Write a Promise by hand

Promises primarily address the problem of callback hell. You can learn about promises by comparing them to, for example, how some HTTP requests would look if they were implemented with promises versus if they were written with callback functions. Here is a simple handwritten promise, let’s unveil the promise as it really is!

  const PENDING = Symbol('pending')
  const FULFILLED = Symbol('resolved')
  const REJECTED = Symbol('reject')
  function MyPromise(fn){
      if(typeoffn! = ='function') {throw new Error('fn must be a function')}let state = PENDING
      let value = null
      let handler = {}
      function resolve(result){
          try{
              state=FULFILLED
              value=result
              handler.onFulfill(value)
          }catch(err){
              reject(err)
          }
      }
      function reject(error){
          state=REJECTED;
          value=error;
      }
      this.then=(onFulfill,onReject) = >{
          switch(state){
              case FULFILLED:onFulfill(value) 
                  break
              case REJECTED:onReject(value)
                  break
              case PENDING:handler = { onFulfill,onReject }
          }
      }
      fn(resolve,reject)
  }
  let p=new MyPromise((resolve,reject) = >{
      setTimeout((a)= >{
          console.log(3.' setTimeout ')
          resolve('hello promise')},3000)})console.log(1,p)
  p.then((val) = >{
      console.log(4,val)
  })
  console.log(2.'ha ha')
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The results

Related to the case

Sleep (3000). Then (()=>{console.log(‘…. ‘) ‘)}) Console. log is executed after 3s

Much like promise, I scribbled a way to implement it, and hope you can correct me or propose a better way to implement it

let sleep=function(ms){
    let resolve=function(){
        console.log('Ho ho ho')};let fn = function(){
        setTimeout((a)= >{
            resolve()
        },ms)
        return fn
    } 
    Function.prototype.then=function(fun){
        resolve=fun;
    }
    return fn();
}
sleep(3000).then((a)= >{console.log('Hahaha')})
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