CODEWITHSUNDEEP
javascriptnode.jsasynchronouses6-promise
cartman
cartman

Reputation: 190

return a promise of an ongoing asynchronous call

I'm trying to launch simultanuously many asynchrnous functions. Some of them make some http calls and I obviously don't want to make the same call multiple times. So, how can this be done using ES6 Promises and if not, what machanism would you recommend to deal with this kind of situations.

Example of the issue:

const func1 = () => async1();

const func2 = () => async2().then(async1);

return Promise.all([func1(), func2()]);

async1 and async2 are asynchronous functions returning Promises. The issue is how to deal with three situations :

The last situation is the only one I call deal with right now to prevent same calls from launching multiple times.

Upvotes: 0

Views: 242

Answers (2)

charlietfl
charlietfl

Reputation: 171679

You can reuse a promise object any time so if you store a request promise the first time it gets called you can return the stored promise on subsequent calls and avoid making multiple requests to same resource

const async1 = (() => {
  let promise;

  return (arg) => {
    console.log(arg, ' = ', promise ? 'Cached request' : 'New request');
    promise = promise || new Promise((res) => setTimeout(res, 500));
    return promise.then(()=>arg);
  }

})()

Promise.all([async1(1),async1(2)]).then(res=>console.log(res))

Upvotes: 1

skyboyer
skyboyer

Reputation: 23695

If I understand it right you want to ensure async1 in your example will not be called twice.

The easy and (quite low)-level of achieving that is memoization.

Say with lodash's _.memoize() it would be

const async1 = _.memoize(async1, () => 1)
// wrapping func1 and func2 is not actually required in this case
const func1 = _.memoize(() => async1()); 
const func2 = _.memoize(() => async2().then(async1));
return Promise.all([func1(), func2()]);

Beware since you are unable to transparently switch from memoized to _un_memoized implementations on fly.

[UPD] since memoization relies on arguments passed you may need to pass resolver callback. Say in your case async1 may either got zero arguments or something coming from previous promise(when used in .then). So arguments are different but since we know all arguments don't matter we can pass resolver that returns constant as a key(say, '1')

Upvotes: 0

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