Mastering JavaScript Promises

Making promise-based actions

Starting off with Q, perform actions that return promises. Let's say, make Node.js action http.get as the promised action:

// using-promise.js
var httpGet = function (opts) {
     var deferred = Q.defer();
     http.get(opts, deferred.resolve);
     return deferred.promise;
};

Later, you can use: httpGet(...).then(function (res) {...}); but you have to make sure that functions return promises. The first Q.defer() returns a set of an empty promise and operations for it. The deferred.promise is the empty promise which fixes a certain value:

// promise-resolve-then-flow.js
var deferred = Q.defer();
deferred.promise.then(function (obj) {
    console.log(obj);
});

deferred.resolve("Hello World");

This prints Hello World to the console. In general, ...