# Stream : Abstract interface for streaming data

A stream is an abstract interface for working with streaming data. The stream module provides an API for implementing the stream interface.

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

To access the stream module:

const stream = require('stream');

The stream module is useful for creating new types of stream instances. It is usually not necessary to use the stream module to consume streams.

# Types Of Streams

There are four fundamental stream types within JSRE:

• Writable: streams to which data can be written.
• Readable: streams from which data can be read.
• Duplex: streams that are both Readable and Writable.
• Transform: Duplex streams that can modify or transform the data as it is written and read.

# Object Mode

All streams created by JSRE APIs operate exclusively on strings and Buffer 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.

The highWaterMark option is a threshold, not a limit: it dictates the amount of data that a stream buffers before it stops asking for more data. It does not enforce a strict memory limitation in general. Specific stream implementations may choose to enforce stricter limits but doing so is optional.

Because Duplexstream are both Readable and Writable, each maintains 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.Socketinstances 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, each side should operate (and buffer) independently of the other.

# API for Stream Consumers

Writable streams 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 stream are both Writableand 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.

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');

# Readable Streams

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

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:

• Adding a 'data' event handler.
• Calling the stream.resume() method.
• Calling the stream.pipe() method to send the data to a Writable.

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 backward 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.

# 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.

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

# Choose One API Style

The Readable stream API 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()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.

# Duplex Streams

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

# Support

The following shows stream module APIs available for each permissions.

# Writable Object

# writable.destroy([error])

• error {Error} Optional, an error to emit with 'error' event.
• Returns: Self object.

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 error. This is a destructive and immediate way to destroy a stream. 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 noop and no further errors except from _destroy may be emitted as 'error'.

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

# writable.end([chunk[, encoding]][, callback])

• chunk {String | Buffer | Any} Optional data to write. For streams not operating in object mode, chunk must be a string or Buffer. For object mode streams, chunk may be any JavaScript value other than null.
• encoding {String} The encoding, if chunk is a string. default: 'utf8'.
• callback {Function} Optional callback for when the stream finishes or errors
  • error {Error} Error object.
• Returns: Self object.

Calling the writable.end() method signals that no more data will be written to the Writable. The optional chunk 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' and the 'error' event.

Calling the stream.write() method after calling stream.end() will raise an error.

# Example

// 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.write(chunk[, encoding][, callback])

• chunk {String | Buffer | Any} Optional data to write. For streams not operating in object mode, chunk must be a string or Buffer. For object mode streams, chunk may be any JavaScript value other than null.
• encoding {String} The encoding, if chunk is a string. default: 'utf8'. A Writable stream in object mode will always ignore the encoding argument.
• callback 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 callback is called asynchronously and before 'error' is emitted.

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, Writable will buffer all written chunks until buffer more then the alertWaterMark configured, at which point it will emit 'error' event if autoAlert configured to true.

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 {
    Task.nextTick(cb);
  }
}

// Wait for cb to be called before doing any other write.
write('hello', () => {
  console.log('Write completed, do more writes now.');
});

# writable.setDefaultEncoding(encoding)

• encoding {String} The new default encoding. The valid encoding can be: 'utf8', 'utf-8', 'hex', 'base64', 'ascii'.
• Returns: {Writable} This.

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

# writable.destroyed

• {Boolean} Is true after writable.destroy() has been called.

# writable.writable

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

# writable.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.

# writable.writableFinished

• {Boolean} Is set to true immediately before the 'finish' event is emitted.

# writable.writableHighWaterMark

• {Integer} Return the value of highWaterMark passed when constructing this Writable.

# writable.writableLength

• {Integer} This property contains the number of bytes in the queue ready to be written. The value provides introspection data regarding the status of the highWaterMark.

# writable.writableNeedDrain

• {Boolean} Is true if the stream's buffer has been full and stream will emit drain.

# writable.writableObjectMode

• {Boolean} Getter for the property objectMode of a given Writable stream.

# Writable Events

# 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.

A Writable stream will always emit the 'close' event if it is created with the emitClose option.

# 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.

# Example

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

# error

• {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 closed when the 'error' event is emitted unless the autoDestroy option was set to false when creating the stream.

After 'error', no further events other than 'close' should be emitted (including 'error' events).

# finish

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

# Example

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

# pipe

• src {Readable} Source stream that is piping to this writable.

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

# Example

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('pipe', (src) => {
  console.log('Something is piping into the writer.');
  assert.equal(src, reader);
});
reader.pipe(writer);

# unpipe

• src {Readable} The source stream that unpiped this writable.

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.

# Example

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

# Writable API Implementers

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.

# new stream.Writable([options])

• options {Object}
  • highWaterMark {Integer} Buffer level when stream.write() starts returning false. default: 16KB or 16 for objectMode streams.
  • decodeStrings {Boolean} Whether to encode strings passed to stream.write() to Buffers (with the encoding specified in the stream.write() call) before passing them to stream._write(). Other types of data are not converted (i.e. Buffers are not decoded into strings). Setting to false will prevent strings from being converted. default: true.
  • defaultEncoding {String} The default encoding that is used when no encoding is specified as an argument to stream.write(). The valid encoding can be: 'utf8', 'utf-8', 'hex', 'base64', 'ascii'. default: 'utf8'.
  • emitClose {Boolean} Whether or not the stream should emit 'close' after it has been destroyed. default: true.
  • autoDestroy {Boolean} Whether this stream should automatically call .destroy() on itself after ending. default: true.
  • alertWaterMark {Integer} JSRE expansion, when stream buffer size exceeds the alertWaterMark, the writing data will be ignored, alertWaterMark must bigger than highWaterMark. default: Infinity - not ingored data forever.
  • autoAlert {Boolean} JSRE expansion, default: true - emit error when buf size reach alertWaterMark otherwise ignore.
  • objectMode {Boolean} Whether or not the stream.write(anyObj) is a valid operation. When set, it becomes possible to write JavaScript values other than string, Bufferif supported by the stream implementation. default: false.
  • construct {Function} Implementation for the stream._construct() method.
  • write {Function} Implementation for the stream._write() method.
  • destroy {Function} Implementation for the stream._destroy() method.
  • final {Function} Implementation for the stream._final() method.

# Example

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

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

Or, when using pre-ES6 style constructors:

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

function MyWritable(options) {
  if (!(this instanceof MyWritable)) {
    return new MyWritable(options);
  }
  Writable.call(this, options);
}
util.inherits(MyWritable, Writable);

Or, using the Simplified Constructor approach:

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

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

# writable._construct(callback)

• callback {Function} Call this function (optionally with an error argument) when the stream has finished initializing.

The _construct() method MUST NOT be called directly. It may be implemented by child classes, and if so, will be called by the internal Writable class methods only.

This optional function will be called in a tick after the stream constructor has returned, delaying any _write(), _final() and _destroy() calls until callback is called. This is useful to initialize state or asynchronously initialize resources before the stream can be used.

# Example

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

class WriteStream extends Writable {
  constructor(filename) {
    super();
    this.filename = filename;
    this.file = undefined;
  }

  _construct(callback) {
    var file = fs.open(this.filename, 'w', 0o666);
    if (!file) {
      callback(new Error('Open file fail.'));
    } else {
      this.file = file;
      callback();
    }
  }

  _write(chunk, encoding, callback) {
    try {
      var size = this.file.write(chunk);
      callback(null, size);
    } catch(e) {
      callback(e);
    }
  }

  _destroy(err, callback) {
    if (this.file) {
      this.file.close();
      this.file = undefined;
      callback(err);
    } else {
      callback(err);
    }
  }
}

# writable._write(chunk, encoding, callback)

• chunk {String | Buffer | Any} The Buffer to be written, converted from the string passed to stream.write(). If the stream's decodeStrings option is false or the stream is operating in object mode, the chunk will not be converted & will be whatever was passed to stream.write().
• encoding {String} If the chunk is a string, then encoding is the character encoding of that string. If chunk is a Buffer, or if the stream is operating in object mode, encoding may be ignored.
• callback Call this function (optionally with an error argument) when processing is complete for the supplied chunk.

All Writable stream implementations must provide a writable._write() method to send data to the underlying resource.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Writable class methods only.

The callback function must be called synchronously inside of writable._write() or asynchronously (i.e. different tick) to signal either that the write completed successfully or failed with an error. The first argument passed to the callback must be the Error object if the call failed or null if the write succeeded.

All calls to writable.write() that occur between the time writable._write() is called and the callback is called will cause the written data to be buffered. When the callback is invoked, the stream might emit a 'drain' event.

The writable._write() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

# writable._destroy(err, callback)

• err {Error} A possible error.
• callback {Function} A callback function that takes an optional error argument.
  • error {Error} Error object.

The _destroy() method is called by writable.destroy(). It can be overridden by child classes but it must not be called directly.

# writable._final(callback)

• callback {Function} Call this function (optionally with an error argument) when finished writing any remaining data.
  • error {Error} Error object.

The _final() method must not be called directly. It may be implemented by child classes, and if so, will be called by the internal Writable class methods only.

This optional function will be called before the stream closes, delaying the 'finish' event until callback is called. This is useful to close resources or write buffered data before a stream ends.

# Errors While Writing

Errors occurring during the processing of the writable._write() and writable._final() methods must be propagated by invoking the callback and passing the error as the first argument. Throwing an Error from within these methods or manually emitting an 'error' event results in undefined behavior.

If a Readable stream pipes into a Writable stream when Writable emits an error, the Readable stream will be unpiped.

# Example

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

const myWritable = new Writable({
  write(chunk, callback) {
    if (chunk.toString().indexOf('a') >= 0) {
      callback(new Error('chunk is invalid'));
    } else {
      callback();
    }
  }
});

# An Example Writable Stream

The following illustrates a rather simplistic (and somewhat pointless) custom Writable stream implementation. While this specific Writable stream instance is not of any real particular usefulness, the example illustrates each of the required elements of a custom Writable stream instance:

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

class MyWritable extends Writable {
  _write(chunk, callback) {
    if (chunk.toString().indexOf('a') >= 0) {
      callback(new Error('chunk is invalid'));
    } else {
      callback();
    }
  }
}

# Readable Object

# readable.destroy([error])

• error Error which will be passed as payload in 'error' event.
• Returns: this.

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 noop and no further errors except from _destroy may be emitted as 'error'.

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

# readable.isPaused()

• Returns: {Boolean} Stream paused or not.

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.

# Example

const readable = new stream.Readable();

readable.isPaused(); // === false
readable.pause();
readable.isPaused(); // === true
readable.resume();
readable.isPaused(); // === false

# readable.pause()

• Returns: 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.

# Example

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);
});

# readable.pipe(destination[, options])

• destination {Writable} The destination for writing data.
• options {Object} Pipe options:
  • end {Boolean} End the writer when the reader ends. default: true.
• Returns: {Writable} The destination, allowing for a chain of pipes if it is a Duplex stream.

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);

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.

# readable.read([size])

• size {Integer} Optional argument to specify how much data to read.
• Returns: {Buffer | String | null} Read data.

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. If encoding is 'utf8' and a byte sequence in read buffer is not valid UTF-8, then the character \uFFFD will be returns.

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.`);
  }
});

The while loop is necessary when processing data with readable.read(). Only after readable.read() returns null, 'readable' will be emitted.

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.resume()

• Returns: 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.');
  });

# readable.setEncoding(encoding)

• encoding {String} The encoding to use.
• Returns: {Readable} This object.

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.

# Example

const read = () => { readAble.push('hello') };
const readAble = new Readable({ read });
readAble.setEncoding('hex');
readAble.on('data', (str) => {
  console.log(str);
});

# readable.unpipe([destination])

• destination {Writable} Optional specific stream to unpipe.
• Returns: this.

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.

# Example

const fs = require('stream').FsReadStream;
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.destroyed

• {Boolean} Is true after readable.destroy() has been called.

# readable.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'.

# readable.readableEncoding

• {null | String} Getter for the property encoding of a given Readable stream. The encoding property can be set using the readable.setEncoding() method.

# readable.readableEnded

• {Boolean} Becomes true when 'end' event is emitted.

# readable.readableFlowing

• {Boolean} This property reflects the current state of a Readable stream as described in the Stream Three States section.

# readable.readableHighWaterMark

• {Integer} Returns the value of highWaterMark passed when constructing this Readable.

# readable.readableLength

• {Integer} This property contains the number of bytes in the queue ready to be read. The value provides introspection data regarding the status of the highWaterMark.

# readable.readableObjectMode

• {Boolean} Getter for the property objectMode of a given Readable stream.

# Readable Events

# 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.

A Readable stream will always emit the 'close' event if it is created with the emitClose option.

# data

• chunk {Buffer | String} 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. If the stream using the readable.setEncoding() method set stream encoding, the character \uFFFD will be returns while a byte sequence in read buffer is not valid encoding.

# Example

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

# 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.

# Example

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.');
});

# error

• err {Error} Error object.

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.

# pause

The 'pause' event is emitted when stream.pause() is called and readableFlowing is not false.

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

# resume

The 'resume' event is emitted when stream.resume() is called and readableFlowing is not true.

# Readable API Implementers

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

Custom Readable streams must call the new stream.Readable(options]) constructor and implement the readable._read() method.

# new stream.Readable([options])

• options
  • highWaterMark {Integer} The maximum number of bytes to store in the internal buffer before ceasing to read from the underlying resource. default: 16KB or 16 for objectMode streams.
  • encoding {String} If specified, then buffers will be decoded to strings using the specified encoding. default: null.
  • objectMode {Boolean} Whether this stream should behave as a stream of objects. Meaning that stream.read(n) returns a single value instead of a Buffer of size n. default: false.
  • emitClose {Boolean} Whether or not the stream should emit 'close' after it has been destroyed. default: true.
  • autoDestroy {Boolean} Whether this stream should automatically call .destroy() on itself after ending. default: true.
  • alertWaterMark {Integer} JSRE expansion, when stream buffer size exceeds the 'alertWaterMark', the underlying reading data will be ignored, 'alertWaterMark' must bigger than 'highWaterMark'. default: Infinity - not ingored data forever.
  • autoAlert {Boolean} JSRE expansion, default: true - emit error when buf size reach alertWaterMark otherwise ignore.
  • construct {Function} Implementation for the stream._construct() method.
  • read {Function} Implementation for the stream._read() method.
  • destroy {Function} Implementation for the stream._destroy() method.

# Example

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

class MyReadable extends Readable {
  constructor(options) {
    // Calls the stream.Readable(options) constructor.
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Readable } = require('stream');
const util = require('util');

function MyReadable(options) {
  if (!(this instanceof MyReadable)) {
    return new MyReadable(options);
  }
  Readable.call(this, options);
}
util.inherits(MyReadable, Readable);

Or, using the Simplified Constructor approach:

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

const myReadable = new Readable({
  read(size) {
    // ...
  }
});

# readable._construct(callback)

• callback {Function} Call this function (optionally with an error argument) when the stream has finished initializing.

The _construct() method MUST NOT be called directly. It may be implemented by child classes, and if so, will be called by the internal Readable class methods only.

This optional function will be scheduled in the next tick by the stream constructor, delaying any _read() and _destroy() calls until callback is called. This is useful to initialize state or asynchronously initialize resources before the stream can be used.

# Example

const { Readable } = require('stream');
const fs = require('fs');

class ReadStream extends Readable {
  constructor(filename) {
    super();
    this.filename = filename;
    this.file = undefined;
  }

  _construct(callback) {
    var file = fs.open(this.filename, 'r', 0o666);
    if (!file) {
      callback(new Error('Open file fail.'));
    } else {
      this.file = file;
      callback();
    }
  }

  _read(n) {
    try {
      var buf = new Buffer(n);
      var num = this.file.read(buf, 0, n);
      this.push(num > 0 ? buf.slice(0, num) : null);
    } catch(e) {
      this.destroy(e);
    }
  }

  _destroy(err, callback) {
    if (this.file) {
      this.file.close();
      this.file = undefined;
      callback(err);
    } else {
      callback(err);
    }
  }
}

# readable._read([size])

• size Number of bytes to read asynchronously

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Readable class methods only.

All Readable stream implementations must provide an implementation of the readable._read() method to fetch data from the underlying resource.

When readable._read() is called, if data is available from the resource, the implementation should begin pushing that data into the read queue using the this.push(dataChunk) method. _read()should continue reading from the resource and pushing data until readable.push() returns false. Only when _read() is called again after it has stopped should it resume pushing additional data onto the queue.

Once the readable._read() method has been called, it will not be called again until more data is pushed through the readable.push() method. Empty data such as empty buffers and strings will not cause readable._read() to be called.

The size argument is advisory. For implementations where a "read" is a single operation that returns data can use the size argument to determine how much data to fetch. Other implementations may ignore this argument and simply provide data whenever it becomes available. There is no need to "wait" until size bytes are available before calling stream.push(chunk).

The readable._read() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

# readable._destroy(err, callback)

• err {Error} A possible error.
• callback {Function} A callback function that takes an optional error argument.
  • error {Error} Error object.

The _destroy() method is called by readable.destroy(). It can be overridden by child classes but it must not be called directly.

# readable.push(chunk[, encoding])

• chunk {Buffer | String | null | Any} Chunk of data to push into the read queue. For streams not operating in object mode, chunk must be a string or Buffer. For object mode streams, chunk may be any JavaScript value.
• encoding {String} Encoding of string chunks. Must be a valid Buffer encoding, such as 'utf8' or 'ascii'.
• Returns: {Boolean} true if additional chunks of data may continue to be pushed; false otherwise.

When chunk is a Buffer,the chunk of data will be added to the internal queue for users of the stream to consume. Passing chunk as null signals the end of the stream (EOF), after which no more data can be written.

When the Readable is operating in paused mode, the data added with readable.push() can be read out by calling the readable.read() method when the 'readable' event is emitted.

When the Readable is operating in flowing mode, the data added with readable.push() will be delivered by emitting a 'data' event.

The readable.push() method is designed to be as flexible as possible. For example, when wrapping a lower-level source that provides some form of pause/resume mechanism, and a data callback, the low-level source can be wrapped by the custom Readable instance:

// `_source` is an object with readStop() and readStart() methods,
// and an `ondata` member that gets called when it has data, and
// an `onend` member that gets called when the data is over.

class SourceWrapper extends Readable {
  constructor(options) {
    super(options);

    this._source = getLowLevelSourceObject();

    // Every time there's data, push it into the internal buffer.
    this._source.ondata = (chunk) => {
      // If push() returns false, then stop reading from source.
      if (!this.push(chunk))
        this._source.readStop();
    };

    // When the source ends, push the EOF-signaling `null` chunk.
    this._source.onend = () => {
      this.push(null);
    };
  }
  // _read() will be called when the stream wants to pull more data in.
  // The advisory size argument is ignored in this case.
  _read(size) {
    this._source.readStart();
  }
}

The readable.push() method is used to push the content into the internal buffer. It can be driven by the readable._read() method.

For streams not operating in object mode, if the chunk parameter of readable.push() is undefined, it will be treated as empty string or buffer. See readable.push('') for more information.

# Errors While Reading

Errors occurring during processing of the readable._read() must be propagated through the readable.destroy(err) method. Throwing an Error from within readable._read() or manually emitting an 'error' event results in undefined behavior.

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

const myReadable = new Readable({
  read(size) {
    const err = checkSomeErrorCondition();
    if (err) {
      this.destroy(err);
    } else {
      // Do some work.
    }
  }
});

# An Example Counting Stream

The following is a basic example of a Readable stream that emits the numerals from 1 to 1,000,000 in ascending order, and then ends.

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

class Counter extends Readable {
  constructor(opt) {
    super(opt);
    this._max = 1000000;
    this._index = 1;
  }

  _read() {
    const i = this._index++;
    if (i > this._max)
      this.push(null);
    else {
      const str = String(i);
      const buf = Buffer.from(str);
      this.push(buf);
    }
  }
}

# Duplex API Implementers

A Duplex stream is one that implements both Readable and Writable, such as a TCP socket connection.

Because JavaScript does not have support for multiple inheritance, the stream.Duplex class is extended to implement a Duplex stream (as opposed to extending the stream.Readable andstream.Writable classes).

The stream.Duplex class prototypically inherits from stream.Readable and parasitically from stream.Writable.

Custom Duplex streams must call the new stream.Duplex(options]) constructor and implement both the [readable._read() and writable._write() methods.

# new stream.Duplex(options)

• options {Object} Passed to both Writable and Readable constructors. Also has the following fields:
  • allowHalfOpen {Boolean} If set to false, then the stream will automatically end the writable side when the readable side ends. default: true.
  • readableObjectMode {Boolean} Sets objectMode for readable side of the stream. Has no effect if objectMode is true. default: false.
  • writableObjectMode {Boolean} Sets objectMode for writable side of the stream. Has no effect if objectMode is true. default: false.
  • readableHighWaterMark {Integer} Sets highWaterMark for the readable side of the stream. Has no effect if highWaterMark is provided.
  • writableHighWaterMark {Integer} Sets highWaterMark for the writable side of the stream. Has no effect if highWaterMark is provided.

# Example

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

class MyDuplex extends Duplex {
  constructor(options) {
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Duplex } = require('stream');
const util = require('util');

function MyDuplex(options) {
  if (!(this instanceof MyDuplex))
    return new MyDuplex(options);
  Duplex.call(this, options);
}
util.inherits(MyDuplex, Duplex);

Or, using the Simplified Constructor approach:

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

const myDuplex = new Duplex({
  read(size) {
    // ...
  },
  write(chunk, encoding, callback) {
    // ...
  }
});

# An Example Duplex Stream

The following illustrates a simple example of a Duplex stream that wraps a hypothetical lower-level source object to which data can be written, and from which data can be read. The following illustrates a simple example of a Duplex stream that buffers incoming written data via the Writable interface that is read back out via the Readable interface.

const { Duplex } = require('stream');
const kSource = Symbol('source');

class MyDuplex extends Duplex {
  constructor(source, options) {
    super(options);
    this[kSource] = source;
  }

  _write(chunk, encode, callback) {
    if (!Buffer.isBuffer(chunk)) {
      encode = encode || 'utf-8';
      chunk = Buffer.from(chunk, encode);
    }
    this[kSource].writeSomeData(chunk);
    callback();
  }

  _read(size) {
    this[kSource].fetchSomeData(size, (data, encoding) => {
      this.push(Buffer.from(data, encoding));
    });
  }
}

The most important aspect of a Duplex stream is that the Readable and Writable sides operate independently of one another despite co-existing within a single object instance.

# Object mode duplex streams

For Duplex streams, objectMode can be set exclusively for either the Readable or Writable side using the readableObjectMode and writableObjectMode options respectively.

In the following example, for instance, a new Transform stream (which is a type of Duplex stream) is created that has an object mode Writable side that accepts JavaScript numbers that are converted to hexadecimal strings on the Readable side.

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

// All Transform streams are also Duplex Streams.
const myTransform = new Transform({
  writableObjectMode: true,

  transform(chunk, encoding, callback) {
    // Coerce the chunk to a number if necessary.
    chunk |= 0;

    // Transform the chunk into something else.
    const data = chunk.toString(16);

    // Push the data onto the readable queue.
    callback(null, '0'.repeat(data.length % 2) + data);
  }
});

myTransform.setEncoding('ascii');
myTransform.on('data', (chunk) => console.log(chunk));

myTransform.write(1);
// Prints: 01
myTransform.write(10);
// Prints: 0a
myTransform.write(100);
// Prints: 64

# Transform API Implementers

A Transform stream is a Duplex stream where the output is computed in some way from the input. Examples include zlib streams or crypto streams that compress, encrypt, or decrypt data.

There is no requirement that the output be the same size as the input, the same number of chunks, or arrive at the same time. For example, a Hash stream will only ever have a single chunk of output which is provided when the input is ended. A zlib stream will produce output that is either much smaller or much larger than its input.

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

The stream.Transform class prototypically inherits from stream.Duplex and implements its own versions of the writable._write() and readable._read() methods. Custom Transform implementations must implement the transform._transform() method and may also implement the transform._flush() method.

Care must be taken when using Transform streams in that data written to the stream can cause the Writable side of the stream to become paused if the output on the Readable side is not consumed.

# new stream.Transform(options)

• options {Object} Passed to both Writable and Readable constructors. Also has the following fields:
  • transform {Function} Implementation for the stream._transform() method.
  • flush {Function} Implementation for the stream._flush() method.

# Example

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

class MyTransform extends Transform {
  constructor(options) {
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Transform } = require('stream');
const util = require('util');

function MyTransform(options) {
  if (!(this instanceof MyTransform)) {
    return new MyTransform(options);
  }
  Transform.call(this, options);
}
util.inherits(MyTransform, Transform);

Or, using the simplified constructor approach:

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

const myTransform = new Transform({
  transform(chunk, encoding, callback) {
    // ...
  }
});

# transform._flush(callback)

• callback {Function} A callback function (optionally with an error argument and data) to be called when remaining data has been flushed.
  • error {Error} Error object.
  • chunk {Buffer | String | null} Data to be written if exist.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Readable class methods only.

In some cases, a transform operation may need to emit an additional bit of data at the end of the stream. For example, a zlib compression stream will store an amount of internal state used to optimally compress the output. When the stream ends, however, that additional data needs to be flushed so that the compressed data will be complete.

Custom Transform implementations may implement the transform._flush() method. This will be called when there is no more written data to be consumed, but before the 'end' event is emitted signaling the end of the Readable stream.

Within the transform._flush() implementation, the transform.push() method may be called zero or more times, as appropriate. The callback function must be called when the flush operation is complete.

The transform._flush() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

# transform._transform(chunk, encoding, callback)

• chunk {Buffer | String | Any} The Buffer to be transformed, converted from the string passed to stream.write(). If the stream's decodeStrings option is false or the stream is operating in object mode, the chunk will not be converted & will be whatever was passed to stream.write().

• encoding {String} If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value buffer. Ignore it in that case.

• callback {Function} A callback function (optionally with an error argument and data) to be called after the supplied chunk has been processed.

  • error {Error} Error object.
  • chunk {Buffer | String | null} Data to be written if exist.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Readable class methods only.

All Transform stream implementations must provide a _transform() method to accept input and produce output. The transform._transform() implementation handles the bytes being written, computes an output, then passes that output off to the readable portion using the transform.push() method.

The transform.push() method may be called zero or more times to generate output from a single input chunk, depending on how much is to be output as a result of the chunk.

It is possible that no output is generated from any given chunk of input data.

The callback function must be called only when the current chunk is completely consumed. The first argument passed to the callback must be an Error object if an error occurred while processing the input or null otherwise. If a second argument is passed to the callback, it will be forwarded on to the transform.push() method. In other words, the following are equivalent:

transform.prototype._transform = function (data, encoding, callback) {
  this.push(data);
  callback();
};

transform.prototype._transform = function (data, encoding, callback) {
  callback(null, data);
};

The transform._transform() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

transform._transform() is never called in parallel; streams implement a queue mechanism, and to receive the next chunk, callback must be called, either synchronously or asynchronously.

# Throttle Readable Class

Throttle is a special Readable class suitable for reading current limit, which can be used to limit the rate of Readable.

# new Throttle(src[, opts[, streamOpts]])

• src {Readable} Source Readable stream object.
• opts {Object} Options.
  • rate {Integer} Read rate: Number of bytes read per second.
• streamOpts {Object} Options for creating this Readable Stream.
• Returns: {Readable} Throttle readable object.

Create a Readable stream object that limits the read rate.

// Create a stream that reads the specified file
var src = fs.createReadStream('./data.txt');

// Create a stream with a read rate of 50 KBytes/s
var thr = new Throttle(src, { rate: 50 * 1024 });

// Start time
var start = Date.now();

// Read data from throttle readable stream
thr.on('data', () => {});
thr.on('end', () => {
  var cost = Date.now() - start;
  console.log('cost:', cost, 'ms');
});

Use this object to limit network access traffic, etc. This class is available in EdgerOS 1.9.8 and later.

# Throttle Readable Object

# throttle.rate

• {Integer} Read rate: Number of bytes read per second.

throttle.rate is a getter and setter, used to get and set read rate, users can dynamically set this parameter.

// Create a stream with a read rate ability
var thr = new Throttle(src);

// Set read rate 128 KBytes/s
thr.rate = 128 * 1024;