ES6---Promise应用: async, await

ES6---Promise应用: async, await

• 前端两项工作 1.ui  2.交互
• Promise的应用 （Promise挺强大的）
• 1. async/await 就是基于promise的实现 （语法糖）
• 2. axios, fetch API(新一代语法取代jquery的ajax), 同样是基于Promise设计
•    (axios返回的内容本来就是一个promise, 一般而言, 还需要再进行一次promise的封装)

 

---

 

1. k2

        var k1 = new Promise((resolve, reject) => {
            resolve('ok');
        })
        console.log(k1);
        async function k2() {

        }
        console.log(k2);

 

console:

 

 2. 

        var k1 = new Promise((resolve, reject) => {
            resolve('ok');
        })
        console.log(k1);
        async function k2() {

        }
        console.log(typeof k2);

 

console:

 

 3. k2()

        var k1 = new Promise((resolve, reject) => {
            resolve('ok');
        })
        console.log(k1);
        async function k2() {

        }
        console.log(k2());

 

console:

 

 

以上：k2和k2()执行后的结果不一样，前者是一个function对象，后者k2()是k2函数执行并返回的值，这个值的类型是promise

 

4.

        var k1 = new Promise((resolve, reject) => {
            resolve('ok');
        })
        console.log(k1);
        //k2和k2()执行后的结果不一样，前者是一个function对象，后者k2()是k2函数执行并返回的值，这个值的类型是promise
        async function k2() {
            return 'ok2';
        }
        console.log(k2());

 

console：

 

 5. 

        var k1 = new Promise((resolve, reject) => {
            resolve('ok');//目的，是不是找人接手
        }).then(result => {
            console.log('收到promise传过来的值:' + result);
        });
        console.log(k1);
        //k2和k2()执行后的结果不一样，前者是一个function对象，后者k2()是k2函数执行并返回的值，这个值的类型是promise
        async function k2() {
            return 'ok2';
        }
        k2().then(null);
        console.log(k2());

 

console:

 

 

6.

        var k1 = new Promise((resolve, reject) => {
            resolve('ok');//目的，是不是找人接手
        }).then(result => {
            console.log('收到promise传过来的值:' + result);
        });
        console.log(k1);
        //k2和k2()执行后的结果不一样，前者是一个function对象
        //而k2()是执行k2这个函数，并返回一个值，这个值的类型是promise
        async function k2() {
            return 'ok2';
        }
        k2().then(result => {
            console.log('收到promise传过来的值:' + result);
        });
        console.log(k2());

 

console:

 

7.

 

        //async 没有then, return promise类型（或者一个带then的类型{}/class）
        async function k3() {
            return new Promise((resolve, reject) => {
                setInterval(() => {
                    resolve('k3 is ok');
                }, 4000);

            })
        }
        k3().then(result => {
            console.log(result);
        }, error => { error })
        console.log(k3());

 

console:

4秒后

 8. 以上aysnc函数进一步简化，对比如下

        async function k4() {
            let tt = await '123b'; //await其实就是then
            console.log(tt);
        }
        k4();

 

console:

 

 

9. 和10一起对比观察

        new Promise((resolve, reject) => {
            resolve('123a');
        }).then(tt => {
            console.log(tt);
            return '456a'
        }).then(pp => {
            console.log(pp);

        })

 

console:

10. 

        async function k4() {
            let tt = await '123b'; //await其实就是then
            console.log(tt);
            let pp = await '456b';
            console.log(pp);
        }
        k4();

 

console:

 

 11. 对比如下:

 

 

12.

        new Promise((resolve, reject) => {
            resolve('123a');
        }).then(tt => {
            console.log(tt);
            return '456a'
        }).then(pp => {
            console.log(pp);
            return new Promise((resolve, reject) => {
                resolve('444444')
            })
        }).then(oo => {
            console.log(oo);
        })

 

console:

 

 

13.

        async function k4() {
            let tt = await '123b'; //await其实就是then
            console.log(tt);
            let pp = await '456b';
            console.log(pp);
            let oo = await new Promise((resolve, reject) => {
                resolve('444444b');
            })
            console.log(oo);
        }
        k4();

 

console:

 

14. 上面的放下面代码图片，对比看下

 

 

15.

        new Promise((resolve, reject) => {
            resolve('123a');
        }).then(tt => {
            console.log(tt);
            return '456a'
        }).then(pp => {
            console.log(pp);
            return new Promise((resolve, reject) => {
                setTimeout(() => {
                    resolve('444444');
                }, 3000);

            })
        }).then(oo => {
            console.log(oo);
        })

        async function k4() {
            let tt = await '123b'; //await其实就是then
            console.log(tt);
            let pp = await '456b';
            console.log(pp);
            let oo = await new Promise((resolve, reject) => {
                setTimeout(() => {
                    resolve('444444b');
                }, 2000);
            })
            console.log(oo);

console: