Async / await assignment to object keys: is it concurrent?
No, every await
will stop the execution until the promise has fulfilled, even mid-expression. It doesn't matter whether they happen to be part of the same statement or not.
If you want to run them in parallel, and wait only once for their result, you have to use await Promise.all(…)
. In your case you'd write
const [result1, result2] = await Promise.all([a(), b()]);
const obj = {result1, result2};
How would you represent the object assignment using
then
/ callbacks?
With temporary variables for each awaited value. Every await
translates into one then
call:
a().then(tmp1 => {
return b().then(tmp2 => {
const obj = {
result1: tmp1,
result2: tmp2
};
return …
});
})
If we wanted to be pedantic, we'd have to pick apart the object creation:
const tmp0 = {};
a().then(tmp1 => {
tmp0.result1 = tmp1;
return b().then(tmp2 => {
tmp0.result2 = tmp2;
const obj = tmp0;
return …
});
})
Comments
-
Babakness almost 2 years
I know that doing this:
const resultA = await a() const resultB = await b() // code here
Is effectively
a().then( resultA => { b().then( resultB => { // code here }) })
Basically, a() runs then b() runs. I nested them to show that both resultA and resultB are in our scope; yet both function didn't run at once.
But what about this:
const obj = { result1: await a(), result2: await b() }
do a() and b() run concurrently?
For reference:
const asyncFunc = async (func) => await func.call() const results = [funcA,funcB].map( asyncFunc )
I know here
funcA
andfuncB
do run concurrently.Bonus:
How would you represent the object assignment
const obj = { result1: await a(), result2: await b() }
using
then
/ callbacks?
UPDATE:
@Bergi is correct in this answer, this occurs sequentially. To share a nice solution for having this work concurrently for an object without having to piece together the object from an array, one can also use
Bluebird
as followsconst obj2 = Bluebird.props(obj)
-
Babakness about 7 yearsYou're right! I just finished writing test code to try the above code using setTimeout and it does indeed run sequentially.