《html5网页开发实例详解》随书源码2

# Async.js Async is a utility module which provides straight-forward, powerful functions for working with asynchronous JavaScript. Although originally designed for use with [node.js](http://nodejs.org), it can also be used directly in the browser. Also supports [component](https://github.com/component/component). 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](http://github.com/caolan/async). Alternatively, you can install using Node Package Manager (npm): npm install async __Development:__ [async.js](https://github.com/caolan/async/raw/master/lib/async.js) - 29.6kb Uncompressed ## In the Browser So far it's been tested in IE6, IE7, IE8, FF3.6 and Chrome 5. Usage: ```html <script type="text/javascript" src="async.js"></script> <script type="text/javascript"> async.map(data, asyncProcess, function(err, results){ alert(results); }); </script> ``` ## Documentation ### Collections * [each](#each) * [map](#map) * [filter](#filter) * [reject](#reject) * [reduce](#reduce) * [detect](#detect) * [sortBy](#sortBy) * [some](#some) * [every](#every) * [concat](#concat) ### Control Flow * [series](#series) * [parallel](#parallel) * [whilst](#whilst) * [doWhilst](#doWhilst) * [until](#until) * [doUntil](#doUntil) * [forever](#forever) * [waterfall](#waterfall) * [compose](#compose) * [applyEach](#applyEach) * [queue](#queue) * [cargo](#cargo) * [auto](#auto) * [iterator](#iterator) * [apply](#apply) * [nextTick](#nextTick) * [times](#times) * [timesSeries](#timesSeries) ### Utils * [memoize](#memoize) * [unmemoize](#unmemoize) * [log](#log) * [dir](#dir) * [noConflict](#noConflict) ## Collections <a name="forEach" /> <a name="each" /> ### each(arr, iterator, callback) Applies an iterator function to each item in an array, in parallel. The iterator is called with an item from the list and a callback for when it has finished. If the iterator passes an error to this callback, the main callback for the each function is immediately called with the error. Note, that since this function applies the iterator to each item in parallel there is no guarantee that the iterator functions will complete in order. __Arguments__ * arr - An array to iterate over. * iterator(item, callback) - A function to apply to each item in the array. The iterator is passed a callback(err) which must be called once it has completed. If no error has occured, the callback should be run without arguments or with an explicit null argument. * callback(err) - A callback which is called after all the iterator functions have finished, or an error has occurred. __Example__ ```js // assuming openFiles is an array of file names and saveFile is a function // to save the modified contents of that file: async.each(openFiles, saveFile, function(err){ // if any of the saves produced an error, err would equal that error }); ``` --------------------------------------- <a name="forEachSeries" /> <a name="eachSeries" /> ### eachSeries(arr, iterator, callback) The same as each only the iterator is applied to each item in the array in series. The next iterator is only called once the current one has completed processing. This means the iterator functions will complete in order. --------------------------------------- <a name="forEachLimit" /> <a name="eachLimit" /> ### eachLimit(arr, limit, iterator, callback) The same as each only no more than "limit" iterators will be simultaneously running at any time. Note that the items are not processed in batches, so there is no guarantee that the first "limit" iterator functions will complete before any others are started. __Arguments__ * arr - An array to iterate over. * limit - The maximum number of iterators to run at any time. * iterator(item, callback) - A function to apply to each item in the array. The iterator is passed a callback(err) which must be called once it has completed. If no error has occured, the callback should be run without arguments or with an explicit null argument. * callback(err) - A callback which is called after all the iterator functions have finished, or an error has occurred. __Example__ ```js // Assume documents is an array of JSON objects and requestApi is a // function that interacts with a rate-limited REST api. async.eachLimit(documents, 20, requestApi, function(err){ // if any of the saves produced an error, err would equal that error }); ``` --------------------------------------- <a name="map" /> ### map(arr, iterator, callback) Produces a new array of values by mapping each value in the given array through the iterator function. The iterator is called with an item from the array and a callback for when it has finished processing. The callback takes 2 arguments, an error and the transformed item from the array. If the iterator passes an error to this callback, the main callback for the map function is immediately called with the error. Note, that since this function applies the iterator to each item in parallel there is no guarantee that the iterator functions will complete in order, however the results array will be in the same order as the original array. __Arguments__ * arr - An array to iterate over. * iterator(item, callback) - A function to apply to each item in the array. The iterator is passed a callback(err, transformed) which must be called once it has completed with an error (which can be null) and a transformed item. * callback(err, results) - A callback which is called after all the iterator functions have finished, or an error has occurred. Results is an array of the transformed items from the original array. __Example__ ```js async.map(['file1','file2','file3'], fs.stat, functi