How to handle multiple awaits in async function

Question

I have multiple API calls to be made which fetch via an API, write data to DB via API, send output to front end via another API.

I have written async function with await like below.

The first two should run one after another but the third one can run independently and doesn't need to wait for the first two fetch statements to complete.

let getToken= await fetch(url_for_getToken);
let getTokenData = await getToken.json();

let writeToDB = await fetch(url_for_writeToDB);
let writeToDBData = await writeToDB.json();

let frontEnd = await fetch(url_for_frontEnd);
let frontEndData = await frontEnd.json();

What is the best way to handle such multiple fetch statements ?

Answer 1

It's easier if you work with promise "creators" (= function that return promises) rather than raw promises. First, define:

const fetchJson = (url, opts) => () => fetch(url, opts).then(r => r.json())

which returns such a "creator". Now, here are two utilities for serial and parallel chaining, which accept both raw promises and "creators":

const call = f => typeof f === 'function' ? f() : f;

const parallel = (...fns)  => Promise.all(fns.map(call));

async function series(...fns) {
    let res = [];

    for (let f of fns)
        res.push(await call(f));

    return res;
}

Then, the main code can be written like this:

let [[getTokenData, writeToDBData], frontEndData] = await parallel(
    series(
        fetchJson(url_for_getToken),
        fetchJson(url_for_writeToDB),
    ),
    fetchJson(url_for_frontEnd),
)

If you don't like the dedicated "creator" wrapper, you can define fetchJson normally

const fetchJson = (url, opts) => fetch(url, opts).then(r => r.json())

and use inline continuations right where series or parallel are called:

let [[getTokenData, writeToDBData], frontEndData] = await parallel(
    series(
        () => fetchJson('getToken'),
        () => fetchJson('writeToDB'),
    ),
    () => fetchJson('frontEnd'), // continuation not necessary, but looks nicer
)

To bring the idea further, we can make series and parallel return "creators" as well rather than promises. This way, we can build arbitrary nested "circuits" of serial and parallel promises and get the results in order. Complete working example:

const call = f => typeof f === 'function' ? f() : f;

const parallel = (...fns)  => () => Promise.all(fns.map(call));

const series = (...fns) => async () => {
    let res = [];

    for (let f of fns)
        res.push(await call(f));

    return res;
};

//

const request = (x, time) => () => new Promise(resolve => {
    console.log('start', x);
    setTimeout(() => {
        console.log('end', x)
        resolve(x)
    }, time)
});

async function main() {
    let chain = series(
        parallel(
            series(
                request('A1', 500),
                request('A2', 200),
            ),
            series(
                request('B1', 900),
                request('B2', 400),
                request('B3', 400),
            ),
        ),
        parallel(
            request('C1', 800),
            series(
                request('C2', 100),
                request('C3', 100),
            )
        ),
    );

    let results = await chain();

    console.log(JSON.stringify(results))
}

main()

.as-console-wrapper { max-height: 100% !important; top: 0; }

Answer 2

There are many ways but the most universal is to wrap each async code path in an async function. This gives you flexibility of mix & matching async return values as you please. In your example you can even inline code with async iife's:

await Promise.all([
  (async() => {
    let getToken = await fetch(url_for_getToken);
    let getTokenData = await getToken.json();

    let writeToDB = await fetch(url_for_writeToDB);
    let writeToDBData = await writeToDB.json();
  })(),
  (async() => {
    let frontEnd = await fetch(url_for_frontEnd);
    let frontEndData = await frontEnd.json();
  })()
]);

Answer 3

You can use .then(), rather than await:

fetch(url_for_getToken)
  .then(getToken => getToken.json())
  .then(async getTokenData => {
    let writeToDB = await fetch(url_for_writeToDB);
    let writeToDBData = await writeToDB.json();
    // Carry on
  })

fetch(url_for_frontEnd)
  .then(frontEnd => frontEnd.json())
  .then(frontEndData => {
    // Carry on  
  })

Answer 4

Run independent request (writeToDB) at the beginning and without await

let writeToDB = fetch(url_for_writeToDB);

let getToken = await fetch(url_for_getToken);
let getTokenData = await getToken.json();

// ...