Node wait for async function before continue

Question

I have a node application that use some async functions.

How can i do for waiting the asynchronous function to complete before proceeding with the rest of the application flow?

Below there is a simple example.

var a = 0;
var b = 1;
a = a + b;

// this async function requires at least 30 sec
myAsyncFunction({}, function(data, err) {
    a = 5;
});

// TODO wait for async function

console.log(a); // it must be 5 and not 1
return a;

In the example, the element "a" to return must be 5 and not 1. It is equal to 1 if the application does not wait the async function.

Thanks

Yes, promises and async/await syntax sugar - as you suggested in the tags - are the way to go. Have you tried applying them to your problem? Please show your efforts. — Bergi
– Bergi, Commented Nov 7, 2017 at 13:29
Possible duplicate of How to return the response from an asynchronous call? — user400654
– user400654, Commented Nov 7, 2017 at 19:37

Abdul Rab Memon · Accepted Answer · 2017-11-07 19:33:12Z

65

Using callback mechanism:

function operation(callback) {

    var a = 0;
    var b = 1;
    a = a + b;
    a = 5;

    // may be a heavy db call or http request?
    // do not return any data, use callback mechanism
    callback(a)
}

operation(function(a /* a is passed using callback */) {
    console.log(a); // a is 5
})

Using async await

async function operation() {
    return new Promise(function(resolve, reject) {
        var a = 0;
        var b = 1;
        a = a + b;
        a = 5;

        // may be a heavy db call or http request?
        resolve(a) // successfully fill promise
    })
}

async function app() {
    var a = await operation() // a is 5
}

app()

edited Nov 7, 2017 at 19:33

Abdul Rab Memon

4355 silver badges19 bronze badges

answered Nov 7, 2017 at 13:39

Ozgur

3,76620 silver badges35 bronze badges

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3 Comments

Porco Over a year ago

I have been searching for a good explanation of Promise and finally found one! Thank you!

ceving Over a year ago

None of the two alternatives actually waits. In particular a returned by await is not 5. Instead it is a promise, which is just another way to require a callback. The right answer would be: you can not wait.

Logan Cundiff Over a year ago

operation function doesn't need to have async before it, since its not using await. Its redundant since async makes it so that the function returns a promise, but operation() already is returning a promise.

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