promise

[xlearns]

执行顺序

• 参考
• 参考

Promise.resolve().then(() => {
    console.log(0);
    return Promise.resolve(4);
}).then((res) => {
    console.log(res)
})

Promise.resolve().then(() => {
    console.log(1);
}).then(() => {
    console.log(2);
}).then(() => {
    console.log(3);
}).then(() => {
    console.log(5);
}).then(() =>{
    console.log(6);
}) 

雏形

 function Promise(fn){
    this.status = 'pending'
    this.value = null
    this.reason = null
    const success=(data)=>{
      if(this.status=='pending'){
         this.value = data
      }
    }
    const error=(data)=>{
      if(this.status=='pending'){
         this.reason = data
      }
    }
    fn(success,error)
  }
  Promise.prototype.then = function(success,error){
    success(this.value)
    error(this.reason)
  }

  let promise1 = new Promise((resolve, reject) => {
    resolve('data')
    reject('error')
  })
  promise1.then(data => {
    console.log(data)
  },()=>{})

状态完善

异步完善

细节完善