JS: Executing events overlapping in time afteranother

Question

I receive events on-the-fly in the correct order in which I want to process them after another (as well on-the-fly - so don't "store" them first).

The following example is not going to work as the function someEventOccured() could be called multiple times in a very close timespan. Thus, the execution of handleEvent would overlap in time (which I don't want). As I am writing to a file in handleEvent() I cannot have simultaneously execute the function and need to maintain the order...

async someEventOccured(data:string){
   await handleEvent(data);
}

I have already tried using .then() and storing the promise, however I have no idea how to wait for the previous .then() to finish...

Your help is very appreciated <3

Bergi · Accepted Answer · 2020-11-22T15:20:45.330

1

The general approach is to have a shared promise to queue onto:

let queue = Promise.resolve();
function someEventOccured(data:string) {
  queue = queue.then(() => {
    return handleEvent(data);
  }).catch(err => {
    // ensure `queue` is never rejected or it'll stop processing events
    // …
  });
}

Of course, this does implicitly store the data events in the closures on the promise chain.

edited Nov 22 '20 at 15:20

answered Nov 22 '20 at 15:04

Bergi

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what if handleEvent is async? – Martin Dallinger Nov 22 '20 at 15:06
@MartinDallinger It will work, as its promise will be wrapped into the `.then`, that automatically awaits it... – FZs Nov 22 '20 at 15:10
@MartinDallinger `handleEvent` is expected to be asynchronous and return a promise, otherwise the whole queuing is pointless – Bergi Nov 22 '20 at 15:15
Thank you - this makes sense now - I was just not used to the syntax and was a little confused as a result. – Martin Dallinger Nov 22 '20 at 15:19

JS: Executing events overlapping in time afteranother

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