async-await-array.reduce.js

#!/usr/bin/env node

// @see https://qiita.com/soarflat/items/1a9613e023200bbebcb3

function sampleResolve(value) {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(value);
        }, 1000);
    })
}

//function sample() {
//    const array = [5, 10, 20];
//    return array.reduce((sum, value) => {
//        return Promise.all([sum, sampleResolve(value)])
//            .then(([sum, value]) => sum + value * 2)
//    }, Promise.resolve(0));
//}

function sample() {
    const array = [5, 10, 20];
    return array.reduce((sum, value) => {
        return sum.then(sum => sampleResolve(value)
            .then(value => sum + value * 2))
    }, Promise.resolve(0));
}


sample().then((v) => {
    console.log(v); // => 70
});