aka:JavaScript:Promises

• What is a promise?
  • "In computer science, future, promise, and delay refer to constructs used for synchronizing in some concurrent programming languages. They describe an object that acts as a proxy for a result that is initially unknown, usually because the computation of its value is yet incomplete." http://en.wikipedia.org/wiki/Futures_and_promises
• Does JavaScript have promises?
  • ES6 has.
    • Some browsers have already implemented the ES6 promises.
    • There is a polyfill for the ES6 promises.
      • https://github.com/jakearchibald/es6-promise
    • Node.js decided not to adopt promises as one of its core features on the early stages. That's why you see plenty of nested callbacks in the community.
• What is Q? I see this letter often when I read something about JavaScript promises.
  • Q is a library that implements promises. There are some such libraries: Q, when, WinJS, and RSVP.js. All these libraries are Promise/A+ compliant. https://github.com/promises-aplus/promises-spec Please note that jQuery's q is not compliant to the standard.
• Can I use Q on node.js?
  • Yes. 'npm install q' will do the work.
  • But, most features of node.js are implemented in a CPS or callback style. So you have little benefit to use promises libraries like Q on node.js.
• No benefit?
  • A benefit is a better code readability. You can avoid so-called callback hell, keeping your code look flat.
• Is that the only benefit?
  • Ahm, actually, asynchronous code is just one of patterns you can use promises. One other example is lazy evaluations: you can postpone actual computation of something into the future when you need the value. I mean you don't use this kind of patterns in web programming.
  • In web programming, communication with servers is a popular pattern to apply asynchronous way of writing such as promises. But in this case, promises are just a syntax sugar of underlying concurrent features such as multithreads or event emitter/loop systems host environments provide.
• Anyway, how can I create a promise object in node with Q?
  • Here

    var q = require('q');
    
    function doSomethingAsync() {
        var deferred = q.defer();
        setTimeout(function () {
            deferred.resolve('hello world');
        }, 500);
        return deferred.promise;
    }
    
    doSomethingAsync().then(function (val) {
        console.log('Promise Resolved', val);
    });
    

• I don't like or understand promise examples that use timers like setTimeout. They look like sophistries.
  • Chill out.
• What is this 'deferred'?
  • Generally, a Deferred is an object that helps create and manipulate promises. A Deferred has a promise property that references the promise that it manages. It also has resolve and reject methods that are responsible for resolving / rejecting the promise.
• Really?
  • Really. See the deferred object of the previous code:

    { promise: [object Object],
      resolve: [Function],
      fulfill: [Function],
      reject: [Function],
      notify: [Function] }