# EventStream

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(http://ci.testling.com/dominictarr/event-stream)

[Streams](http://nodejs.org/api/stream.html "Stream") are node's best and most misunderstood idea, and 
_<em>EventStream</em>_ is a toolkit to make creating and working with streams <em>easy</em>.  

Normally, streams are only used for IO,  
but in event stream we send all kinds of objects down the pipe.  
If your application's <em>input</em> and <em>output</em> are streams,  
shouldn't the <em>throughput</em> be a stream too?  

The *EventStream* functions resemble the array functions,  
because Streams are like Arrays, but laid out in time, rather than in memory.  

<em>All the `event-stream` functions return instances of `Stream`</em>.

`event-stream` creates 
[0.8 streams](https://github.com/joyent/node/blob/v0.8/doc/api/stream.markdown)
, which are compatible with [0.10 streams](http://nodejs.org/api/stream.html "Stream").

>NOTE: I shall use the term <em>"through stream"</em> to refer to a stream that is writable <em>and</em> readable.  

### [simple example](https://github.com/dominictarr/event-stream/blob/master/examples/pretty.js):

``` js

//pretty.js

if(!module.parent) {
  var es = require('event-stream')
  var inspect = require('util').inspect

  process.stdin                        //connect streams together with `pipe`
    .pipe(es.split())                  //split stream to break on newlines
    .pipe(es.map(function (data, cb) { //turn this async function into a stream
      cb(null
        , inspect(JSON.parse(data)))   //render it nicely
    }))
    .pipe(process.stdout)              // pipe it to stdout !
}
```
run it ...

``` bash  
curl -sS registry.npmjs.org/event-stream | node pretty.js
```
 
[node Stream documentation](http://nodejs.org/api/stream.html)

## through (write?, end?)

Re-emits data synchronously. Easy way to create synchronous through streams.
Pass in optional `write` and `end` methods. They will be called in the 
context of the stream. Use `this.pause()` and `this.resume()` to manage flow.
Check `this.paused` to see current flow state. (write always returns `!this.paused`)

this function is the basis for most of the synchronous streams in `event-stream`.

``` js

es.through(function write(data) {
    this.emit('data', data)
    //this.pause() 
  },
  function end () { //optional
    this.emit('end')
  })

```

## map (asyncFunction)

Create a through stream from an asynchronous function.  

``` js
var es = require('event-stream')

es.map(function (data, callback) {
  //transform data
  // ...
  callback(null, data)
})

```

Each map MUST call the callback. It may callback with data, with an error or with no arguments, 

  * `callback()` drop this data.  
    this makes the map work like `filter`,  
    note:`callback(null,null)` is not the same, and will emit `null`

  * `callback(null, newData)` turn data into newData
    
  * `callback(error)` emit an error for this item.

>Note: if a callback is not called, `map` will think that it is still being processed,   
>every call must be answered or the stream will not know when to end.  
>
>Also, if the callback is called more than once, every call but the first will be ignored.

## mapSync (syncFunction)

Same as `map`, but the callback is called synchronously. Based on `es.through`

## split (matcher)

Break up a stream and reassemble it so that each line is a chunk. matcher may be a `String`, or a `RegExp` 

Example, read every line in a file ...

``` js
fs.createReadStream(file, {flags: 'r'})
  .pipe(es.split())
  .pipe(es.map(function (line, cb) {
    //do something with the line 
    cb(null, line)
  }))
```

`split` takes the same arguments as `string.split` except it defaults to '\n' instead of ',', and the optional `limit` parameter is ignored.
[String#split](https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/String/split)

## join (separator)

Create a through stream that emits `separator` between each chunk, just like Array#join.

(for legacy reasons, if you pass a callback instead of a string, join is a synonym for `es.wait`)

## merge (stream1,...,streamN) or merge (streamArray) 
> concat → merge

Merges streams into one and returns it.
Incoming data will be emitted as soon it comes into - no ordering will be applied (for example: `data1 data1 data2 data1 data2` - where `data1` and `data2` is data from two streams).
Counts how many streams were passed to it and emits end only when all streams emitted end.

```js
es.merge(
  process.stdout,
  process.stderr
).pipe(fs.createWriteStream('output.log'));
```

It can also take an Array of streams as input like this: 
```js
es.merge([
  fs.createReadStream('input1.txt'),
  fs.createReadStream('input2.txt')
]).pipe(fs.createWriteStream('output.log'));
```

## replace (from, to)

Replace all occurrences of `from` with `to`. `from` may be a `String` or a `RegExp`.  
Works just like `string.split(from).join(to)`, but streaming.


## parse

Convenience function for parsing JSON chunks. For newline separated JSON,
use with `es.split`.  By default it logs parsing errors by `console.error`;
for another behaviour, transforms created by `es.parse({error: true})` will
emit error events for exceptions thrown from `JSON.parse`, unmodified.

``` js
fs.createReadStream(filename)
  .pipe(es.split()) //defaults to lines.
  .pipe(es.parse())
```

## stringify

convert javascript objects into lines of text. The text will have whitespace escaped and have a `\n` appended, so it will be compatible with `es.parse`

``` js
objectStream
  .pipe(es.stringify())
  .pipe(fs.createWriteStream(filename))
```

## readable (asyncFunction) 

create a readable stream (that respects pause) from an async function.  
while the stream is not paused,  
the function will be polled with `(count, callback)`,  
and `this`  will be the readable stream.

``` js

es.readable(function (count, callback) {
  if(streamHasEnded)
    return this.emit('end')
  
  //...
  
  this.emit('data', data) //use this way to emit multiple chunks per call.
      
  callback() // you MUST always call the callback eventually.
             // the function will not be called again until you do this.
})
```
you can also pass the data and the error to the callback.  
you may only call the callback once.  
calling the same callback more than once will have no effect.  

## readArray (array)

Create a readable stream from an Array.

Just emit each item as a data event, respecting `pause` and `resume`.

``` js
  var es = require('event-stream')
    , reader = es.readArray([1,2,3])

  reader.pipe(...)
```

If you want the stream behave like a 0.10 stream you will need to wrap it using [`Readable.wrap()`](http://nodejs.org/api/stream.html#stream_readable_wrap_stream) function. Example:

``` js
	var s = new stream.Readable({objectMode: true}).wrap(es.readArray([1,2,3]));
```

## writeArray (callback)

create a writeable stream from a callback,  
all `data` events are stored in an array, which is passed to the callback when the stream ends.

``` js
  var es = require('event-stream')
    , reader = es.readArray([1, 2, 3])
    , writer = es.writeArray(function (err, array){
      //array deepEqual [1, 2, 3]
    })

  reader.pipe(writer)
```

## pause  () 

A stream that buffers all chunks when paused.


``` js
  var ps = es.pause()
  ps.pause() //buffer the stream, also do not allow 'end' 
  ps.resume() //allow chunks through
```

## duplex (writeStream, readStream)

Takes a writable stream and a readable stream and makes them appear as a readable writable stream.

It is assumed that the two streams are connected to each other in some way.  

(This is used by `pipeline` and `child`.)

``` js
  var grep = cp.exec('grep Stream')

  es.duplex(grep.stdin, grep.stdout)
```

## child (child_process)

Create a through stream from a child process ...

``` js
  var cp = require('child_process')

  es.child(cp.exec('grep Stream')) // a through stream

```

## wait (callback)

waits for stream to emit 'end'.
joins chunks of a stream into a single string or buffer. 
takes an optional callback, which will be passed the 
complete string/buffer when it receives the 'end' event.

also, emits a single 'data' event.

``` js

readStream.pipe(es.wait(function (err, body) {
  // have complete text here.
}))

```

# Other Stream Modules

These modules are not included as a part of *EventStream* but may be
useful when working with streams.

## [reduce (syncFunction, initial)](https://github.com/parshap/node-stream-reduce)

Like `Array.prototype.reduce` but for streams. Given a sync reduce
function and an initial value it will return a through stream that emits
a single data event with the reduced value once the input stream ends.

``` js
var reduce = require("stream-reduce");
process.stdin.pipe(reduce(function(acc, data) {
  return acc + data.length;
}, 0)).on("data", function(length) {
  console.log("stdin size:", length);
});
```