Catching errors from nested async/await functions

Question

I have a function chain in a node 4.3 script that looks something like, callback -> promise -> async/await -> async/await -> async/await

like so:

const topLevel = (resolve, reject) => {
    const foo = doThing(data)
    .then(results => {
        resolve(results)
    })
    .catch(err => {
        reject(err)
    })
}

async function doThing(data) {
    const thing = await doAnotherThing(data)
    return thing
}

async function doAnotherThing(data) {
    const thingDone = await etcFunction(data)
    return thingDone
}

(The reason it isn't async/await all the way through is that the top level function is a task queue library, and ostensibly can't be run async/await style)

If etcFunction() throws, does the error bubble up all the way to the top-level Promise?

If not, how can I bubble-up errors? Do I need to wrap each await in a try/catch and throw from there, like so?

async function doAnotherThing(data) {
   try {
     await etcFunction(data)
   } catch(err) {
     throw err  
   }
}

Yolanthe

What is makePromise? – Vander

"the top level function is a task queue library, and ostensibly can't be run async/await style" - there's no reason why a queue library couldn't use async functions as tasks. You really shouldn't have to deal with callbacks. If you need to use a particular queue library that employs callback style, use a wrapper function. – Vander

@Vander makePromise is actually an async function, but since it's called from a non-async environment, i'm handling it like a promise. re: i agree, I'm going to try and promise-ify it eventually, but can I have reliable error bubbling in the mean time? – Yolanthe

@Vander i removed the makePromise bit from the snippet to clarify--the first function called in the top-level is an async function i'm treating as a regular Promise – Yolanthe

So it's a promise-returning function and needs to be makePromise() not makePromise. I see. – Vander

@Vander yes, thanks for catching that & the other errors- i should've just copied the actual code instead of trying to recreate in pseudocode – Yolanthe

FYI, async/await is not part of ES7. It will be released next year. – Dordrecht

Answer 1

If etcFunction() throws, does the error bubble up all the way through the async functions?

Yes. The promise returned by the outermost function will be rejected. There's no need to do try { … } catch(e) { throw e; }, that's just as pointless as it would be in synchronous code.

… bubble up all the way to the top-level Promise?

No. Your topLevel contains multiple mistakes. If you don't return the doThing(data) from the then callback, it will be ignored (not even awaited) and the rejection stays unhandled. You'll have to use

.then(data => { return doThing(data); })
// or
.then(data => doThing(data))
// or just
.then(doThing) // recommended

And in general, your function should look like this:

function toplevel(onsuccess, onerror) {
    makePromise()
    .then(doThing)
    .then(onsuccess, onerror);
}

No unnecessary function expressions, no .then(…).catch(…) antipattern (that could lead to onsuccess and onerror to both be called).

Vander

Answer 2

I know this question is old, but as your question is written, wouldn't the doAnotherThing() function be unnecessary because it just wraps etcFunction()?

So your code could be simplified to this:

async function topLevel(){
  let thing = await doThing(data)
  let thingDone = await etcFunction(data)
  //Everything is done here...
}

//Everything starts here...
topLevel()

Foofaraw

I think this is mock code to demonstrate nested await calls – Barrelchested

Answer 3

I just had a similar issue where my nested errors didn't appear to bubble up to my top level function.

The fix for me was to remove the "try / catch" from my nested function and allow the error to just be thrown.

Hemmer