Dave's pwomise. Remember that the catch is not implemented correctly, recall the tree structure of Promise.

promiseunderthehood.js

class Pwomise {
  constructor (fn) {
    fn(this._resolve.bind(this), this._reject.bind(this))
    this._thens = []
    this._catches = []
  }

  _resolve (value) {
    while (this._thens.length > 0) {
      this._thens.pop()(value)
    }
  }

  _reject (err) {
    this._catches.pop()(err)
  }

  then (fn) {
    this._thens.push(fn)
  }

  catch (fn) {
    this._catches.push(fn)
  }
}

const p = new Pwomise((resolve, reject) => {
  setTimeout(() => {
    resolve('Hello Promises')
  }, 1000)
})

p.then(msg => {
  console.log(msg)
})