A _very_ minimal implementation of a [PassThrough
stream](https://nodejs.org/api/stream.html#stream_class_stream_passthrough)

[It's very
fast](https://docs.google.com/spreadsheets/d/1oObKSrVwLX_7Ut4Z6g3fZW-AX1j1-k6w-cDsrkaSbHM/edit#gid=0)
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.

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:

- [minizlib](http://npm.im/minizlib)
- [fs-minipass](http://npm.im/fs-minipass)
- [tar](http://npm.im/tar)
- [minipass-collect](http://npm.im/minipass-collect)
- [minipass-flush](http://npm.im/minipass-flush)
- [minipass-pipeline](http://npm.im/minipass-pipeline)
- [tap](http://npm.im/tap)
- [tap-parser](http://npm.im/tap-parser)
- [treport](http://npm.im/treport)
- [minipass-fetch](http://npm.im/minipass-fetch)
- [pacote](http://npm.im/pacote)
- [make-fetch-happen](http://npm.im/make-fetch-happen)
- [cacache](http://npm.im/cacache)
- [ssri](http://npm.im/ssri)
- [npm-registry-fetch](http://npm.im/npm-registry-fetch)
- [minipass-json-stream](http://npm.im/minipass-json-stream)
- [minipass-sized](http://npm.im/minipass-sized)

## Differences from Node.js Streams

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 node-core streams and
intend to use Minipass streams in your programs.

You can avoid most of these differences entirely (for a very
small performance penalty) by setting `{async: true}` in the
constructor options.

### Timing

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.

Example:

```js
const Minipass = require('minipass')
const stream = new Minipass({ async: true })
stream.on('data', () => console.log('data event'))
console.log('before write')
stream.write('hello')
console.log('after write')
// output:
// before write
// data event
// after write
```

### Exception: Async Opt-In

If you wish to have a Minipass stream with behavior that more
closely mimics Node.js core streams, you can set the stream in
async mode either by setting `async: true` in the constructor
options, or by setting `stream.async = true` later on.

```js
const Minipass = require('minipass')
const asyncStream = new Minipass({ async: true })
asyncStream.on('data', () => console.log('data event'))
console.log('before write')
asyncStream.write('hello')
console.log('after write')
// output:
// before write
// after write
// data event <-- this is deferred until the next tick
```

Switching _out_ of async mode is unsafe, as it could cause data
corruption, and so is not enabled.  Example:

```js
const Minipass = require('minipass')
const stream = new Minipass({ encoding: 'utf8' })
stream.on('data', chunk => console.log(chunk))
stream.async = true
console.log('before writes')
stream.write('hello')
setStreamSyncAgainSomehow(stream) // <-- this doesn't actually exist!
stream.write('world')
console.log('after writes')
// hypothetical output would be:
// before writes
// world
// after writes
// hello
// NOT GOOD!
```

To avoid this problem, once set into async mode, any attempt to
make the stream sync again will be ignored.

```js
const Minipass = require('minipass')
const stream = new Minipass({ encoding: 'utf8' })
stream.on('data', chunk => console.log(chunk))
stream.async = true
console.log('before writes')
stream.write('hello')
stream.async = false // <-- no-op, stream already async
stream.write('world')
console.log('after writes')
// actual output:
// before writes
// after writes
// hello
// world
```

### No High/Low Water Marks

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 nothing is ever buffered unnecessarily, there is much less
copying data, and less bookkeeping about buffer capacity levels.

### Hazards of Buffering (or: Why Minipass Is So Fast)

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:

```js
const {PassThrough} = require('stream')
const p1 = new PassThrough({ highWaterMark: 1024 })
const p2 = new PassThrough({ highWaterMark: 1024 })
const p3 = new PassThrough({ highWaterMark: 1024 })
const p4 = new PassThrough({ highWaterMark: 1024 })

p1.pipe(p2).pipe(p3).pipe(p4)
p4.on('data', () => console.log('made it through'))

// this returns false and buffers, then writes to p2 on next tick (1)
// p2 returns false and buffers, pausing p1, then writes to p3 on next tick (2)
// p3 returns false and buffers, pausing p2, then writes to p4 on next tick (3)
// p4 returns false and buffers, pausing p3, then emits 'data' and 'drain'
// on next tick (4)
// p3 sees p4's 'drain' event, and calls resume(), emitting 'resume' and
// 'drain' on next tick (5)
// p2 sees p3's 'drain', calls resume(), emits 'resume' and 'drain' on next tick (6)
// p1 sees p2's 'drain', calls resume(), emits 'resume' and 'drain' on next
// tick (7)

p1.write(Buffer.alloc(2048)) // returns false
```

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:

```js
const m1 = new Minipass()
const m2 = new Minipass()
const m3 = new Minipass()
const m4 = new Minipass()

m1.pipe(m2).pipe(m3).pipe(m4)
m4.on('data', () => console.log('made it through'))

// m1 is flowing, so it writes the data to m2 immediately
// m2 is flowing, so it writes the data to m3 immediately
// m3 is flowing, so it writes the data to m4 immediately
// m4 is flowing, so it fires the 'data' event immediately, returns true
// m4's write returned true, so m3 is still flowing, returns true
// m3's write returned true, so m2 is still flowing, returns true
// m2's write returned true, so m1 is still flowing, returns true
// No event deferrals or buffering along the way!

m1.write(Buffer.alloc(2048)) // returns true
```

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.

### Immediately emit `end` for empty streams (when not paused)

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.

However, this is _usually_ not a problem because:

### Emit `end` When Asked

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.

### Emit `error` When Asked

The most recent error object passed to the `'error'` event is
stored on the stream.  If a new `'error'` event handler is added,
and an error was previously emitted, then the event handler will
be called immediately (or on `process.nextTick` in the case of
async streams).

This makes it much more difficult to end up trying to interact
with a broken stream, if the error handler is added after an
error was previously emitted.

### Impact of "immediate flow" on Tee-streams

A "tee stream" is a stream piping to multiple destinations:

```js
const tee = new Minipass()
t.pipe(dest1)
t.pipe(dest2)
t.write('foo') // goes to both 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.

```js
// WARNING! WILL LOSE DATA!
const src = new Minipass()
src.write('foo')
src.pipe(dest1) // 'foo' chunk flows to dest1 immediately, and is gone
src.pipe(dest2) // gets nothing!
```

One solution is to create a dedicated tee-stream junction that pipes to
both locations, and then pipe to _that_ instead.

```js
// Safe example: tee to both places
const src = new Minipass()
src.write('foo')
const tee = new Minipass()
tee.pipe(dest1)
tee.pipe(dest2)
src.pipe(tee) // tee gets 'foo', pipes to both locations
```

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:

```js
// WARNING! WILL LOSE DATA!
const src = new Minipass()
src.write('foo')
src.on('data', handler1) // receives 'foo' right away
src.on('data', handler2) // nothing to see here!
```

Using a dedicated tee-stream can be used in this case as well:

```js
// Safe example: tee to both data handlers
const src = new Minipass()
src.write('foo')
const tee = new Minipass()
tee.on('data', handler1)
tee.on('data', handler2)
src.pipe(tee)
```

All of the hazards in this section are avoided by setting `{
async: true }` in the Minipass constructor, or by setting
`stream.async = true` afterwards.  Note that this does add some
overhead, so should only be done in cases where you are willing
to lose a bit of performance in order to avoid having to refactor
program logic.

## USAGE

It's a stream!  Use it like a stream and it'll most likely do what you
want.

```js
const Minipass = require('minipass')
const mp = new Minipass(options) // optional: { encoding, objectMode }
mp.write('foo')
mp.pipe(someOtherStream)
mp.end('bar')
```

### OPTIONS

* `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.
* `async` Defaults to `false`.  Set to `true` to defer data
  emission until next tick.  This reduces performance slightly,
  but makes Minipass streams use timing behavior closer to Node
  core streams.  See [Timing](#timing) for more details.

### API

Implements the user-facing portions of Node.js's `Readable` and `Writable`
streams.

### Methods

* `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.  When
  data is emitted, it is immediately written to any and all pipe
  destinations.  (Or written on next tick in `async` mode.)
* `unpipe(dest)` - Stop piping to the destination stream.  This
  is immediate, meaning that any asynchronously queued data will
  _not_ make it to the destination when running in `async` mode.
    * `options.end` - Boolean, end the destination stream when
      the source stream ends.  Default `true`.
    * `options.proxyErrors` - Boolean, proxy `error` events from
      the source stream to the destination stream.  Note that
      errors are _not_ proxied after the pipeline terminates,
      either due to the source emitting `'end'` or manually
      unpiping with `src.unpipe(dest)`.  Default `false`.
* `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.

### Properties

* `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](http://npm.im/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](http://npm.im/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`.

### Events

* `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'`.
* `finish` An end-ish event that follows the same logic as `end` and is
  emitted in the same conditions where `end` is emitted.  Emitted after
  `'prefinish'`.
* `close` An 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.)

### Static Methods

* `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.)

## EXAMPLES

Here are some examples of things you can do with Minipass streams.

### simple "are you done yet" promise

```js
mp.promise().then(() => {
  // stream is finished
}, er => {
  // stream emitted an error
})
```

### collecting

```js
mp.collect().then(all => {
  // all is an array of all the data emitted
  // encoding is supported in this case, so
  // so the result will be a collection of strings if
  // an encoding is specified, or buffers/objects if not.
  //
  // In an async function, you may do
  // const data = await stream.collect()
})
```

### collecting into a single blob

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:

```js
mp.concat().then(onebigchunk => {
  // onebigchunk is a string if the stream
  // had an encoding set, or a buffer otherwise.
})
```

### iteration

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.

```js
const mp = new Minipass({ objectMode: true })
mp.write('a')
mp.write('b')
for (let letter of mp) {
  console.log(letter) // a, b
}
mp.write('c')
mp.write('d')
for (let letter of mp) {
  console.log(letter) // c, d
}
mp.write('e')
mp.end()
for (let letter of mp) {
  console.log(letter) // e
}
for (let letter of mp) {
  console.log(letter) // nothing
}
```

Asynchronous iteration will continue until the end event is reached,
consuming all of the data.

```js
const mp = new Minipass({ encoding: 'utf8' })

// some source of some data
let i = 5
const inter = setInterval(() => {
  if (i-- > 0)
    mp.write(Buffer.from('foo\n', 'utf8'))
  else {
    mp.end()
    clearInterval(inter)
  }
}, 100)

// consume the data with asynchronous iteration
async function consume () {
  for await (let chunk of mp) {
    console.log(chunk)
  }
  return 'ok'
}

consume().then(res => console.log(res))
// logs `foo\n` 5 times, and then `ok`
```

### subclass that `console.log()`s everything written into it

```js
class Logger extends Minipass {
  write (chunk, encoding, callback) {
    console.log('WRITE', chunk, encoding)
    return super.write(chunk, encoding, callback)
  }
  end (chunk, encoding, callback) {
    console.log('END', chunk, encoding)
    return super.end(chunk, encoding, callback)
  }
}

someSource.pipe(new Logger()).pipe(someDest)
```

### same thing, but using an inline anonymous class

```js
// js classes are fun
someSource
  .pipe(new (class extends Minipass {
    emit (ev, ...data) {
      // let's also log events, because debugging some weird thing
      console.log('EMIT', ev)
      return super.emit(ev, ...data)
    }
    write (chunk, encoding, callback) {
      console.log('WRITE', chunk, encoding)
      return super.write(chunk, encoding, callback)
    }
    end (chunk, encoding, callback) {
      console.log('END', chunk, encoding)
      return super.end(chunk, encoding, callback)
    }
  }))
  .pipe(someDest)
```

### subclass that defers 'end' for some reason

```js
class SlowEnd extends Minipass {
  emit (ev, ...args) {
    if (ev === 'end') {
      console.log('going to end, hold on a sec')
      setTimeout(() => {
        console.log('ok, ready to end now')
        super.emit('end', ...args)
      }, 100)
    } else {
      return super.emit(ev, ...args)
    }
  }
}
```

### transform that creates newline-delimited JSON

```js
class NDJSONEncode extends Minipass {
  write (obj, cb) {
    try {
      // JSON.stringify can throw, emit an error on that
      return super.write(JSON.stringify(obj) + '\n', 'utf8', cb)
    } catch (er) {
      this.emit('error', er)
    }
  }
  end (obj, cb) {
    if (typeof obj === 'function') {
      cb = obj
      obj = undefined
    }
    if (obj !== undefined) {
      this.write(obj)
    }
    return super.end(cb)
  }
}
```

### transform that parses newline-delimited JSON

```js
class NDJSONDecode extends Minipass {
  constructor (options) {
    // always be in object mode, as far as Minipass is concerned
    super({ objectMode: true })
    this._jsonBuffer = ''
  }
  write (chunk, encoding, cb) {
    if (typeof chunk === 'string' &&
        typeof encoding === 'string' &&
        encoding !== 'utf8') {
      chunk = Buffer.from(chunk, encoding).toString()
    } else if (Buffer.isBuffer(chunk))
      chunk = chunk.toString()
    }
    if (typeof encoding === 'function') {
      cb = encoding
    }
    const jsonData = (this._jsonBuffer + chunk).split('\n')
    this._jsonBuffer = jsonData.pop()
    for (let i = 0; i < jsonData.length; i++) {
      try {
        // JSON.parse can throw, emit an error on that
        super.write(JSON.parse(jsonData[i]))
      } catch (er) {
        this.emit('error', er)
        continue
      }
    }
    if (cb)
      cb()
  }
}
```
