4.Promise 三种状态

一、promise有几种状态 PromiseState

1、 pending

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

2、fulfilled

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

3、rejected

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

二、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);
})

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

三、状态的触发

 resolve  then  -->触发之后的结果是Promise,状态fulfilled
 reject   catch -->触发之后的结果是Promise,状态fulfilled

var p = Promise.resolve(10);
var r = Promise.reject(20);
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)

四、then和catch返回值的状态的变化

1-1 pending—>resolve

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

1-2 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 rejected

1-3 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)

1-4 catch之后的报错

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