Using async with map to execute multiple promises

Question

I am trying to execute multiple promises using async with map:

const testAsyncFunction = async function testAsyncFunction() {
  const array = ["var1", "var2"];
  const promises = array.map(
    (variable) =>
      async function () {
        // This doesn't get logged
        console.log(
          "🚀 ~ file: coursesServices.js ~ line 853 ~ testAsyncFunction ~ variable",
          variable
        );
        return variable + "TEST";
      }
  );
  const promises_results = await Promise.all(promises);
  // This gets logged
  console.log(
    "🚀 ~ file: coursesServices.js ~ line 861 ~ testAsyncFunction ~ promises_results",
    promises_results
  );
};
testAsyncFunction();

The problem here is that the code inside the map function never gets executed.

This is what gets logged in the console:

🚀 ~ file: coursesServices.js ~ line 861 ~ testAsyncFunction ~ promises_results [ [AsyncFunction (anonymous)], [AsyncFunction (anonymous)] ]

I don't see what I'm doing wrong exactly. But, I have a feeling that Promise.all wouldn't work on an array of async functions.

A function is not a Promise. Either return an explicit Promise in the .map() callback (.map(value => new Promise(...)) or execute the anonymous function in the callback and return their return value (.map(value => (async function() { ... })())). — Andreas
– Andreas, Commented Jun 17, 2021 at 12:14
@Andreas or just use an async callback in the .map() instead of returning an async function: .map(async (value) => { /* ... */ }) — VLAZ
– VLAZ, Commented Jun 17, 2021 at 12:18

prasanth · Accepted Answer · 2021-06-17 12:24:08Z

1

You should use new promise inside the map. otherwise the promise.all does't not care about the map

(async() => {
  const arr = [1, 2, 3]
  const res = arr.map((a) => {
    return new Promise((resolve) => {
      setTimeout(() => {  //async call
        resolve(a + 'finished')
      }, 1000)
    })
  })
  console.log(await Promise.all(res))
})()

answered Jun 17, 2021 at 12:24

prasanth

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1 Comment

VLAZ Over a year ago

No need for explicit new Promise() because the .map() already has an async function.

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