# EventStream
<img src=https://secure.travis-ci.org/dominictarr/event-stream.png?branch=master>
[]
(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);
});
```