Filtering an array with a function that returns a promise

Question

Given

let arr = [1,2,3];

function filter(num) {
  return new Promise((res, rej) => {
    setTimeout(() => {
      if( num === 3 ) {
        res(num);
         


        

Answer 1

In case someone is interested in modern typescript solution (with fail symbol used for filtering):

const failSymbol = Symbol();

export async function filterAsync(
  itemsToFilter: T[],
  filterFunction: (item: T) => Promise,
): Promise {
  const itemsOrFailFlags = await Promise.all(
    itemsToFilter.map(async (item) => {
      const hasPassed = await filterFunction(item);

      return hasPassed ? item : failSymbol;
    }),
  );

  return itemsOrFailFlags.filter(
    (itemOrFailFlag) => itemOrFailFlag !== failSymbol,
  ) as T[];
}