[Javascript] Q (promise, defer) usage (Q 用法)

整理分享簡單用法。

可以參考 q 原始碼網頁 。

 Basic 

var Q = require("q");

function test() {
    // Do something
    return something;
}

function deferredTest() {

    var deferred = Q.defer();

    // Do something

    // If success
    deferred.resolve();
    // If success & want the value to be returned
    deferred.resolve(value);

    // If failed
    deferred.reject();
    // If failed & want the error to be returned & handled
    deferred.reject(error);

    return deferred.promise;

}

Q.fcall(test);
// Equals
deferredTest();

 Usage 1 

deferredTest().then(
    function(successValue) { /* Do something */ },
    function(rejectError) { /* Do something */ }
);

 Usage 2 

deferredTest()
.then(successHandler)
.fail(rejectHandler) // .catch(rejectedHandler) may be used in modern engines
.fin(finallyHandler);  // .finally(finallyHandler) may be used in ...

 Usage 3 : outputPromise == successValue

var outputPromise = deferredTest().then(function(successValue) {
    return successValue;
});

 Advanced : Q.all

var promises = [];

promises.push(deferredFunction1, deferredFunction2, ...);

Q.all(promises).then(handler);

Difference between Q.all and Q.allSettled 

Q.all：promises 中，只要任何一個 rejected，馬上 rejected，不等沒做完的。
Q.allSettled：是不管每一個 promise 是 fulfilled 或是 rejected，都會等全部做完。