# MapStream

Refactored out of [event-stream](https://github.com/dominictarr/event-stream)

##map (asyncFunction)

Create a through stream from an asyncronous function.  

``` js
var map = require('map-stream')

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.


