promise.js

const PENDING = Symbol();
const REJECTED = Symbol();
const FULLFILLED = Symbol();

const MyPromise = function(fn) {
  this.state = PENDING;
  this.value = '';

  const resolve = (value) => {
    this.state = FULLFILLED;
    this.value = value;
  }

  const reject = (error) => {
    this.state = REJECTED;
    this.value = error;
  }

  this.then = (onFullFill, onReject) => {
    if (this.state == FULLFILLED) {
      onFullFill(this.value);
    } else {
      onReject(this.value);
    }
  }

  try {
    fn(resolve, reject);
  } catch(error) {
    reject(error);
  }
}

// test
let p = new MyPromise((resolve, reject) => {
  resolve('hello');
})
p.then(res => {
  console.log(res);  // hello
})