whydoyouwork.js

function doReplaceAsync() {
  return myAsyncFunc()
    .then(function(character) {
      return input.replace(/a/g, character);
    });
}

function myAsyncFunc() {
  return new Promise(resolve => {
    for(let i = 0; i < 1000000; i++) {
      resolve('e');
    }
  });
}

doReplaceASync().then(result => console.log(result));