CODEWITHSUNDEEP
javascriptasynchronouses6-promise
Trees4theForest
Trees4theForest

Reputation: 1396

Acting on Synchronous Promises

I have a few javascript functions that return promises. I'm using .then to act on the data returned from those functions. However, is the order guaranteed in the below code? Will result2 get processed after result?

const test = promiseReturningFunction(data);
test.then((result) => {
  doStuff(result);
});
const test2 = promiseReturningFunction2(data2);
test2.then((result2) => {
  doStuff(result2);
});

Or should this be rewritten along the lines of:

const test = promiseReturningFunction(data);
test.then((result) => {
  doStuff(result);
}).then(() => {
  const test2 = promiseReturningFunction2(data2);
  test2.then((result2) => {
    doStuff(result2);
  });
});

In short: is .then blocking?

Upvotes: 0

Views: 41

Answers (3)

Michał Perłakowski
Michał Perłakowski

Reputation: 92639

No, .then() is not blocking. In the first code snippet, you don't know if result will be processed first or result2.

You could try the async functions syntax, it's easier to understand:

(async () => {
  const result = await promiseReturningFunction(data);
  doStuff(result);
  const result2 = await promiseReturningFunction2(data2);
  doStuff(result2);
})();

await means that the execution of this code will be stopped until the promise returned by promiseReturningFunction is resolved.

Upvotes: 2

Bergi
Bergi

Reputation: 665316

No, the order of then callbacks on independent promises is not guaranteed. This is pretty much necessary to allow concurrent processing of multiple asynchronous tasks.

If you need a particular order, you need to explicit chain your promise callbacks.

In your case, if you just need the two doStuff calls to run after each other, but want to concurrently process the two promiseReturningFunctions, you can use

const p1 = promiseReturningFunction(data);
const p2 = test.then(doStuff);
const p3 = promiseReturningFunction2(data2);
Promise.all([p2, p3]).then((stuff, result2) => doStuff(result2));

If you want to have the two promiseReturningFunctions run sequentially, then your second snippet is fine.

Upvotes: 2

Saikat Hajra
Saikat Hajra

Reputation: 680

First one is asynchronous there is no guaranty for result1 after result2 

promiseReturningFunction(data);
.then((result) => {
  doStuff(result);
})
.then(()=>promiseReturningFunction2(data2))
.then((result2) => {
  doStuff(result2);
})
.catch((err)=> /* Error handling **/);

Try this structure please

make sure doStaff is not any I/O operation or else you have make them promise returnable function

Upvotes: 0

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