returningpromises.md

Returning promises

One Paragraph Explainer

When an error occurs, whether from a synchronous or asynchronous flow, it's imperative to have a full stacktrace of the error flow. Surprisingly, if an async function returns a promise (e.g., calls other async function) without awaiting, should an error occur then the caller function won't appear in the stacktrace. This will leave the person who diagnoses the error with partial information - All the more if the error cause lies within that caller function. There is a feature v8 called "zero-cost async stacktraces" that allow stacktraces not to be cut on the most recent await. But due to non-trivial implementation details, it will not work if the return value of a function (sync or async) is a promise. So, to avoid holes in stacktraces when returned promises would be rejected, we must always explicitly resolve promises with await before returning them from functions

Code example Anti-Pattern: Calling async function without awaiting

Javascript

async function throwAsync(msg) {
  await null // need to await at least something to be truly async (see note #2)
  throw Error(msg)
}

async function returnWithoutAwait () {
  return throwAsync('missing returnWithoutAwait in the stacktrace')
}

// 👎 will NOT have returnWithoutAwait in the stacktrace
returnWithoutAwait().catch(console.log)

would log

Error: missing returnWithoutAwait in the stacktrace
    at throwAsync ([...])

Code example: Calling and awaiting as appropriate

Javascript

async function throwAsync(msg) {
  await null // need to await at least something to be truly async (see note #2)
  throw Error(msg)
}

async function returnWithAwait() {
  return await throwAsync('with all frames present')
}

// 👍 will have returnWithAwait in the stacktrace
returnWithAwait().catch(console.log)

would log

Error: with all frames present
    at throwAsync ([...])
    at async returnWithAwait ([...])

Code example Anti-Pattern: Returning a promise without tagging the function as async

Javascript

async function throwAsync () {
  await null // need to await at least something to be truly async (see note #2)
  throw Error('missing syncFn in the stacktrace')
}

function syncFn () {
  return throwAsync()
}

async function asyncFn () {
  return await syncFn()
}

// 👎 syncFn would be missing in the stacktrace because it returns a promise while been sync
asyncFn().catch(console.log)

would log

Error: missing syncFn in the stacktrace
    at throwAsync ([...])
    at async asyncFn ([...])

Code example: Tagging the function that returns a promise as async

Javascript

async function throwAsync () {
  await null // need to await at least something to be truly async (see note #2)
  throw Error('with all frames present')
}

async function changedFromSyncToAsyncFn () {
  return await throwAsync()
}

async function asyncFn () {
  return await changedFromSyncToAsyncFn()
}

// 👍 now changedFromSyncToAsyncFn would present in the stacktrace
asyncFn().catch(console.log)

would log

Error: with all frames present
    at throwAsync ([...])
    at changedFromSyncToAsyncFn ([...])
    at async asyncFn ([...])

Code Example Anti-pattern #3: direct usage of async callback where sync callback is expected

Javascript

async function getUser (id) {
  await null
  if (!id) throw Error('stacktrace is missing the place where getUser has been called')
  return {id}
}

const userIds = [1, 2, 0, 3]

// 👎 the stacktrace would include getUser function but would give no clue on where it has been called
Promise.all(userIds.map(getUser)).catch(console.log)

would log

Error: stacktrace is missing the place where getUser has been called
    at getUser ([...])
    at async Promise.all (index 2)

Side-note: it may looks like Promise.all (index 2) can help understanding the place where getUser has been called, but due to a completely different bug in v8, (index 2) is a line from internals of v8

Code example: wrap async callback in a dummy async function before passing it as a sync callback

Javascript

Note 1: in case if you control the code of the function that would call the callback - just change that function to async and add await before the callback call. Below I assume that you are not in charge of the code that is calling the callback (or it's change is unacceptable for example because of backward compatibility)

Note 2: quite often usage of async callback in places where sync one is expected would not work at all. This is not about how to fix the code that is not working - it's about how to fix stacktrace in case if code is already working as expected

async function getUser (id) {
  await null
  if (!id) throw Error('with all frames present')
  return {id}
}

const userIds = [1, 2, 0, 3]

// 👍 now the line below is in the stacktrace
Promise.all(userIds.map(async id => await getUser(id))).catch(console.log)

would log

Error: with all frames present
    at getUser ([...])
    at async ([...])
    at async Promise.all (index 2)

where thanks to explicit await in map, the end of the line at async ([...]) would point to the exact place where getUser has been called

Side-note: if async function that wrap getUser would miss await before return (anti-pattern #1 + anti-pattern #3) then only one frame would left in the stacktrace:

[...]

// 👎 anti-pattern 1 + anti-pattern 3 - only one frame left in stacktrace
Promise.all(userIds.map(async id => getUser(id))).catch(console.log)

would log

Error: [...]
    at getUser ([...])

Advanced explanation

The mechanisms behind sync functions stacktraces and async functions stacktraces in v8 implementation are quite different: sync stacktrace is based on stack provided by operating system Node.js is running on (just like in most programming languages). When an async function is executing, the stack of operating system is popping it out as soon as the function is getting to it's first await. So async stacktrace is a mix of operating system stack and a rejected promise resolution chain. Zero-cost async stacktraces implementation is extending the promise resolution chain only when the promise is getting awaited ¹. Because only async functions may await, sync function would always be missed in async stacktrace if any async operation has been performed after the function has been called ²

The tradeoff

Every await creates a new microtask in the event loop, so adding more awaits to the code would introduce some performance penalty. Nevertheless, the performance penalty introduced by network or database is tremendously larger so additional awaits penalty is not something that should be considered during web servers or CLI development unless for a very hot code per request or command. So removing awaits in return awaits should be one of the last places to search for noticeable performance boost and definitely should never be done up-front

Why return await was considered as anti-pattern in the past

There is a number of excellent articles explained why return await should never be used outside of try block and even an ESLint rule that disallows it. The reason for that is the fact that since async/await become available with transpilers in Node.js 0.10 (and got native support in Node.js 7.6) and until "zero-cost async stacktraces" was introduced in Node.js 10 and unflagged in Node.js 12, return await was absolutely equivalent to return for any code outside of try block. It may still be the same for some other ES engines. This is why resolving promises before returning them is the best practice for Node.js and not for the EcmaScript in general

Notes:

1. One other reason why async stacktrace has such tricky implementation is the limitation that stacktrace must always be built synchronously, on the same tick of event loop ¹
2. Without await in throwAsync the code would be executed in the same phase of event loop. This is a degenerated case when OS stack would not get empty and stacktrace be full even without explicitly awaiting the function result. Usually usage of promises include some async operations and so parts of the stacktrace would get lost
3. Zero-cost async stacktraces still would not work for complicated promise usages e.g. single promise awaited many times in different places

References:

1. Blog post on zero-cost async stacktraces in v8

2. Document on zero-cost async stacktraces with mentioned here implementation details