第十三章 Promise

1-1状态

三种状态
    pending、fulfilled、rejected

var p = new Promise((resolve,reject)=>{
        })
        var f = new Promise((resolve,reject)=>{
            resolve(10);
        })
        var r = new Promise((resolve,reject)=>{
            reject(20)
        })

1-2 状态的变化

状态的变化是不可逆的

pending--->fulfilled
 var p = new Promise((resolve,reject)=>{
           setTimeout(()=>{
               resolve(10);
           },2000)
       })

pending-->rejected
var r = new Promise((resolve,reject)=>{
           setTimeout(() => {
               reject(20)
           }, 2000);
       })

1-3 触发时状态的转变

reslove then -->触发后的结果是promise状态是fulfilled
reject   catch-->触发后的结果是promise，状态fulfilled

var p = Promise.resolve(10);
        var r = Promise.reject(10);
        // console.log(p);
        var result = p.then(res=>{
            console.log(res);
        })
        console.log(result);
        var result02 = r.catch(res=>{
            console.log(res);
        })
        console.log(result02);

1-4 then和catch返回值状态的变化

（1）pending—>resolve

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

（2）then函数中报错,返回值的状态为rejected

var p = Promise.resolve(100).then(res=>{
            // console.log(res);
            throw "error"
        })
        // console.log(p);  //Promise fulfilled

(3)then函数返回值报错的处理

var p = Promise.resolve(10).then(res=>{
    console.log(res);
    throw "error"
})
/* 
p  Promise   rejected 
 */
var res = p.then(()=>{
    console.log("20");
}).catch(res=>{
    console.log(res)
})
console.log(res)

(4)catch之后报错处理

var r =  Promise.reject(10).catch(res=>{
    console.log(res);  //10
    throw "error";
})
r.then(()=>{
    console.log(200);
}).catch(err=>{
    console.log(err);
})