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Finally in Promises & Try/Catch

Anna Rankin on September 19, 2018

Lately, I've been experimenting more with the async/await keywords in JavaScript. I noticed that I sometimes struggle to reconcile the strategies I...

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Ben Halpern • Sep 19 '18

Finally, I pinged my tech lead detailing my annoyance

Alain Van Hout • Sep 19 '18

Good catch!

Anna Rankin • Sep 19 '18

😜heeheehee

JasonSooter • Sep 26 '18

You gave it the good old college try with that one!

Frank Robert • Sep 20 '18

Great write-up! I've only ever found a single good use for finally. You could do performance checks using console.time.

// logs async evaluation in ms
const promises = [new Promise(), new Promise()];

for (const promise of promises) {
  console.time('timer');
  try {
    await promise();
  } catch(error) {
    console.error(error);
  } finally {
    console.timeEnd('timer');
  }
}

Meg Stepp • Dec 28 '18

I also use finally to toggle loading indicators to keep things DRY in my then/catches. If I'm doing the same line of code in both, I move it to the finally block.

JasonSooter • Sep 26 '18

Nice tip!

John Stewart • Sep 19 '18

Such a well written article and I learned something new!

Anna Rankin • Sep 19 '18

Daww thank you! 😁 Love that gif

kurisutofu • Sep 20 '18

When I saw the code for try-catch-finally, my first thought was that you should not have a "return" in the try and catch if you are using finally.
My understanding was that finally has the "final say".

I find interesting the way promises handle that. Which I think makes sense too ... each action "block" would basically be a local try-catch-finally except the "catches" are grouped at the end for handling.

Anna Rankin • Sep 25 '18

Sorry I missed your comment - I agree, the multiple returns with the catch block are pretty wacky. The way Promises work makes sense looking at it now, but my brain totally rebelled on first look 😵Thanks kuristofu!

Anna Rankin • Sep 19 '18

It's a neat tool - and it's fun what you can learn when you try weird things lol. One thing I will note is that you might run into difficulty using Promise.finally in applications that are tested with Jest.

rhymes • Sep 19 '18

Thanks Anna, it's quite different on how finally works in contrast to other general purpose languages. Nice to know :-) Hopefully 99% of the time people will use finally just to cleanup...