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A very minimal implementation of a PassThrough stream
It's very fast for objects, strings, and buffers.
Supports pipe()ing (including multi-pipe() and backpressure transmission), buffering data until either a data
event handler or pipe()
is added (so you don't lose the first chunk), and most other cases where PassThrough is a good idea.
There is a read()
method, but it's much more efficient to consume data from this stream via ‘'data’events or by calling
pipe()into some other stream. Calling
read()` requires the buffer to be flattened in some cases, which requires copying memory.
There is also no unpipe()
method. Once you start piping, there is no stopping it!
If you set objectMode: true
in the options, then whatever is written will be emitted. Otherwise, it'll do a minimal amount of Buffer copying to ensure proper Streams semantics when read(n)
is called.
objectMode
can also be set by doing stream.objectMode = true
, or by writing any non-string/non-buffer data. objectMode
cannot be set to false once it is set.
This is not a through
or through2
stream. It doesn't transform the data, it just passes it right through. If you want to transform the data, extend the class, and override the write()
method. Once you're done transforming the data however you want, call super.write()
with the transform output.
For some examples of streams that extend Minipass in various ways, check out:
There are several things that make Minipass streams different from (and in some ways superior to) Node.js core streams.
Please read these caveats if you are familiar with noode-core streams and intend to use Minipass streams in your programs.
Minipass streams are designed to support synchronous use-cases. Thus, data is emitted as soon as it is available, always. It is buffered until read, but no longer. Another way to look at it is that Minipass streams are exactly as synchronous as the logic that writes into them.
This can be surprising if your code relies on PassThrough.write()
always providing data on the next tick rather than the current one, or being able to call resume()
and not have the entire buffer disappear immediately.
However, without this synchronicity guarantee, there would be no way for Minipass to achieve the speeds it does, or support the synchronous use cases that it does. Simply put, waiting takes time.
This non-deferring approach makes Minipass streams much easier to reason about, especially in the context of Promises and other flow-control mechanisms.
Node.js core streams will optimistically fill up a buffer, returning true
on all writes until the limit is hit, even if the data has nowhere to go. Then, they will not attempt to draw more data in until the buffer size dips below a minimum value.
Minipass streams are much simpler. The write()
method will return true
if the data has somewhere to go (which is to say, given the timing guarantees, that the data is already there by the time write()
returns).
If the data has nowhere to go, then write()
returns false, and the data sits in a buffer, to be drained out immediately as soon as anyone consumes it.
Since data written to a Minipass stream is immediately written all the way through the pipeline, and write()
always returns true/false based on whether the data was fully flushed, backpressure is communicated immediately to the upstream caller. This minimizes buffering.
Consider this case:
Along the way, the data was buffered and deferred at each stage, and multiple event deferrals happened, for an unblocked pipeline where it was perfectly safe to write all the way through!
Furthermore, setting a highWaterMark
of 1024
might lead someone reading the code to think an advisory maximum of 1KiB is being set for the pipeline. However, the actual advisory buffering level is the sum of highWaterMark
values, since each one has its own bucket.
Consider the Minipass case:
It is extremely unlikely that you don't want to buffer any data written, or ever buffer data that can be flushed all the way through. Neither node-core streams nor Minipass ever fail to buffer written data, but node-core streams do a lot of unnecessary buffering and pausing.
As always, the faster implementation is the one that does less stuff and waits less time to do it.
If a stream is not paused, and end()
is called before writing any data into it, then it will emit end
immediately.
If you have logic that occurs on the end
event which you don't want to potentially happen immediately (for example, closing file descriptors, moving on to the next entry in an archive parse stream, etc.) then be sure to call stream.pause()
on creation, and then stream.resume()
once you are ready to respond to the end
event.
One hazard of immediately emitting ‘'end’` is that you may not yet have had a chance to add a listener. In order to avoid this hazard, Minipass streams safely re-emit the ‘'end’event if a new listener is added after
'end'` has been emitted.
Ie, if you do ‘stream.on('end’, someFunction), and the stream has already emitted
end, then it will call the handler right away. (You can think of this somewhat like attaching a new
.then(fn)` to a previously-resolved Promise.)
To prevent calling handlers multiple times who would not expect multiple ends to occur, all listeners are removed from the ‘'end’` event whenever it is emitted.
A "tee stream" is a stream piping to multiple destinations:
Since Minipass streams immediately process any pending data through the pipeline when a new pipe destination is added, this can have surprising effects, especially when a stream comes in from some other function and may or may not have data in its buffer.
The solution is to create a dedicated tee-stream junction that pipes to both locations, and then pipe to that instead.
The same caveat applies to ‘on('data’)` event listeners. The first one added will immediately receive all of the data, leaving nothing for the second:
Using a dedicated tee-stream can be used in this case as well:
It's a stream! Use it like a stream and it'll most likely do what you want.
encoding
How would you like the data coming out of the stream to be encoded? Accepts any values that can be passed to Buffer.toString()
.objectMode
Emit data exactly as it comes in. This will be flipped on by default if you write() something other than a string or Buffer at any point. Setting objectMode: true
will prevent setting any encoding value.Implements the user-facing portions of Node.js's Readable
and Writable
streams.
write(chunk, [encoding], [callback])
- Put data in. (Note that, in the base Minipass class, the same data will come out.) Returns false
if the stream will buffer the next write, or true if it's still in "flowing" mode.end([chunk, [encoding]], [callback])
- Signal that you have no more data to write. This will queue an end
event to be fired when all the data has been consumed.setEncoding(encoding)
- Set the encoding for data coming of the stream. This can only be done once.pause()
- No more data for a while, please. This also prevents end
from being emitted for empty streams until the stream is resumed.resume()
- Resume the stream. If there's data in the buffer, it is all discarded. Any buffered events are immediately emitted.pipe(dest)
- Send all output to the stream provided. There is no way to unpipe. When data is emitted, it is immediately written to any and all pipe destinations.on(ev, fn)
, emit(ev, fn)
- Minipass streams are EventEmitters. Some events are given special treatment, however. (See below under "events".)promise()
- Returns a Promise that resolves when the stream emits end
, or rejects if the stream emits error
.collect()
- Return a Promise that resolves on end
with an array containing each chunk of data that was emitted, or rejects if the stream emits error
. Note that this consumes the stream data.concat()
- Same as collect()
, but concatenates the data into a single Buffer object. Will reject the returned promise if the stream is in objectMode, or if it goes into objectMode by the end of the data.read(n)
- Consume n
bytes of data out of the buffer. If n
is not provided, then consume all of it. If n
bytes are not available, then it returns null. Note consuming streams in this way is less efficient, and can lead to unnecessary Buffer copying.destroy([er])
- Destroy the stream. If an error is provided, then an ‘'error’event is emitted. If the stream has a
close()method, and has not emitted a
'close'event yet, then
stream.close()will be called. Any Promises returned by
.promise(),
.collect()or
.concat()` will be rejected. After being destroyed, writing to the stream will emit an error. No more data will be emitted if the stream is destroyed, even if it was previously buffered.bufferLength
Read-only. Total number of bytes buffered, or in the case of objectMode, the total number of objects.encoding
The encoding that has been set. (Setting this is equivalent to calling setEncoding(enc)
and has the same prohibition against setting multiple times.)flowing
Read-only. Boolean indicating whether a chunk written to the stream will be immediately emitted.emittedEnd
Read-only. Boolean indicating whether the end-ish events (ie, end
, prefinish
, finish
) have been emitted. Note that listening on any end-ish event will immediateyl re-emit it if it has already been emitted.writable
Whether the stream is writable. Default true
. Set to false
when end()
readable
Whether the stream is readable. Default true
.buffer
A yallist linked list of chunks written to the stream that have not yet been emitted. (It's probably a bad idea to mess with this.)pipes
A yallist linked list of streams that this stream is piping into. (It's probably a bad idea to mess with this.)destroyed
A getter that indicates whether the stream was destroyed.paused
True if the stream has been explicitly paused, otherwise false.objectMode
Indicates whether the stream is in objectMode
. Once set to true
, it cannot be set to false
.data
Emitted when there's data to read. Argument is the data to read. This is never emitted while not flowing. If a listener is attached, that will resume the stream.end
Emitted when there's no more data to read. This will be emitted immediately for empty streams when end()
is called. If a listener is attached, and end
was already emitted, then it will be emitted again. All listeners are removed when end
is emitted.prefinish
An end-ish event that follows the same logic as end
and is emitted in the same conditions where end
is emitted. Emitted after ‘'end’. *
finishAn end-ish event that follows the same logic as
endand is emitted in the same conditions where
endis emitted. Emitted after
'prefinish'. *
closeAn indication that an underlying resource has been released. Minipass does not emit this event, but will defer it until after
end` has been emitted, since it throws off some stream libraries otherwise.drain
Emitted when the internal buffer empties, and it is again suitable to write()
into the stream.readable
Emitted when data is buffered and ready to be read by a consumer.resume
Emitted when stream changes state from buffering to flowing mode. (Ie, when resume
is called, pipe
is called, or a data
event listener is added.)Minipass.isStream(stream)
Returns true
if the argument is a stream, and false otherwise. To be considered a stream, the object must be either an instance of Minipass, or an EventEmitter that has either a pipe()
method, or both write()
and end()
methods. (Pretty much any stream in node-land will return true
for this.)Here are some examples of things you can do with Minipass streams.
This is a bit slower because it concatenates the data into one chunk for you, but if you're going to do it yourself anyway, it's convenient this way:
You can iterate over streams synchronously or asynchronously in platforms that support it.
Synchronous iteration will end when the currently available data is consumed, even if the end
event has not been reached. In string and buffer mode, the data is concatenated, so unless multiple writes are occurring in the same tick as the read()
, sync iteration loops will generally only have a single iteration.
To consume chunks in this way exactly as they have been written, with no flattening, create the stream with the { objectMode: true }
option.
Asynchronous iteration will continue until the end event is reached, consuming all of the data.