Handwriting promise and async await

promise

Have a look first promise To achieve the effect of

const Pro =  new Promise((resolve,reject) => {
    
  setTimeout(() => {
    
    resolve(123)
  }, 5000)
})
Pro.then((res) => {
     
  console.log(res,'----')
})
//  Run code 5 Seconds later, the console prints  123 "----"

Realize it by yourself

const PromiseFater = function(callBack) {
    
  this.callBack = callBack
  this.then = (back) => {
    
    this.callBack(back)
  }
}

const p = new PromiseFater((resolve, reject) => {
    
  console.log(1)
  setTimeout(() => {
    
    resolve(123)
  }, 5000)
})
p.then((res) => {
    
  console.log(res, '---')
})
//  Run code 5 Seconds later, the console prints  123 "---"

The above is just another way to explain promise Implementation principle of , Let's add reject

This code does not support continuous .then().then(), Just analyzed .then().catch() How to achieve it , Don't want to make too much code , This is inconvenient to see

class  MyPromise {
    
  constructor (callBack) {
    
    if (typeof callBack !== 'function') {
    
      throw new Error(' You must pass in a function ')
    }
     //  Add state 
     this.status = 'PENDING'
     this.thenFun = [] //  Deposit .then Incoming back Method 
     this.catchFun = [] //  Deposit .catch Incoming back Method 
     try {
    
      callBack(this.resolve, this.reject) 
    } catch (err) {
    
      this.reject(err)
    }
  }

  reject = (val) => {
    
    this.status = 'fail'
    this.catchFun(val)
  }
  resolve = (val) => {
    
    this.status = 'success'
    this.thenFun(val)
  }
  then = (back) => {
    
    if (typeof back !== 'function') {
    
      throw new Error('then Parameters must be passed into a function ')
    }
    this.thenFun = back
    return this
  }
  catch = (back) => {
    
    if (typeof back !== 'function') {
    
      throw new Error('catch Parameters must be passed into a function ')
    }
    this.catchFun = back
    return this
  }
}

const promise = new MyPromise((resolve, reject) => {
    
  setTimeout(() => {
    
    reject(' Failed tests ')
  }, 2000);
})
promise
  .then((res) => {
    
    console.log(res)
  })
  .catch((res) => {
    
    console.log(res)
  })
//  function , Print on the console   Failed tests 


 If you put 
const promise = new MyPromise((resolve, reject) => {
    
  setTimeout(() => {
    
    reject(' Failed tests ')
  }, 2000);
})
 Switch to 
const promise = new MyPromise((resolve, reject) => {
    
  setTimeout(() => {
    
    resolve(' Successful test ')
  }, 2000);
})
//  function , Print on the console   Successful test 

If you want to achieve continuous .then().then(), You can put the above thenFun Change to thenArr To save each then Incoming method , Call once through the state , But why , It's not good to write like this , use async await He doesn't smell good