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    <p>What really made it all click for me was <a href="http://www.slideshare.net/domenicdenicola/callbacks-promises-and-coroutines-oh-my-the-evolution-of-asynchronicity-in-javascript" rel="nofollow noreferrer">this presentation</a> by Domenic Denicola.</p> <p>In a <a href="https://gist.github.com/3889970" rel="nofollow noreferrer">github gist</a>, he gave the description I like most, it's very concise:</p> <blockquote> <p><em>The point of promises is to give us back functional composition and error bubbling in the async world.</em></p> </blockquote> <p>In other word, promises are a way that lets us write <em>asynchronous</em> code that is almost as easy to write as if it was <em>synchronous</em>.</p> <p>Consider this example, with promises:</p> <pre><code>getTweetsFor("domenic") // promise-returning async function .then(function (tweets) { var shortUrls = parseTweetsForUrls(tweets); var mostRecentShortUrl = shortUrls[0]; return expandUrlUsingTwitterApi(mostRecentShortUrl); // promise-returning async function }) .then(doHttpRequest) // promise-returning async function .then( function (responseBody) { console.log("Most recent link text:", responseBody); }, function (error) { console.error("Error with the twitterverse:", error); } ); </code></pre> <p>It works as if you were writing this synchronous code:</p> <pre><code>try { var tweets = getTweetsFor("domenic"); // blocking var shortUrls = parseTweetsForUrls(tweets); var mostRecentShortUrl = shortUrls[0]; var responseBody = doHttpRequest(expandUrlUsingTwitterApi(mostRecentShortUrl)); // blocking x 2 console.log("Most recent link text:", responseBody); } catch (error) { console.error("Error with the twitterverse: ", error); } </code></pre> <p>(If this still sounds complicated, watch that presentation!)</p> <p>Regarding Deferred, it's a way to <code>.resolve()</code> or <code>.reject()</code> promises. In the <a href="http://wiki.commonjs.org/wiki/Promises/B" rel="nofollow noreferrer">Promises/B</a> spec, it is called <code>.defer()</code>. In jQuery, it's <code>$.Deferred()</code>.</p> <p>Please note that, as far as I know, the Promise implementation in jQuery is broken (see that gist), at least as of jQuery 1.8.2.<br> It supposedly implements <a href="http://wiki.commonjs.org/wiki/Promises/A" rel="nofollow noreferrer">Promises/A thenables</a>, but you don't get the correct error handling you should, in the sense that the whole "async try/catch" functionality won't work. Which is a pity, because having a "try/catch" with async code is utterly cool.</p> <p>If you are going to use Promises (you should try them out with your own code!), use <a href="https://github.com/kriskowal/q" rel="nofollow noreferrer">Kris Kowal's Q</a>. The jQuery version is just some callback aggregator for writing cleaner jQuery code, but misses the point.</p> <p>Regarding Future, I have no idea, I haven't seen that in any API.</p> <p><strong>Edit:</strong> <a href="http://www.youtube.com/watch?v=hf1T_AONQJU" rel="nofollow noreferrer">Domenic Denicola's youtube talk on Promises</a> from <a href="https://stackoverflow.com/users/286588/farm">@Farm</a>'s comment below.</p> <p>A quote from Michael Jackson (yes, <em>Michael Jackson</em>) from the video:</p> <blockquote> <p>I want you to burn this phrase in your mind: <em>A promise is an <strong>asynchronous value</em></strong>.</p> </blockquote> <p>This is an excellent description: a promise is like a variable from the future - a first-class reference to something that, at some point, will exist (or happen).</p>
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    1. POWhat are the differences between Deferred, Promise and Future in JavaScript?
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    1. USCamilo Martin
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    1. COVery good presentation!
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      1. USEran Medan
    2. COA great explanation of Futures (now implemented in the DOM!) by a member of the W3 and Chrome core team is to be found here: http://www.xanthir.com/b4PY0
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    3. CO@oligofren Thanks for the link, that seems nice! By the way, what a mysteriously annoying favicon lol.
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