How can promise resolve all data with multiple layer of callbacks in NodeJS

Question

Suppose I have the following source code in NodeJS using promise (connecting to mongodb with 3 collections, well, this is not important)

function getData(){
    return new promise((resolve, reject) => {
        var dataArray = [];
        getCollection1().then((rows1) => {
            getCollection2().then((rows2) => {
                var val = someDataProcessingFunction(rows1, rows2);
                dataArray.push(val);

                resolve(dataArray);
            }, (err) => {
                reject(err);
            }
        }, (err) => {
            reject(err);
        });

        getCollection3().then((rows3) => {
            rows3.forEach((row3) => {
                dataArray.push(row3);
            });
            resolve(dataArray);
        }, (err) => {
            reject(err);
        });
    });
}

The problem is, there is multiple places where dataArray is pushed, but I don't know when all the pushing is completed. so that I don't know when to resolve, to return the data to user, for example, by response.send in express. As to the code above while I am using resolves several times, I will only get part of the whole data returned.

I can think out two workarounds, neither is good. one is to surround all resolves in setTimeout, which means I must delay the response to user.

another is to count all the push times at the beginning, every time subtract 1 when one item is pushed. when the times goes 0, do resolve. but it is often difficult to calculate the number before doing the real loops.

Is there better solutions? Thank you and love you :)

Answer 1

First of all, avoid the Promise constructor antipattern and use return values instead! For multiple layers of asynchronous functions, just use multiple layers of promises. To await multiple promises at once, use Promise.all:

function getData(){
    return Promise.all([
        getCollection1(),
        getCollection2(),
        getCollection3()
    ]).then(([rows1, rows2, rows3]) => {
        return [someDataProcessingFunction(rows1, rows2)].concat(rows3);
    });
}