Async的流程控制使用文档

Nodejs著名的流程控制库

# Async.js [](doc.xuehai.net) Async is a utility module which provides straight-forward, powerful functions for working with asynchronous JavaScript. Although originally designed for use with [Node.js](doc.xuehai.net), it can also be used directly in the browser. Also supports [component](doc.xuehai.net). Async provides around 20 functions that include the usual 'functional' suspects (`map`, `reduce`, `filter`, `each`…) as well as some common patterns for asynchronous control flow (`parallel`, `series`, `waterfall`…). All these functions assume you follow the Node.js convention of providing a single callback as the last argument of your `async` function. ## Quick Examples ```javascript async.map(['file1','file2','file3'], fs.stat, function(err, results){ // results is now an array of stats for each file }); async.filter(['file1','file2','file3'], fs.exists, function(results){ // results now equals an array of the existing files }); async.parallel([ function(){ ... }, function(){ ... } ], callback); async.series([ function(){ ... }, function(){ ... } ]); ``` There are many more functions available so take a look at the docs below for a full list. This module aims to be comprehensive, so if you feel anything is missing please create a GitHub issue for it. ## Common Pitfalls ### Binding a context to an iterator This section is really about `bind`, not about `async`. If you are wondering how to make `async` execute your iterators in a given context, or are confused as to why a method of another library isn't working as an iterator, study this example: ```js // Here is a simple object with an (unnecessarily roundabout) squaring method var AsyncSquaringLibrary = { squareExponent: 2, square: function(number, callback){ var result = Math.pow(number, this.squareExponent); setTimeout(function(){ callback(null, result); }, 200); } }; async.map([1, 2, 3], AsyncSquaringLibrary.square, function(err, result){ // result is [NaN, NaN, NaN] // This fails because the `this.squareExponent` expression in the square // function is not evaluated in the context of AsyncSquaringLibrary, and is // therefore undefined. }); async.map([1, 2, 3], AsyncSquaringLibrary.square.bind(AsyncSquaringLibrary), function(err, result){ // result is [1, 4, 9] // With the help of bind we can attach a context to the iterator before // passing it to async. Now the square function will be executed in its // 'home' AsyncSquaringLibrary context and the value of `this.squareExponent` // will be as expected. }); ``` ## Download The source is available for download from [GitHub](doc.xuehai.net). Alternatively, you can install using Node Package Manager (`npm`): npm install async __Development:__ [async.js](doc.xuehai.net.

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