Promise.All conditionally await promises

Question

I have a list of async functions that each have a condtion such that the function should be called if and only if the corresponding condition is met and I want to run all of the functions parallely. Example:

await Promise.all([asyncFunc1(), asyncFunc2(), ...])

Each function in the list should be called if and only if the corresponding conditions: cond1, cond2, ... are met. What is the best way of doing this?

It makes no sense to not wait for a function that you're calling. Do you mean you actually don't want to *call* the respective function based on the conditions? — Bergi, Dec 11 '20 at 13:42
@Pointy Mabye I've misunderstood this answer: https://stackoverflow.com/a/35612484/7964141 Is there a way to run all of the asynchronous functions in parallel? — , Dec 11 '20 at 13:58
Launching a bunch of asynchronous operations *may* result in parallel execution *outside* the context of JavaScript, but that's up to whatever service the operations involve. As far as JavaScript itself is concerned, the operations will result in events when they succeed or fail, and those events will be processed one at a time as they occur. — Pointy, Dec 11 '20 at 13:59
@Pointy That makes a lot of sense. Thanks for the clarification! — , Dec 11 '20 at 14:02

score 1 · Answer 1 · answered Dec 11 '20 at 13:33

1

I came up with this very simple solution:

const promiseList = []
if(cond1) promiseList.push(asyncFun1())
if(cond2) promiseList.push(asyncFun2())
...
await Promise.all(promiseList)

answered Dec 11 '20 at 13:33

FZs · Accepted Answer · 2020-12-13T21:47:56.873

Put your conditions into an array, and filter the function list with it:

const conditions = [cond1, cond2, cond3, cond4]
const functions = [asyncFunc1, asyncFunc2, () => asyncFunc3(foo), bar.asyncFunc4.bind(bar, baz)]

await Promise.all(
  functions
    .filter((_, i) => conditions[i])
    .map(fn => fn())
)

To pass parameters to your async functions (or preserve their this context), .bind them, or wrap them in other functions.

Promise.All conditionally await promises

2 Answers2