Q.js手册

This is Q version 1, from the v1 branch in Git. This documentation applies to the latest of both the version 1 and version 0.9 release trains. These releases are stable. There will be no further releases of 0.9 after 0.9.7 which is nearly equivalent to version 1.0.0. All further releases of q@~1.0 will be backward compatible. The version 2 release train introduces significant and backward-incompatible changes and is experimental at this time.

If a function cannot return a value or throw an exception without blocking, it can return a promise instead. A promise is an object that represents the return value or the thrown exception that the function may eventually provide. A promise can also be used as a proxy for a remote object to overcome latency.

On the first pass, promises can mitigate the “Pyramid of Doom”: the situation where code marches to the right faster than it marches forward.

step1(function (value1) {
    step2(value1, function(value2) {
        step3(value2, function(value3) {
            step4(value3, function(value4) {
                // Do something with value4
            });
        });
    });
});

使用诺言库, 你可以使金字塔扁平.

Q.fcall(promisedStep1)
.then(promisedStep2)
.then(promisedStep3)
.then(promisedStep4)
.then(function (value4) {
    // Do something with value4
})
.catch(function (error) {
    // Handle any error from all above steps
})
.done();

With this approach, you also get implicit error propagation, just like try,catch, and finally. An error in promisedStep1 will flow all the way to the catch function, where it’s caught and handled. (Here promisedStepN is a version of stepN that returns a promise.)

The callback approach is called an “inversion of control”.A function that accepts a callback instead of a return value is saying, “Don’t call me, I’ll call you.”. Promises un-invert the inversion, cleanly separating the input arguments from control flow arguments. This simplifies the use and creation of API’s, particularly variadic, rest and spread arguments.