How to sync JavaScript callbacks?

Question

I\'ve been developing in JavaScript for quite some time but net yet a cowboy developer, as one of the many things that always haunts me is synching JavaScript\'s callbacks.<

Answer 1

There are many, many ways to achieve this, I hope these suggestions help!

First, I would transform the callback into a promise! Here is one way to do that:

function aPromise(arg) {
    return new Promise((resolve, reject) => {
        aCallback(arg, (err, result) => {
            if(err) reject(err);
            else resolve(result);
        });
    })
}

Next, use reduce to process the elements of an array one by one!

const arrayOfArg = ["one", "two", "three"];
const promise = arrayOfArg.reduce(
    (promise, arg) => promise.then(() => aPromise(arg)), // after the previous promise, return the result of the aPromise function as the next promise
    Promise.resolve(null) // initial resolved promise
    );
promise.then(() => {
    // carry on
});

If you want to process all elements of an array at the same time, use map an Promise.all!

const arrayOfArg = ["one", "two", "three"];
const promise = Promise.all(arrayOfArg.map(
    arg => aPromise(arg)
));
promise.then(() => {
    // carry on
});

If you are able to use async / await then you could just simply do this:

const arrayOfArg = ["one", "two", "three"];
for(let arg of arrayOfArg) {
    await aPromise(arg); // wow
}

// carry on

You might even use my very cool synchronize-async library like this:

const arrayOfArg = ["one", "two", "three"];
const context = {}; // can be any kind of object, this is the threadish context

for(let arg of arrayOfArg) {
    synchronizeCall(aPromise, arg); // synchronize the calls in the given context
}

join(context).then(() => { // join will resolve when all calls in the context are finshed
    // carry on
});

And last but not least, use the fine async library if you really don't want to use promises.

const arrayOfArg = ["one", "two", "three"];
async.each(arrayOfArg, aCallback, err => {
    if(err) throw err; // handle the error!
    // carry on
});