pomise_lite.js

function myPromise(fn){
  let done = false;
  let next = [];
  fn(() => {
    done = true;
    while (next.length > 0) next.pop()();
  });
  
  return {
    then: fn => {
      if (done) {
        fn();
      } 
      else {
        next.push(fn);
      }
    }
  };
}