106 lines
3.5 KiB
Markdown
106 lines
3.5 KiB
Markdown
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<a href="http://promises-aplus.github.com/promises-spec"><img src="http://promises-aplus.github.com/promises-spec/assets/logo-small.png" alt="Promises/A+ logo" align="right" /></a>
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[](https://travis-ci.org/cujojs/when)
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when.js
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=======
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When.js is a rock solid, battle-tested [Promises/A+](http://promises-aplus.github.com/promises-spec) and `when()` implementation, including a complete [ES6 Promise shim](docs/es6-promise-shim.md). It's a powerful combination of small size, high performance, debuggability, and rich features:
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* Resolve arrays and hashes of promises, as well as infinite promise sequences
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* Execute tasks in parallel or sequentially
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* Transform Node-style and other callback-based APIs into promise-based APIs
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When.js is one of the many stand-alone components of [cujoJS](http://cujojs.com), the JavaScript Architectural Toolkit.
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Check it out:
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- [What's new](CHANGES.md)
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- [API docs](docs/api.md#api)
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- Read more about how [promises simplify async programming](http://know.cujojs.com/tutorials/async/simplifying-async-with-promises)
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Installation
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------------
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#### AMD
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Availble as `when` through [bower](http://bower.io), or just clone the repo and load `when.js` from the root.
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```
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bower install --save when
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```
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#### CommonJS/Node
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```
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npm install --save when
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```
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[More help & other environments »](docs/installation.md)
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Usage
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-----
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Promises can be used to help manage complex and/or nested callback flows in a simple manner. To get a better handle on how promise flows look and how they can be helpful, there are a couple examples below (using commonjs).
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This first example will print `"hello world!!!!"` if all went well, or `"drat!"` if there was a problem. It also uses [rest](https://github.com/cujojs/rest) to make an ajax request to a (fictional) external service.
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```js
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var rest = require('rest');
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fetchRemoteGreeting()
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.then(addExclamation)
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.catch(handleError)
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.done(function(greeting) {
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console.log(greeting);
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});
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function fetchRemoteGreeting() {
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// returns a when.js promise for 'hello world'
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return rest('http://example.com/greeting');
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}
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function addExclamation(greeting) {
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return greeting + '!!!!'
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}
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function handleError(e) {
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return 'drat!';
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}
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```
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The second example shows off the power that comes with when's promise logic. Here, we get an array of numbers from a remote source and reduce them. The example will print `150` if all went well, and if there was a problem will print a full stack trace.
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```js
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var when = require('when');
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var rest = require('rest');
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when.reduce(when.map(getRemoteNumberList(), times10), sum)
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.done(function(result) {
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console.log(result);
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});
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function getRemoteNumberList() {
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// Get a remote array [1, 2, 3, 4, 5]
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return rest('http://example.com/numbers').then(JSON.parse);
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}
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function sum(x, y) { return x + y; }
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function times10(x) {return x * 10; }
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```
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License
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-------
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Licensed under MIT. [Full license here »](LICENSE.txt)
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Contributing
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------------
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Please see the [contributing guide](CONTRIBUTING.md) for more information on running tests, opening issues, and contributing code to the project.
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References
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----------
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Much of this code was inspired by the async innards of [wire.js](https://github.com/cujojs/wire), and has been influenced by the great work in [Q](https://github.com/kriskowal/q), [Dojo's Deferred](https://github.com/dojo/dojo), and [uber.js](https://github.com/phiggins42/uber.js).
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