Stream
目录
- Organization of this document
- Types of streams
- API for stream consumers
- Writable streams
- Readable streams
- Duplex and transform streams
- stream.finished(stream[, options], callback)
- stream.pipeline(source[, ...transforms], destination, callback)
- stream.pipeline(streams, callback)
- stream.compose(...streams)
- stream.Readable.from(iterable[, options])
- stream.Readable.fromWeb(readableStream[, options])
- stream.Readable.isDisturbed(stream)
- stream.isErrored(stream)
- stream.isReadable(stream)
- stream.Readable.toWeb(streamReadable[, options])
- stream.Writable.fromWeb(writableStream[, options])
- stream.Writable.toWeb(streamWritable)
- stream.Duplex.from(src)
- stream.Duplex.fromWeb(pair[, options])
- stream.Duplex.toWeb(streamDuplex)
- stream.addAbortSignal(signal, stream)
- API for stream implementers
- Additional notes
Added in: v0.10.0
源代码: lib/stream.js
A stream is an abstract interface for working with streaming data in Node.js.
The node:stream
module provides an API for implementing 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
.
To access the node:stream
module:
JS
The node:stream
module is useful for creating new types of stream instances.
It is usually not necessary to use the node:stream
module to consume streams.
Organization of this document
This document contains two primary sections and a third section for notes. The first section explains how to use existing streams within an application. The second section explains how to create new types of streams.
Types of streams
There are four fundamental stream types within Node.js:
Writable
: streams to which data can be written (for example,fs.createWriteStream()
).Readable
: streams from which data can be read (for example,fs.createReadStream()
).Duplex
: streams that are bothReadable
andWritable
(for example,net.Socket
).Transform
:Duplex
streams that can modify or transform the data as it is written and read (for example,zlib.createDeflate()
).
Additionally, this module includes the utility functions
stream.pipeline()
, stream.finished()
, stream.Readable.from()
and stream.addAbortSignal()
.
Streams Promises API
Added in: v15.0.0
The stream/promises
API provides an alternative set of asynchronous utility
functions for streams that return Promise
objects rather than using
callbacks. The API is accessible via require('node:stream/promises')
or require('node:stream').promises
.
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.
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 Duplex
and Transform
streams 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.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, each side should
operate (and buffer) independently of the other.
The mechanics of the internal buffering are an internal implementation detail
and may be changed at any time. However, for certain advanced implementations,
the internal buffers can be retrieved using writable.writableBuffer
or
readable.readableBuffer
. Use of these undocumented properties is discouraged.
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:
JS
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('node: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:
- HTTP requests, on the client
- HTTP responses, on the server
- fs write streams
- zlib streams
- crypto streams
- TCP sockets
- child process stdin
process.stdout
,process.stderr
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:
JS
C stream.Writable
Added in: v0.9.4
Event: 'close'
历史
版本 | 更改 |
---|---|
v10.0.0 | Add `emitClose` option to specify if `'close'` is emitted on destroy. |
v0.9.4 | Added in: v0.9.4 |
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.
Event: 'drain'
Added in: v0.9.4
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.
JS
Event: 'error'
Added in: v0.9.4
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).
Event: 'finish'
Added in: v0.9.4
The 'finish'
event is emitted after the stream.end()
method
has been called, and all data has been flushed to the underlying system.
JS
Event: 'pipe'
Added in: v0.9.4
src
stream.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.
JS
Event: 'unpipe'
Added in: v0.9.4
src
stream.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.
JS
writable.cork()
Added in: v0.11.2
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 accommodate a situation in which
several small chunks are written to the stream in rapid succession. Instead of
immediately forwarding them to the underlying destination, writable.cork()
buffers all the chunks until writable.uncork()
is called, which will pass them
all to writable._writev()
, if present. This prevents a head-of-line blocking
situation where data is being buffered while waiting for the first small chunk
to be processed. However, use of writable.cork()
without implementing
writable._writev()
may have an adverse effect on throughput.
See also: writable.uncork()
, writable._writev()
.
writable.destroy([error])
历史
版本 | 更改 |
---|---|
v14.0.0 | Work as a no-op on a stream that has already been destroyed. |
v8.0.0 | Added in: v8.0.0 |
Destroy the stream. Optionally emit an 'error'
event, and emit a 'close'
event (unless emitClose
is set to false
). After this call, the writable
stream has ended and subsequent calls to write()
or end()
will result in
an ERR_STREAM_DESTROYED
error.
This is a destructive and immediate way to destroy a stream. Previous calls to
write()
may not have drained, and may trigger an ERR_STREAM_DESTROYED
error.
Use end()
instead of destroy if data should flush before close, or wait for
the 'drain'
event before destroying the stream.
CJS
CJS
CJS
Once destroy()
has been called any further calls will be a no-op and no
further errors except from _destroy()
may be emitted as 'error'
.
Implementors should not override this method,
but instead implement writable._destroy()
.
writable.closed
Added in: v18.0.0
Is true
after 'close'
has been emitted.
writable.destroyed
Added in: v8.0.0
Is true
after writable.destroy()
has been called.
CJS
writable.end([chunk[, encoding]][, callback])
历史
版本 | 更改 |
---|---|
v15.0.0 | The `callback` is invoked before 'finish' or on error. |
v14.0.0 | The `callback` is invoked if 'finish' or 'error' is emitted. |
v10.0.0 | This method now returns a reference to `writable`. |
v8.0.0 | The `chunk` argument can now be a `Uint8Array` instance. |
v0.9.4 | Added in: v0.9.4 |
chunk
string
|Buffer
|Uint8Array
|any
Optional data to write. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value other thannull
.encoding
string
The encoding ifchunk
is a stringcallback
Function
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.
Calling the stream.write()
method after calling
stream.end()
will raise an error.
JS
writable.setDefaultEncoding(encoding)
历史
版本 | 更改 |
---|---|
v6.1.0 | This method now returns a reference to `writable`. |
v0.11.15 | Added in: v0.11.15 |
The writable.setDefaultEncoding()
method sets the default encoding
for a
Writable
stream.
writable.uncork()
Added in: v0.11.2
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, defer calls to writable.uncork()
using
process.nextTick()
. Doing so allows batching of all
writable.write()
calls that occur within a given Node.js event loop phase.
JS
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.
JS
See also: writable.cork()
.
writable.writable
Added in: v11.4.0
Is true
if it is safe to call writable.write()
, which means
the stream has not been destroyed, errored, or ended.
writable.writableAborted
Added in: v18.0.0, v16.17.0
Returns whether the stream was destroyed or errored before emitting 'finish'
.
writable.writableEnded
Added in: v12.9.0
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.writableCorked
Added in: v13.2.0, v12.16.0
Number of times writable.uncork()
needs to be
called in order to fully uncork the stream.
writable.errored
Added in: v18.0.0
Returns error if the stream has been destroyed with an error.
writable.writableFinished
Added in: v12.6.0
Is set to true
immediately before the 'finish'
event is emitted.
writable.writableHighWaterMark
Added in: v9.3.0
Return the value of highWaterMark
passed when creating this Writable
.
writable.writableLength
Added in: v9.4.0
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.writableNeedDrain
Added in: v15.2.0, v14.17.0
Is true
if the stream's buffer has been full and stream will emit 'drain'
.
writable.writableObjectMode
Added in: v12.3.0
Getter for the property objectMode
of a given Writable
stream.
writable.write(chunk[, encoding][, callback])
历史
版本 | 更改 |
---|---|
v8.0.0 | The `chunk` argument can now be a `Uint8Array` instance. |
v6.0.0 | Passing `null` as the `chunk` parameter will always be considered invalid now, even in object mode. |
v0.9.4 | Added in: v0.9.4 |
chunk
string
|Buffer
|Uint8Array
|any
Optional data to write. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value other thannull
.encoding
string
|null
The encoding, ifchunk
is a string. Default:'utf8'
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; otherwisetrue
.
The writable.write()
method writes some data to the stream, and calls the
supplied callback
once the data has been fully handled. If an error
occurs, the callback
will be called with the error as its
first argument. The callback
is called asynchronously and before 'error'
is
emitted.
The return value is true
if the internal buffer is less than 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.
Once write()
returns false, do not write more chunks
until the 'drain'
event is emitted. While calling write()
on a stream that
is not draining is allowed, Node.js will buffer all written chunks until
maximum memory usage occurs, at which point it will abort unconditionally.
Even before it aborts, high memory usage will cause poor garbage collector
performance and high RSS (which is not typically released back to the system,
even after the memory is no longer required). Since TCP sockets may never
drain if the remote peer does not read the data, writing a socket that is
not draining may lead to a remotely exploitable vulnerability.
Writing data while the stream is not draining is particularly
problematic for a Transform
, because the Transform
streams are paused
by default until they are piped or a 'data'
or 'readable'
event handler
is added.
If the data to be written can be generated or fetched on demand, it is
recommended to encapsulate the logic into a Readable
and use
stream.pipe()
. However, if calling write()
is preferred, it is
possible to respect backpressure and avoid memory issues using the
'drain'
event:
JS
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:
- HTTP responses, on the client
- HTTP requests, on the server
- fs read streams
- zlib streams
- crypto streams
- TCP sockets
- child process stdout and stderr
process.stdin
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 aWritable
.
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.
Adding a 'readable'
event handler automatically makes the stream
stop flowing, and the data has 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
.
JS
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.
C stream.Readable
Added in: v0.9.4
Event: 'close'
历史
版本 | 更改 |
---|---|
v10.0.0 | Add `emitClose` option to specify if `'close'` is emitted on destroy. |
v0.9.4 | Added in: v0.9.4 |
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.
Event: 'data'
Added in: v0.9.4
chunk
Buffer
|string
|any
The chunk of data. For streams that are not operating in object mode, the chunk will be either a string orBuffer
. For streams that are in object mode, the chunk can be any JavaScript value other thannull
.
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
.
JS
Event: 'end'
Added in: v0.9.4
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.
JS
Event: 'error'
Added in: v0.9.4
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: 'pause'
Added in: v0.9.4
The 'pause'
event is emitted when stream.pause()
is called
and readableFlowing
is not false
.
Event: 'readable'
历史
版本 | 更改 |
---|---|
v10.0.0 | The `'readable'` is always emitted in the next tick after `.push()` is called. |
v10.0.0 | Using `'readable'` requires calling `.read()`. |
v0.9.4 | Added in: v0.9.4 |
The 'readable'
event is emitted when there is data available to be read from
the stream or when the end of the stream has been reached. Effectively, the
'readable'
event indicates that the stream has new information. If data is
available, stream.read()
will return that data.
JS
If the end of the stream has been reached, calling
stream.read()
will return null
and trigger the 'end'
event. This is also true if there never was any data to be read. For instance,
in the following example, foo.txt
is an empty file:
JS
The output of running this script is:
BASH
In some cases, attaching a listener for the 'readable'
event will cause some
amount of data to be read into an internal buffer.
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()
.
Event: 'resume'
Added in: v0.9.4
The 'resume'
event is emitted when stream.resume()
is
called and readableFlowing
is not true
.
readable.destroy([error])
历史
版本 | 更改 |
---|---|
v14.0.0 | Work as a no-op on a stream that has already been destroyed. |
v8.0.0 | Added in: v8.0.0 |
Destroy the stream. Optionally emit an 'error'
event, and emit a 'close'
event (unless emitClose
is set to false
). After this call, the readable
stream will release any internal resources and subsequent calls to push()
will be ignored.
Once destroy()
has been called any further calls will be a no-op and no
further errors except from _destroy()
may be emitted as 'error'
.
Implementors should not override this method, but instead implement
readable._destroy()
.
readable.closed
Added in: v18.0.0
Is true
after 'close'
has been emitted.
readable.destroyed
Added in: v8.0.0
Is true
after readable.destroy()
has been called.
readable.isPaused()
Added in: v0.11.14
- Returns:
boolean
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.
JS
readable.pause()
Added in: v0.9.4
- 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.
JS
The readable.pause()
method has no effect if there is a 'readable'
event listener.
readable.pipe(destination[, options])
Added in: v0.9.4
destination
stream.Writable
The destination for writing dataoptions
Object
Pipe optionsend
boolean
End the writer when the reader ends. Default:true
.
- Returns:
stream.Writable
The destination, allowing for a chain of pipes if it is aDuplex
or aTransform
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
:
JS
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:
JS
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:
JS
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])
Added in: v0.9.4
size
number
Optional argument to specify how much data to read.- Returns:
string
|Buffer
|null
|any
The readable.read()
method reads data out of the internal buffer and
returns it. If no data is available to be read, null
is returned. By default,
the data is 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 size
argument must be less than or equal to 1 GiB.
The readable.read()
method should only be called on Readable
streams
operating in paused mode. In flowing mode, readable.read()
is called
automatically until the internal buffer is fully drained.
JS
Each call to readable.read()
returns a chunk of data, or null
. The chunks
are not concatenated. A while
loop is necessary to consume all data
currently in the buffer. When reading a large file .read()
may return null
,
having consumed all buffered content so far, but there is still more data to
come not yet buffered. In this case a new 'readable'
event will be emitted
when there is more data in the buffer. Finally the 'end'
event will be
emitted when there is no more data to come.
Therefore to read a file's whole contents from a readable
, it is necessary
to collect chunks across multiple 'readable'
events:
JS
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.readable
Added in: v11.4.0
Is true
if it is safe to call readable.read()
, which means
the stream has not been destroyed or emitted 'error'
or 'end'
.
readable.readableAborted
Added in: v16.8.0
Returns whether the stream was destroyed or errored before emitting 'end'
.
readable.readableDidRead
Added in: v16.7.0, v14.18.0