Karl Morrison Karl Morrison - 1 month ago 17
Node.js Question

Chaining async functions?

let val = 0;

async function first() {
console.log('1a', val);
second();
console.log('1b', val);
}

async function second() {
console.log('2a', val);
third();
console.log('2b', val);
}

async function third() {
console.log('3a', val);
val = await new Promise(function (resolve, reject) {
setTimeout(function () {
resolve(3);
}, 1000);
});
console.log('3b', val);
}

console.log('0a', val);
first();
console.log('0b', val);


I am expecting:

0a 0
1a 0
2a 0
3a 0
3b 3
2b 3
1b 3
0b 3


But I am receiving:

0a 0
1a 0
2a 0
3a 0
2b 0
1b 0
0b 0
3b 3


I'm guessing there is a fundamental thing about async which I am unaware of?

Answer

You will need to use

async function first() {
    console.log('1a', val);
    await second();
//  ^^^^^
    console.log('1b', val);
}

async function second() {
    console.log('2a', val);
    await third();
//  ^^^^^
    console.log('2b', val);
}

to chain the b logs after the asynchronous execution of the called functions.

And it's the same for

console.log('0a', val);
first();
console.log('0b', val);

only that you cannot use await here since you are not inside an aync function. Making a function async doesn't mean that it magically blocks on everything asynchronous inside it, on the contrary - its result becomes a promise that is always asynchronous, and you can use the await keyword. So to make 0b wait, you can use either

console.log('0a', val);
first().then(function() {
    console.log('0b', val);
}, function(err) {
    console.error(err);
});

or

(async function() {
    try {
        console.log('0a', val);
        first();
        console.log('0b', val);
    } catch(err) {
        console.log(err);
    }
}());
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