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
}
}
When an error is thrown in an async function, you can catch it with a try {} catch {}. So this works as you'd expect: This is syntax sugar for what you might have been doing with promises earlier: In fact, anywhere you use the keyword await, you can remove await and do the traditional .then () and .catch () calls.
Top level await has poor support so, in general, you can only use await inside an async function. You have to call an async function for it to do anything. The async function has to get a promise to await from somewhere and usually that is by calling another function.
Async/await has a clean syntax, but you still have to handle thrown exceptions in async functions. Handling error with .catch in promise .then chain can be difficult unless you implement custom error classes. Using the handle utility function, we are able to avoid Unhandled promise rejection error and also handle error granularly.
This means the caller could use try / catch within an async function that await s the call to userProfile () or by using .catch () on the promise returned by userProfile (). The caller's option to not handle the error and allow the error to continue propagating normally is also preserved.
If
etcFunction()
throws, does the error bubble up all the way through theasync function
s?
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).
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