promise.js

const PENDING = 0;
const FULLFILLED = 1;
const REJECTED = 2;

function CustomPromise(executor) {
  let state = PENDING;
  let value = null;
  let handlers = [];
  let catches = [];
  
  function resolve(result) {
    if(state !== PENDING) return;
    
    state = FULFILLED;
    value = result;
    
    handlers.forEach((h) => h(value));
  }
  
  function reject(err) {
    if(state !== PENDING) return;
    
    state = REJECTED;
    value = err;
    catches.forEach((c) => c(err));
  }
  
  this.then = function (callback) {
    if(state === FULFILLED) {
      callback(value);
    } else {
      handlers.push(callback);
    }
  }
  
  executor(resolve, reject);
}

const doWork = (res, rej) => {
  setTimeout(() => { res("Hello World") }, 1000);
}

let someText = new CustomPromise(doWork);

someText.then((val) => {
  console.log("1st log:" + val);
})

setTimeout(() => {
  someText.then((val) => {
    console.log("3st log:" + val);
  })
},3000)