): this
once(event: "resume", listener: () => void): this
once(event: string | symbol, listener: (...args: any[]) => void): this
pause(): this

The readable.pause() method will cause a stream in flowing mode to stop emitting 'data' events, switching out of flowing mode. Any data that becomes available will remain in the internal buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
  readable.pause();
  console.log('There will be no additional data for 1 second.');
  setTimeout(() => {
    console.log('Now data will start flowing again.');
    readable.resume();
  }, 1000);
});

The readable.pause() method has no effect if there is a 'readable'event listener.

prependListener(event: "close", listener: () => void): this
prependListener(event: "data", listener: (chunk: any) => void): this
prependListener(event: "end", listener: () => void): this
prependListener(event: "error", listener: (err: Error) => void): this
prependListener(event: "pause", listener: () => void): this
prependListener(event: "readable", listener: () => void): this
prependListener(event: "resume", listener: () => void): this
prependListener(event: string | symbol, listener: (...args: any[]) => void): this
prependOnceListener(event: "close", listener: () => void): this
prependOnceListener(event: "data", listener: (chunk: any) => void): this
prependOnceListener(event: "end", listener: () => void): this
prependOnceListener(event: "error", listener: (err: Error) => void): this
prependOnceListener(event: "pause", listener: () => void): this
prependOnceListener(event: "readable", listener: () => void): this
prependOnceListener(event: "resume", listener: () => void): this
prependOnceListener(event: string | symbol, listener: (...args: any[]) => void): this
push(chunk: any, encoding?: BufferEncoding): boolean
read(size?: number): any

The readable.read() method pulls some data out of the internal buffer and returns it. If no data available to be read, null is returned. By default, the data will be returned as a Buffer object unless an encoding has been specified using the readable.setEncoding() method or the stream is operating in object mode.

The optional size argument specifies a specific number of bytes to read. Ifsize bytes are not available to be read, null will be returned _unless_the stream has ended, in which case all of the data remaining in the internal buffer will be returned.

If the size argument is not specified, all of the data contained in the internal buffer will be returned.

The size argument must be less than or equal to 1 GiB.

The readable.read() method should only be called on Readable streams operating in paused mode. In flowing mode, readable.read() is called automatically until the internal buffer is fully drained.

const readable = getReadableStreamSomehow();

// 'readable' may be triggered multiple times as data is buffered in
readable.on('readable', () => {
  let chunk;
  console.log('Stream is readable (new data received in buffer)');
  // Use a loop to make sure we read all currently available data
  while (null !== (chunk = readable.read())) {
    console.log(`Read ${chunk.length} bytes of data...`);
  }
});

// 'end' will be triggered once when there is no more data available
readable.on('end', () => {
  console.log('Reached end of stream.');
});

Each call to readable.read() returns a chunk of data, or null. The chunks are not concatenated. A while loop is necessary to consume all data currently in the buffer. When reading a large file .read() may return null, having consumed all buffered content so far, but there is still more data to come not yet buffered. In this case a new 'readable' event will be emitted when there is more data in the buffer. Finally the 'end' event will be emitted when there is no more data to come.

Therefore to read a file's whole contents from a readable, it is necessary to collect chunks across multiple 'readable' events:

const chunks = [];

readable.on('readable', () => {
  let chunk;
  while (null !== (chunk = readable.read())) {
    chunks.push(chunk);
  }
});

readable.on('end', () => {
  const content = chunks.join('');
});

A Readable stream in object mode will always return a single item from a call to readable.read(size), regardless of the value of thesize argument.

If the readable.read() method returns a chunk of data, a 'data' event will also be emitted.

Calling {@link read} after the 'end' event has been emitted will return null. No runtime error will be raised.

removeListener(event: "close", listener: () => void): this
removeListener(event: "data", listener: (chunk: any) => void): this
removeListener(event: "end", listener: () => void): this
removeListener(event: "error", listener: (err: Error) => void): this
removeListener(event: "pause", listener: () => void): this
removeListener(event: "readable", listener: () => void): this
removeListener(event: "resume", listener: () => void): this
removeListener(event: string | symbol, listener: (...args: any[]) => void): this
resume(): this

The readable.resume() method causes an explicitly paused Readable stream to resume emitting 'data' events, switching the stream into flowing mode.

The readable.resume() method can be used to fully consume the data from a stream without actually processing any of that data:

getReadableStreamSomehow()
  .resume()
  .on('end', () => {
    console.log('Reached the end, but did not read anything.');
  });

The readable.resume() method has no effect if there is a 'readable'event listener.

setEncoding(encoding: BufferEncoding): this

The readable.setEncoding() method sets the character encoding for data read from the Readable stream.

By default, no encoding is assigned and stream data will be returned asBuffer objects. Setting an encoding causes the stream data to be returned as strings of the specified encoding rather than as Bufferobjects. For instance, calling readable.setEncoding('utf8') will cause the output data to be interpreted as UTF-8 data, and passed as strings. Callingreadable.setEncoding('hex') will cause the data to be encoded in hexadecimal string format.

The Readable stream will properly handle multi-byte characters delivered through the stream that would otherwise become improperly decoded if simply pulled from the stream as Buffer objects.

const readable = getReadableStreamSomehow();
readable.setEncoding('utf8');
readable.on('data', (chunk) => {
  assert.equal(typeof chunk, 'string');
  console.log('Got %d characters of string data:', chunk.length);
});
unpipe(destination?: WritableStream): this

The readable.unpipe() method detaches a Writable stream previously attached using the {@link pipe} method.

If the destination is not specified, then all pipes are detached.

If the destination is specified, but no pipe is set up for it, then the method does nothing.

const fs = require('fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt',
// but only for the first second.
readable.pipe(writable);
setTimeout(() => {
  console.log('Stop writing to file.txt.');
  readable.unpipe(writable);
  console.log('Manually close the file stream.');
  writable.end();
}, 1000);
unshift(chunk: any, encoding?: BufferEncoding): void

Passing chunk as null signals the end of the stream (EOF) and behaves the same as readable.push(null), after which no more data can be written. The EOF signal is put at the end of the buffer and any buffered data will still be flushed.

The readable.unshift() method pushes a chunk of data back into the internal buffer. This is useful in certain situations where a stream is being consumed by code that needs to "un-consume" some amount of data that it has optimistically pulled out of the source, so that the data can be passed on to some other party.

The stream.unshift(chunk) method cannot be called after the 'end' event has been emitted or a runtime error will be thrown.

Developers using stream.unshift() often should consider switching to use of a Transform stream instead. See the API for stream implementers section for more information.

// Pull off a header delimited by \n\n.
// Use unshift() if we get too much.
// Call the callback with (error, header, stream).
const { StringDecoder } = require('string_decoder');
function parseHeader(stream, callback) {
  stream.on('error', callback);
  stream.on('readable', onReadable);
  const decoder = new StringDecoder('utf8');
  let header = '';
  function onReadable() {
    let chunk;
    while (null !== (chunk = stream.read())) {
      const str = decoder.write(chunk);
      if (str.match(/\n\n/)) {
        // Found the header boundary.
        const split = str.split(/\n\n/);
        header += split.shift();
        const remaining = split.join('\n\n');
        const buf = Buffer.from(remaining, 'utf8');
        stream.removeListener('error', callback);
        // Remove the 'readable' listener before unshifting.
        stream.removeListener('readable', onReadable);
        if (buf.length)
          stream.unshift(buf);
        // Now the body of the message can be read from the stream.
        callback(null, header, stream);
      } else {
        // Still reading the header.
        header += str;
      }
    }
  }
}

Unlike {@link push}, stream.unshift(chunk) will not end the reading process by resetting the internal reading state of the stream. This can cause unexpected results if readable.unshift() is called during a read (i.e. from within a {@link read} implementation on a custom stream). Following the call to readable.unshift() with an immediate {@link_ push} will reset the reading state appropriately, however it is best to simply avoid calling readable.unshift() while in the process of performing a read.

wrap(stream: ReadableStream): this

Prior to Node.js 0.10, streams did not implement the entire stream module API as it is currently defined. (See Compatibility for more information.)

When using an older Node.js library that emits 'data' events and has a {@link pause} method that is advisory only, thereadable.wrap() method can be used to create a Readable stream that uses the old stream as its data source.

It will rarely be necessary to use readable.wrap() but the method has been provided as a convenience for interacting with older Node.js applications and libraries.

const { OldReader } = require('./old-api-module.js');
const { Readable } = require('stream');
const oreader = new OldReader();
const myReader = new Readable().wrap(oreader);

myReader.on('readable', () => {
  myReader.read(); // etc.
});
[Symbol.asyncIterator](): AsyncIterableIterator<any>

Static Methods

from(iterable: Iterable<any> | AsyncIterable<any>, options?: ReadableOptions): Readable

A utility method for creating Readable Streams out of iterators.

isDisturbed(stream: Readable | ReadableStream): boolean

Returns whether the stream has been read from or cancelled.