Given the following code:
var arr = [1,2,3,4,5];
var results: number[] = await arr.map(async (item): Promise => {
await callAsynchr
There's another solution for it if you are not using native Promises but Bluebird.
You could also try using Promise.map(), mixing the array.map and Promise.all
In you case:
var arr = [1,2,3,4,5];
var results: number[] = await Promise.map(arr, async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
});
The problem here is that you are trying to await
an array of promises rather than a promise. This doesn't do what you expect.
When the object passed to await
is not a Promise, await
simply returns the value as-is immediately instead of trying to resolve it. So since you passed await
an array (of Promise objects) here instead of a Promise, the value returned by await is simply that array, which is of type Promise<number>[]
.
What you need to do here is call Promise.all
on the array returned by map
in order to convert it to a single Promise before await
ing it.
According to the MDN docs for Promise.all:
The
Promise.all(iterable)
method returns a promise that resolves when all of the promises in the iterable argument have resolved, or rejects with the reason of the first passed promise that rejects.
So in your case:
var arr = [1, 2, 3, 4, 5];
var results: number[] = await Promise.all(arr.map(async (item): Promise<number> => {
await callAsynchronousOperation(item);
return item + 1;
}));
This will resolve the specific error you are encountering here.
If you map to an array of Promises, you can then resolve them all to an array of numbers. See Promise.all.
I'd recommend using Promise.all as mentioned above, but if you really feel like avoiding that approach, you can do a for or any other loop:
const arr = [1,2,3,4,5];
let resultingArr = [];
for (let i in arr){
await callAsynchronousOperation(i);
resultingArr.push(i + 1)
}
Solution below to process all elements of an array asynchronously AND preserve the order:
const arr = [1, 2, 3, 4, 5, 6, 7, 8];
const randomDelay = () => new Promise(resolve => setTimeout(resolve, Math.random() * 1000));
const calc = async n => {
await randomDelay();
return n * 2;
};
const asyncFunc = async () => {
const unresolvedPromises = arr.map(n => calc(n));
const results = await Promise.all(unresolvedPromises);
};
asyncFunc();
Also codepen.
Notice we only "await" for Promise.all. We call calc without "await" multiple times, and we collect an array of unresolved promises right away. Then Promise.all waits for resolution of all of them and returns an array with the resolved values in order.