Promise in Arrays

promise-in-arrays.js

function promiseFunction(v) {
    return new Promise((resolve) => {
        setTimeout(() => resolve(v), 1000);
    });
}

// f1() will print `all done` first and then print 1,2,3,4 at once after a second.
function f1() {
    [1, 2, 3, 4]. forEach(async(i) => {
        const v = await promiseFunction(i);
        console.log('-v-', v);
    });
    console.log('all done');
}

// f2() will print 1,2,3,4 at once after a second then print `all done`.
async function f2() {
    await Promise.all([1, 2, 3, 4].map(async(i) => {
        const v = await promiseFunction(i);
        console.log('-v-', v);
    }));
    console.log('all done');
}

// f3() will print 1,2,3,4 in every second, and then print `all done`.
async function f3() {
    await [1, 2, 3, 4].reduce((p, i) => {
        return p.then(async () => {
            const v = await promiseFunction(i);
            console.log('-v-', v);
        });
    }, Promise.resolve());
    console.log('all done');
}

// What async function returns is actually a promise