JS Promises - if-else flow when only single path is async

问题

I'm rewriting some legacy code, which was build using sync ajax (so so bad). Now I'm using Promises (specifically Bluebird). There are a lot of situations, where function has a lot of paths, from which only one is async.

The problem is that I have to identify all paths and manually return new promise from each of them. Like this:

function syncFn() {
    // sync code return
}
function asyncFn() {
    // some async code here that return Promise
}
function myMegaFunction() {
    if ( ... ) {
        return Promise.resolve(syncFn());
    }
    else if ( ... ) {
        if ( ... ) {
            return Promise.resolve(syncFn());
        }
        else if ( ... ) {
            return Promise.resolve(syncFn());
        }
        else {
            return asyncFn(); // only async function here
        }
    }
    else {
        return Promise.resolve();
    }
}

Is there some way or pattern, that could simplify this? Maybe something like - if return undefined, then wrap as empty promise?

回答1:

You can use the Promise.method helper for this:

var myMegaFunction = Promise.method(function myMegaFunction() {
    // return values
    // throw errors
    // return promises
});

…and myMegaFunction(…) will always return a promise. It automatically catches exceptions, and wraps the result of your function in a Promise.resolve(…).

回答2:

Simpler

function myMegaFunction() {
    if ( ... ) {
        syncFn();
    }
    else if ( ... ) {
        if ( ... ) {
            syncFn();
        }
        else if ( ... ) {
            syncFn();
        }
        else {
            return asyncFn(); // only async function here
        }
    }
    return Promise.resolve();
}

Not by much ... but simpler

As you are returning asyncFn in the one place it needs to be done, you can simply put a return Promise.resolve() ... actually ... let me edit that, I just noticed something ...

function myMegaFunction() {
    if ( ... ) { //A
        ;
    }
    else if ( ... ) { //B
        if ( ... ) { //C
            ;
        }
        else if ( ... ) { // D
            ;
        }
        else {
            return asyncFn(); // only async function here
        }
    }
    synchFn();
    return Promise.resolve();
}

could be written as

function myMegaFunction() {
    if(!A && B && !C && !D) {
        return asyncFn();
    }
    synchFn();
    return Promise.resolve();
}

edit:: not quite - but close - that synchFn is a bitch

function myMegaFunction() {
    if(!A && B && !C && !D) {
        return asyncFn();
    }
    if(A || B) {
        synchFn();
    }
    return Promise.resolve();
}

来源：https://stackoverflow.com/questions/31559165/js-promises-if-else-flow-when-only-single-path-is-async

标签

javascript

promise

bluebird