36 lines
919 B
Markdown
36 lines
919 B
Markdown
# MapStream
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Refactored out of [event-stream](https://github.com/dominictarr/event-stream)
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##map (asyncFunction)
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Create a through stream from an asyncronous function.
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``` js
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var map = require('map-stream')
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map(function (data, callback) {
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//transform data
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// ...
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callback(null, data)
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})
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```
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Each map MUST call the callback. It may callback with data, with an error or with no arguments,
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* `callback()` drop this data.
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this makes the map work like `filter`,
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note:`callback(null,null)` is not the same, and will emit `null`
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* `callback(null, newData)` turn data into newData
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* `callback(error)` emit an error for this item.
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>Note: if a callback is not called, `map` will think that it is still being processed,
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>every call must be answered or the stream will not know when to end.
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>
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>Also, if the callback is called more than once, every call but the first will be ignored.
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