javascript nodejs保证稍后会执行所有操作

Question

I have multiple promises like this:

module.exports = function (a, b, c) {
    return someAsync(() => {
            someFunc(a);
        })
        .then(() => myPromises(a, b, c))
        .then(result => {
            console.log('log the result: ', JSON.stringify(result)); // empty
        })
};


const myPromises = function myPromises(a, b, c){
    Promise.all([
        somePromise(a),
        someOtherPromise(b)
    ]).then(function (data) {
        Promise.all([
            anotherPromise(data[0]),
            yetanotherPromise(data[1])
        ]).then(function (finalResult) {
            console.log('check out finalResult: ', JSON.stringify(finalResult)); // i see the expected results here
            return finalResult;
        }).catch(err => { return err });
    }).catch(err => { return err });
};

why is the console.log('log the result: ', JSON.stringify(result)); returning empty? in other words, why is this line getting executed before the promise.all is finished? how can I make it to wait for promise all and then execute?

Answer 1

Not sure which promise you want to return, but the general idea would be to return the Promise.all there, instead of just calling Promise.all.

const myPromises = function myPromises(a, b, c){
  return Promise.all([

Answer 2

The problem is your second promise.all isn't returning anything for it to pass to the main Promise.all Try this edited piece of code

module.exports = function (a, b, c) {
    return someAsync(() => {
            someFunc(a);
        })
        .then(() => myPromises(a, b, c))
        .then(result => {
            console.log('log the result: ', JSON.stringify(result)); // empty
        })
};


const myPromises = function myPromises(a, b, c){
    return Promise.all([ //missing return
        somePromise(a),
        someOtherPromise(b)
    ]).then(function (data) { //data from first promise all
        return Promise.all([ //missing return: passes data to next then
            anotherPromise(data[0]),
            yetanotherPromise(data[1])
        ])
     })
     .then(function (finalResult) { //data from second promise all
            console.log('check out finalResult:',JSON.stringify(finalResult)); // i see the expected results here
            return finalResult; //passes data to next then or top level return whichever available
    })
    .catch(err => { return err }); // top level catch works for the entire chain
};

The reason for your code not working is that promise chain is single flow, you didn't return anything from second promise for it to pass it to the outer promise.all chain