1-5 promise进阶

1.promise的几种状态及状态的切换
2.then和catch的返回值

1.promise的三种状态  
    - pending  fulfilled rejected 
2.触发状态的
    pending -->fulfilled
  pending -->rejected
  变化的状态是不可逆的,一旦变化就不会返回。 
3.then和catch对状态的影响  (then和catch的链式调用)

1 状态的变化

1-1 pending 冻结

var p = new Promise((resolve,reject)=>{
})
console.log(p)

1-2 pending - resolve

var p = new Promise((resolve,reject)=>{
            setTimeout(()=>{
                resolve()
            })
 })
console.log(p)
 setTimeout(()=>{
      console.log(p)
 })

2 触发状态状

1.pending状态不会触发resolve和reject
2.resolve会触发之后的then回调函数
3.reject会触发catch回调函数

var p = Promise.resolve(100);
var r = Promise.reject(200);

3 then和catch改变状态 返回值

1.then正常返回resolve,有报错则返回reject
2.catch正常返回resolve,有报错则返回reject

3-1 pending —>resolve

var p = Promise.resolve(100).then(res=>{
             console.log(res)
})
console.log(p)  //返回

//没有报错可以继续then
p.then(()=>{
    console.log(200)
})

3-2 then函数中有报错,返回reject

var p = Promise.resolve(100).then(res=>{
             console.log(res)
             throw "不是函数"
 })
console.log(p)

3-3 有报错继续then函数

         var p = Promise.resolve(100).then(res=>{
             throw "error"
         })
         p.then(()=>{
             console.log("good")
         }).catch(res=>{
             console.log(res)
         })

3-4 catch之报错

var p = Promise.reject("err").catch(err=>{
             console.log(err)
             throw "fun"
})
console.log(p) //返回的promise的状态为  reject