2 promise状态

1-几种状态

// promise三种状态  pending fulfilled rejected
        //penging 待定 PromisrState
        var p = new Promise((resolve,reject)=>{
        })
        console.log(p);
                // fulfilled 已完成
        var f = new Promise((resolve,reject)=>{
            resolve(10)
        })
        console.log(f);

2-状态变化

//状态变化不可逆
        // pending -- fulfilled
        // pending -- rejected
var p = new Promise((resolve,reject)=>{
            setTimeout(()=>{
                resolve(10)
            },2000)
        })
        console.log(p);//pending -- fulfilled
        var r = new Promise((resolve,reject)=>{
            setTimeout(()=>{
                reject(20)
            },2000)
        })
        console.log(r);//pending -- rejected

3-状态触发

 <!-- 
        then 触发resolve catch触发 reject
         then   --触发之后的结果是promise，还是fulfilled
         catch  --触发之后的结果是promise, 还是fulfilled   
    -->
    <script>
        var p = Promise.resolve(10);
        console.log(p);             
        var result = p.then(res=>{
            console.log(res);
        })
        console.log(result);
        var r = Promise.reject(20);
        console.log(r);
        var re = r.catch(res=>{
            console.log(res);
        })
        console.log(re);

4-then-catch的返回值状态的变化

1—then

1--  pending --fulfilled
<!-- pending --fulfilled -->
    <script>
        var p = Promise.resolve(100).then(res=>{
            console.log(res);
        })
        console.log(p);  //Promise fulfilled
    </script>

2--  pending --rejected then函数中报错
<script>
        var p = Promise.resolve(100).then(res=>{
            console.log(res);
            throw "error"
        })
        console.log(p);//Promise rejected
    </script>

2—catch

5- then函数返回报错的处理

1、
<script>
        var p = Promise.resolve(10).then(res=>{
            console.log(res);
            throw "error"
        })
        var re = p.then(()=>{
            console.log(20);
        }).catch(res=>{
            console.log(res);
        })
        console.log(re);
  </script>

2、
<script>
        var r = Promise.reject(20).catch(res=>{
            console.log(res);  //20
        })
        console.log(r); //Promise fulfilled
        r.catch(()=>{    //catch不能触发 
            console.log("error"); //不输出
        }).then(()=>{   //then触发
            console.log(200); //200
        })
    </script>

6- catch函数返回报错的处理

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