3.async -- await

3-1 async

        /* async可以将一个普通函数的返回值变成promise */
        async function go(){
            return "p"
        }
        console.log(go());//promise
        go().then(res=>{
            console.log(res)//p
        })

3-2 await

        /* 
        1、await关键字只能在async函数使用,不能在普通函数中使用
        2、await  相当于then,获取获取promise--resolve的结果
         */
         async function show(){
             console.log("start");
             var p =  await go();
             console.log(p);
             console.log("end")
         }
         async function go(){
             return "mid"
         }
         show();
//start
mid
end

        /* await  后面如果是promise会得到then的结果 */
        async function show(){
            console.log("start");
            var p  =await Promise.resolve("mid")
            console.log(p);
            console.log("end")
        }
        show();
//start
 mid
 end

        /* await
        1、相当于执行了promise的then函数
        2、await后面不一定跟promise
        3、await阻塞函数的执行,优先执行同步的代码再去之后await执行完毕之后的内容
         */
         async function show(){
             console.log("start");
             await mid();
             console.log("end")
         }
         async function mid(){
             console.log("mid");
         }
         show();
         console.log("a1");
//
start
mid
al
end

    <!-- await会阻塞函数的执行,会优先执行await后面同步的代码,异步代码等待同步代码执行完毕之后再去执行 -->
    <script>
        async function a1(){
            console.log("a1");
            await a2();
            console.log("end")
        }
        async function a2(){
            console.log("a2");
        }
        a1();
        console.log("a3");
        Promise.resolve().then(()=>{
            console.log("a4")
        })
        /* 
        a1
        a2
        a3
        end
        a4
         */
    </script>

3-3 await http请求

        /* await--async可以将异步变成同步代码 */
        var url = 'http://47.108.197.28:3000/top/playlist';
        async function show(){
            var res = await $.ajax({url});
            var id = res.playlists[0].id
            var url02 = `http://47.108.197.28:3000/playlist/detail?id=${id}`;
            var detail = await $.ajax({url:url02});
            console.log(detail)
        }
        show();