handler.asynchronous.js

import assert from "assert";
import each from "each";

const result = await each(
  [{id: 'a'}, {id: 'b'}, {id: 'c'}, {id: 'd'}],
  (item, index) =>
    new Promise((resolve) =>
      setTimeout(resolve(`${item.id}@${index}`), 100)
    )
);

assert.deepStrictEqual(
  result, 
  ['a@0', 'b@1', 'c@2', 'd@3']
);