Node.js v10.14.1 英文文档


目录

Stream#

Stability: 2 - Stable

A stream is an abstract interface for working with streaming data in Node.js. The stream module provides a base API that makes it easy to build objects that implement the stream interface.

There are many stream objects provided by Node.js. For instance, a request to an HTTP server and process.stdout are both stream instances.

Streams can be readable, writable, or both. All streams are instances of EventEmitter.

The stream module can be accessed using:

const stream = require('stream');

While it is important to understand how streams work, the stream module itself is most useful for developers that are creating new types of stream instances. Developers who are primarily consuming stream objects will rarely need to use the stream module directly.

Organization of this Document#

This document is divided into two primary sections with a third section for additional notes. The first section explains the elements of the stream API that are required to use streams within an application. The second section explains the elements of the API that are required to implement new types of streams.

Types of Streams#

There are four fundamental stream types within Node.js:

Additionally this module includes the utility functions pipeline and finished.

Object Mode#

All streams created by Node.js APIs operate exclusively on strings and Buffer (or Uint8Array) objects. It is possible, however, for stream implementations to work with other types of JavaScript values (with the exception of null, which serves a special purpose within streams). Such streams are considered to operate in "object mode".

Stream instances are switched into object mode using the objectMode option when the stream is created. Attempting to switch an existing stream into object mode is not safe.

Buffering#

Both Writable and Readable streams will store data in an internal buffer that can be retrieved using writable.writableBuffer or readable.readableBuffer, respectively.

The amount of data potentially buffered depends on the highWaterMark option passed into the stream's constructor. For normal streams, the highWaterMark option specifies a total number of bytes. For streams operating in object mode, the highWaterMark specifies a total number of objects.

Data is buffered in Readable streams when the implementation calls stream.push(chunk). If the consumer of the Stream does not call stream.read(), the data will sit in the internal queue until it is consumed.

Once the total size of the internal read buffer reaches the threshold specified by highWaterMark, the stream will temporarily stop reading data from the underlying resource until the data currently buffered can be consumed (that is, the stream will stop calling the internal readable._read() method that is used to fill the read buffer).

Data is buffered in Writable streams when the writable.write(chunk) method is called repeatedly. While the total size of the internal write buffer is below the threshold set by highWaterMark, calls to writable.write() will return true. Once the size of the internal buffer reaches or exceeds the highWaterMark, false will be returned.

A key goal of the stream API, particularly the stream.pipe() method, is to limit the buffering of data to acceptable levels such that sources and destinations of differing speeds will not overwhelm the available memory.

Because Duplex and Transform streams are both Readable and Writable, each maintain two separate internal buffers used for reading and writing, allowing each side to operate independently of the other while maintaining an appropriate and efficient flow of data. For example, net.Socket instances are Duplex streams whose Readable side allows consumption of data received from the socket and whose Writable side allows writing data to the socket. Because data may be written to the socket at a faster or slower rate than data is received, it is important for each side to operate (and buffer) independently of the other.

API for Stream Consumers#

Almost all Node.js applications, no matter how simple, use streams in some manner. The following is an example of using streams in a Node.js application that implements an HTTP server:

const http = require('http');

const server = http.createServer((req, res) => {
  // req is an http.IncomingMessage, which is a Readable Stream
  // res is an http.ServerResponse, which is a Writable Stream

  let body = '';
  // Get the data as utf8 strings.
  // If an encoding is not set, Buffer objects will be received.
  req.setEncoding('utf8');

  // Readable streams emit 'data' events once a listener is added
  req.on('data', (chunk) => {
    body += chunk;
  });

  // the 'end' event indicates that the entire body has been received
  req.on('end', () => {
    try {
      const data = JSON.parse(body);
      // write back something interesting to the user:
      res.write(typeof data);
      res.end();
    } catch (er) {
      // uh oh! bad json!
      res.statusCode = 400;
      return res.end(`error: ${er.message}`);
    }
  });
});

server.listen(1337);

// $ curl localhost:1337 -d "{}"
// object
// $ curl localhost:1337 -d "\"foo\""
// string
// $ curl localhost:1337 -d "not json"
// error: Unexpected token o in JSON at position 1

Writable streams (such as res in the example) expose methods such as write() and end() that are used to write data onto the stream.

Readable streams use the EventEmitter API for notifying application code when data is available to be read off the stream. That available data can be read from the stream in multiple ways.

Both Writable and Readable streams use the EventEmitter API in various ways to communicate the current state of the stream.

Duplex and Transform streams are both Writable and Readable.

Applications that are either writing data to or consuming data from a stream are not required to implement the stream interfaces directly and will generally have no reason to call require('stream').

Developers wishing to implement new types of streams should refer to the section API for Stream Implementers.

Writable Streams#

Writable streams are an abstraction for a destination to which data is written.

Examples of Writable streams include:

Some of these examples are actually Duplex streams that implement the Writable interface.

All Writable streams implement the interface defined by the stream.Writable class.

While specific instances of Writable streams may differ in various ways, all Writable streams follow the same fundamental usage pattern as illustrated in the example below:

const myStream = getWritableStreamSomehow();
myStream.write('some data');
myStream.write('some more data');
myStream.end('done writing data');

Class: stream.Writable#

Event: 'close'#

The 'close' event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

Not all Writable streams will emit the 'close' event.

Event: 'drain'#

If a call to stream.write(chunk) returns false, the 'drain' event will be emitted when it is appropriate to resume writing data to the stream.

// Write the data to the supplied writable stream one million times.
// Be attentive to back-pressure.
function writeOneMillionTimes(writer, data, encoding, callback) {
  let i = 1000000;
  write();
  function write() {
    let ok = true;
    do {
      i--;
      if (i === 0) {
        // last time!
        writer.write(data, encoding, callback);
      } else {
        // see if we should continue, or wait
        // don't pass the callback, because we're not done yet.
        ok = writer.write(data, encoding);
      }
    } while (i > 0 && ok);
    if (i > 0) {
      // had to stop early!
      // write some more once it drains
      writer.once('drain', write);
    }
  }
}
Event: 'error'#

The 'error' event is emitted if an error occurred while writing or piping data. The listener callback is passed a single Error argument when called.

The stream is not closed when the 'error' event is emitted.

Event: 'finish'#

The 'finish' event is emitted after the stream.end() method has been called, and all data has been flushed to the underlying system.

const writer = getWritableStreamSomehow();
for (let i = 0; i < 100; i++) {
  writer.write(`hello, #${i}!\n`);
}
writer.end('This is the end\n');
writer.on('finish', () => {
  console.error('All writes are now complete.');
});
Event: 'pipe'#

The 'pipe' event is emitted when the stream.pipe() method is called on a readable stream, adding this writable to its set of destinations.

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('pipe', (src) => {
  console.error('something is piping into the writer');
  assert.equal(src, reader);
});
reader.pipe(writer);
Event: 'unpipe'#

The 'unpipe' event is emitted when the stream.unpipe() method is called on a Readable stream, removing this Writable from its set of destinations.

This is also emitted in case this Writable stream emits an error when a Readable stream pipes into it.

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('unpipe', (src) => {
  console.error('Something has stopped piping into the writer.');
  assert.equal(src, reader);
});
reader.pipe(writer);
reader.unpipe(writer);
writable.cork()#

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

The primary intent of writable.cork() is to avoid a situation where writing many small chunks of data to a stream do not cause a backup in the internal buffer that would have an adverse impact on performance. In such situations, implementations that implement the writable._writev() method can perform buffered writes in a more optimized manner.

See also: writable.uncork().

writable.destroy([error])#

Destroy the stream, and emit the passed 'error' and a 'close' event. After this call, the writable stream has ended and subsequent calls to write() or end() will result in an ERR_STREAM_DESTROYED error. Implementors should not override this method, but instead implement writable._destroy().

writable.end([chunk][, encoding][, callback])#
  • chunk <string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.
  • encoding <string> The encoding, if chunk is a string
  • callback <Function> Optional callback for when the stream is finished
  • Returns: <this>

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. If provided, the optional callback function is attached as a listener for the 'finish' event.

Calling the stream.write() method after calling stream.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!
writable.setDefaultEncoding(encoding)#

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

writable.uncork()#

The writable.uncork() method flushes all data buffered since stream.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 all writable.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().

writable.writableHighWaterMark#

Return the value of highWaterMark passed when constructing this Writable.

writable.writableLength#

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.

writable.write(chunk[, encoding][, callback])#
  • chunk <string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.
  • encoding <string> The encoding, if chunk is a string
  • callback <Function> Callback for when this chunk of data is flushed
  • Returns: <boolean> false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

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 may or may not be called with the error as its first argument. To reliably detect write errors, add a listener for the 'error' event.

The return value is true if the internal buffer is less than the highWaterMark 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 an '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 stream.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.

Readable Streams#

Readable streams are an abstraction for a source from which data is consumed.

Examples of Readable streams include:

All Readable streams implement the interface defined by the stream.Readable class.

Two Reading Modes#

Readable streams effectively operate in one of two modes: flowing and paused. These modes are separate from object mode. A Readable stream can be in object mode or not, regardless of whether it is in flowing mode or paused mode.

  • In flowing mode, data is read from the underlying system automatically and provided to an application as quickly as possible using events via the EventEmitter interface.

  • In paused mode, the stream.read() method must be called explicitly to read chunks of data from the stream.

All Readable streams begin in paused mode but can be switched to flowing mode in one of the following ways:

The Readable can switch back to paused mode using one of the following:

  • If there are no pipe destinations, by calling the stream.pause() method.
  • If there are pipe destinations, by removing all pipe destinations. Multiple pipe destinations may be removed by calling the stream.unpipe() method.

The important concept to remember is that a Readable will not generate data until a mechanism for either consuming or ignoring that data is provided. If the consuming mechanism is disabled or taken away, the Readable will attempt to stop generating the data.

For backwards compatibility reasons, removing 'data' event handlers will not automatically pause the stream. Also, if there are piped destinations, then calling stream.pause() will not guarantee that the stream will remain paused once those destinations drain and ask for more data.

If a Readable is switched into flowing mode and there are no consumers available to handle the data, that data will be lost. This can occur, for instance, when the readable.resume() method is called without a listener attached to the 'data' event, or when a 'data' event handler is removed from the stream.

Adding a 'readable' event handler automatically make the stream to stop flowing, and the data to be consumed via readable.read(). If the 'readable' event handler is removed, then the stream will start flowing again if there is a 'data' event handler.

Three States#

The "two modes" of operation for a Readable stream are a simplified abstraction for the more complicated internal state management that is happening within the Readable stream implementation.

Specifically, at any given point in time, every Readable is in one of three possible states:

  • readable.readableFlowing === null
  • readable.readableFlowing === false
  • readable.readableFlowing === true

When readable.readableFlowing is null, no mechanism for consuming the stream's data is provided. Therefore, the stream will not generate data. While in this state, attaching a listener for the 'data' event, calling the readable.pipe() method, or calling the readable.resume() method will switch readable.readableFlowing to true, causing the Readable to begin actively emitting events as data is generated.

Calling readable.pause(), readable.unpipe(), or receiving backpressure will cause the readable.readableFlowing to be set as false, temporarily halting the flowing of events but not halting the generation of data. While in this state, attaching a listener for the 'data' event will not switch readable.readableFlowing to true.

const { PassThrough, Writable } = require('stream');
const pass = new PassThrough();
const writable = new Writable();

pass.pipe(writable);
pass.unpipe(writable);
// readableFlowing is now false

pass.on('data', (chunk) => { console.log(chunk.toString()); });
pass.write('ok');  // will not emit 'data'
pass.resume();     // must be called to make stream emit 'data'

While readable.readableFlowing is false, data may be accumulating within the stream's internal buffer.

Choose One API Style#

The Readable stream API evolved across multiple Node.js versions and provides multiple methods of consuming stream data. In general, developers should choose one of the methods of consuming data and should never use multiple methods to consume data from a single stream. Specifically, using a combination of on('data'), on('readable'), pipe(), or async iterators could lead to unintuitive behavior.

Use of the readable.pipe() method is recommended for most users as it has been implemented to provide the easiest way of consuming stream data. Developers that require more fine-grained control over the transfer and generation of data can use the EventEmitter and readable.on('readable')/readable.read() or the readable.pause()/readable.resume() APIs.

Class: stream.Readable#

Event: 'close'#

The 'close' event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

Not all Readable streams will emit the 'close' event.

Event: 'data'#
  • chunk <Buffer> | <string> | <any> The chunk of data. For streams that are not operating in object mode, the chunk will be either a string or Buffer. For streams that are in object mode, the chunk can be any JavaScript value other than null.

The 'data' event is emitted whenever the stream is relinquishing ownership of a chunk of data to a consumer. This may occur whenever the stream is switched in flowing mode by calling readable.pipe(), readable.resume(), or by attaching a listener callback to the 'data' event. The 'data' event will also be emitted whenever the readable.read() method is called and a chunk of data is available to be returned.

Attaching a 'data' event listener to a stream that has not been explicitly paused will switch the stream into flowing mode. Data will then be passed as soon as it is available.

The listener callback will be passed the chunk of data as a string if a default encoding has been specified for the stream using the readable.setEncoding() method; otherwise the data will be passed as a Buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});
Event: 'end'#

The 'end' event is emitted when there is no more data to be consumed from the stream.

The 'end' event will not be emitted unless the data is completely consumed. This can be accomplished by switching the stream into flowing mode, or by calling stream.read() repeatedly until all data has been consumed.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});
readable.on('end', () => {
  console.log('There will be no more data.');
});
Event: 'error'#

The 'error' event may be emitted by a Readable implementation at any time. Typically, this may occur if the underlying stream is unable to generate data due to an underlying internal failure, or when a stream implementation attempts to push an invalid chunk of data.

The listener callback will be passed a single Error object.

Event: 'readable'#

The 'readable' event is emitted when there is data available to be read from the stream. In some cases, attaching a listener for the 'readable' event will cause some amount of data to be read into an internal buffer.

const readable = getReadableStreamSomehow();
readable.on('readable', function() {
  // there is some data to read now
  let data;

  while (data = this.read()) {
    console.log(data);
  }
});

The 'readable' event will also be emitted once the end of the stream data has been reached but before the 'end' event is emitted.

Effectively, the 'readable' event indicates that the stream has new information: either new data is available or the end of the stream has been reached. In the former case, stream.read() will return the available data. In the latter case, stream.read() will return null. For instance, in the following example, foo.txt is an empty file:

const fs = require('fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
  console.log(`readable: ${rr.read()}`);
});
rr.on('end', () => {
  console.log('end');
});

The output of running this script is:

$ node test.js
readable: null
end

In general, the readable.pipe() and 'data' event mechanisms are easier to understand than the 'readable' event. However, handling 'readable' might result in increased throughput.

If both 'readable' and 'data' are used at the same time, 'readable' takes precedence in controlling the flow, i.e. 'data' will be emitted only when stream.read() is called. The readableFlowing property would become false. If there are 'data' listeners when 'readable' is removed, the stream will start flowing, i.e. 'data' events will be emitted without calling .resume().

readable.destroy([error])#
  • error <Error> Error which will be passed as payload in 'error' event
  • Returns: <this>

Destroy the stream, and emit 'error' and 'close'. After this call, the readable stream will release any internal resources and subsequent calls to push() will be ignored. Implementors should not override this method, but instead implement readable._destroy().

readable.isPaused()#

The readable.isPaused() method returns the current operating state of the Readable. This is used primarily by the mechanism that underlies the readable.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
readable.pause()#

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.

readable.pipe(destination[, options])#

The readable.pipe() method attaches a Writable stream to the readable, causing it to switch automatically into flowing mode and push all of its data to the attached Writable. The flow of data will be automatically managed so that the destination Writable stream is not overwhelmed by a faster Readable stream.

The following example pipes all of the data from the readable into a file named file.txt:

const fs = require('fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt'
readable.pipe(writable);

It is possible to attach multiple Writable streams to a single Readable stream.

The readable.pipe() method returns a reference to the destination stream making it possible to set up chains of piped streams:

const fs = require('fs');
const r = fs.createReadStream('file.txt');
const z = zlib.createGzip();
const w = fs.createWriteStream('file.txt.gz');
r.pipe(z).pipe(w);

By default, stream.end() is called on the destination Writable stream when the source Readable stream emits 'end', so that the destination is no longer writable. To disable this default behavior, the end option can be passed as false, causing the destination stream to remain open:

reader.pipe(writer, { end: false });
reader.on('end', () => {
  writer.end('Goodbye\n');
});

One important caveat is that if the Readable stream emits an error during processing, the Writable destination is not closed automatically. If an error occurs, it will be necessary to manually close each stream in order to prevent memory leaks.

The process.stderr and process.stdout Writable streams are never closed until the Node.js process exits, regardless of the specified options.

readable.read([size])#

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. If size 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 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.on('readable', () => {
  let chunk;
  while (null !== (chunk = readable.read())) {
    console.log(`Received ${chunk.length} bytes of data.`);
  }
});

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

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

Calling stream.read([size]) after the 'end' event has been emitted will return null. No runtime error will be raised.

readable.readableHighWaterMark#

Returns the value of highWaterMark passed when constructing this Readable.

readable.readableLength#

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.

readable.resume()#

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.

readable.setEncoding(encoding)#

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 as Buffer objects. Setting an encoding causes the stream data to be returned as strings of the specified encoding rather than as Buffer objects. For instance, calling readable.setEncoding('utf8') will cause the output data to be interpreted as UTF-8 data, and passed as strings. Calling readable.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);
});
readable.unpipe([destination])#

The readable.unpipe() method detaches a Writable stream previously attached using the stream.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);
readable.unshift(chunk)#
  • chunk <Buffer> | <Uint8Array> | <string> | <any> Chunk of data to unshift onto the read queue. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.

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 stream.push(chunk), 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 stream._read() implementation on a custom stream). Following the call to readable.unshift() with an immediate stream.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.

readable.wrap(stream)#

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 stream.pause() method that is advisory only, the readable.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.
});
readable[Symbol.asyncIterator]()#

Stability: 1 - Experimental

const fs = require('fs');

async function print(readable) {
  readable.setEncoding('utf8');
  let data = '';
  for await (const k of readable) {
    data += k;
  }
  console.log(data);
}

print(fs.createReadStream('file')).catch(console.log);

If the loop terminates with a break or a throw, the stream will be destroyed. In other terms, iterating over a stream will consume the stream fully. The stream will be read in chunks of size equal to the highWaterMark option. In the code example above, data will be in a single chunk if the file has less then 64kb of data because no highWaterMark option is provided to fs.createReadStream().

Duplex and Transform Streams#

Class: stream.Duplex#

Duplex streams are streams that implement both the Readable and Writable interfaces.

Examples of Duplex streams include:

Class: stream.Transform#

Transform streams are Duplex streams where the output is in some way related to the input. Like all Duplex streams, Transform streams implement both the Readable and Writable interfaces.

Examples of Transform streams include:

transform.destroy([error])#

Destroy the stream, and emit 'error'. After this call, the transform stream would release any internal resources. Implementors should not override this method, but instead implement readable._destroy(). The default implementation of _destroy() for Transform also emit 'close'.

stream.finished(stream, callback)#

  • stream <Stream> A readable and/or writable stream.
  • callback <Function> A callback function that takes an optional error argument.

A function to get notified when a stream is no longer readable, writable or has experienced an error or a premature close event.

const { finished } = require('stream');

const rs = fs.createReadStream('archive.tar');

finished(rs, (err) => {
  if (err) {
    console.error('Stream failed', err);
  } else {
    console.log('Stream is done reading');
  }
});

rs.resume(); // drain the stream

Especially useful in error handling scenarios where a stream is destroyed prematurely (like an aborted HTTP request), and will not emit 'end' or 'finish'.

The finished API is promisify'able as well;

const finished = util.promisify(stream.finished);

const rs = fs.createReadStream('archive.tar');

async function run() {
  await finished(rs);
  console.log('Stream is done reading');
}

run().catch(console.error);
rs.resume(); // drain the stream

stream.pipeline(...streams[, callback])#

  • ...streams <Stream> Two or more streams to pipe between.
  • callback <Function> A callback function that takes an optional error argument.

A module method to pipe between streams forwarding errors and properly cleaning up and provide a callback when the pipeline is complete.

const { pipeline } = require('stream');
const fs = require('fs');
const zlib = require('zlib');

// Use the pipeline API to easily pipe a series of streams
// together and get notified when the pipeline is fully done.

// A pipeline to gzip a potentially huge tar file efficiently:

pipeline(
  fs.createReadStream('archive.tar'),
  zlib.createGzip(),
  fs.createWriteStream('archive.tar.gz'),
  (err) => {
    if (err) {
      console.error('Pipeline failed', err);
    } else {
      console.log('Pipeline succeeded');
    }
  }
);

The pipeline API is promisify'able as well:

const pipeline = util.promisify(stream.pipeline);

async function run() {
  await pipeline(
    fs.createReadStream('archive.tar'),
    zlib.createGzip(),
    fs.createWriteStream('archive.tar.gz')
  );
  console.log('Pipeline succeeded');
}

run().catch(console.error);

API for Stream Implementers#

The stream module API has been designed to make it possible to easily implement streams using JavaScript's prototypal inheritance model.

First, a stream developer would declare a new JavaScript class that extends one of the four basic stream classes (stream.Writable, stream.Readable, stream.Duplex, or stream.Transform), making sure they call the appropriate parent class constructor:

const { Writable } = require('stream');

class MyWritable extends Writable {
  constructor(options) {
    super(options);
    // ...
  }
}

The new stream class must then implement one or more specific methods, depending on the type of stream being created, as detailed in the chart below:

Use-caseClassMethod(s) to implement
Reading onlyReadable_read
Writing onlyWritable_write, _writev, _final
Reading and writingDuplex_read, _write, _writev, _final
Operate on written data, then read the resultTransform_transform, _flush, _final

The implementation code for a stream should never call the "public" methods of a stream that are intended for use by consumers (as described in the API for Stream Consumers section). Doing so may lead to adverse side effects in application code consuming the stream.

Simplified Construction#

For many simple cases, it is possible to construct a stream without relying on inheritance. This can be accomplished by directly creating instances of the stream.Writable, stream.Readable, stream.Duplex or stream.Transform objects and passing appropriate methods as constructor options.

const { Writable } = require('stream');

const myWritable = new Writable({
  write(chunk, encoding, callback) {
    // ...
  }
});

Implementing a Writable Stream#

The stream.Writable class is extended to implement a Writable stream.

Custom Writable streams must call the new stream.Writable([options]) constructor and implement the writable._write() method. The writable._writev() method may also be implemented.

Constructor: new stream.Writable([options])#