QuestionToDo.js

var cacheData = {
  'url1' : {a: 3}
}

// not completely async 

function ajax (url, fn) {
  if (cacheData[url]) {
    fn(cacheData[url])
    return 
  }
  
  setTimeout(() => fn({a: 2}), 0)
}

function result(data) {
  console.log( a, data );
}

var a = 0;

// make result call async

ajax( "url2", result );
a++;


// permutation ['abc'] => ['abc', 'acb', 'bca', 'bac', 'cab', 'cba']
// combination ['abc'] => ['a', 'b', 'c', 'ab', 'bc', 'ac', 'abc']