async/await 使用

1.在函数之间加上async意味着：函数将返回一个Promise

                async f() {
                    return '444';
                },

                f().then(res=>{
                    console.log(res) //打印444
                }); 

不断的使用then()链式操作：

                async f1() {
                    return '1111';
                },
                async f2() {
                    return '2222';
                },
                async f3() {
                    return '3333';
                },
                obj() {

                    this.f1()
                        .then(res => {
                            console.log(res);///111
                            return this.f2();
                        })
                        .then((res) => {
                            console.log(res);///222
                            return this.f3();
                        }).then((res) => {
                            console.log(res);///333
                        });
                }

2.await的基本语法

                async f() {
                    let promise = new Promise((resolve, reject) => {
                        console.log('1111')
                        setTimeout(() => resolve("done!"), 2000)
                    });
                    let result = await promise; //等待promise的resolve执行完再执行
                    console.log(result); // "done!"  2s后打印
                },
                f();

函数执行将会在 let result = await promise 这一行暂停，直到Promise返回结果，因此上述代码将会2秒后打印出done!

再看：

                fn(num) {
                    return new Promise((resolve, reject) => {
                        setTimeout(() => {
                            resolve(2 * num)
                        }, 2000);
                    })
                },


                async testResult() {
                    let first = await this.fn(30);
                    let second = await this.fn(50);
                    let third = await this.fn(20);
                    console.log(first + second + third);//6s后打印出200
                }

          this.testResult()

调用testResult 函数，它里面遇到了await, await 表示等一下，代码就暂停到这里，不再向下执行了，它等什么呢？等后面的promise对象执行完毕，然后拿到promise resolve 的值并进行返回，返回值拿到之后，它继续向下执行。

再看过明了的：

                fn1() {
                    return new Promise((resolve, reject) => {
                        setTimeout(() => {
                            resolve(console.log('我是fn1,2s后输出'))
                        }, 2000);
                    })
                },
                async fn2() {
                    return new Promise((resolve, reject) => {
                        setTimeout(() => {
                            resolve(console.log('我是fn2,4s后输出'))
                        }, 2000);
                    })
                },
                async fn3() {
                    return new Promise((resolve, reject) => {
                        setTimeout(() => {
                            resolve(console.log('我是fn3,6s后输出'))
                        }, 2000);
                    })
                },


                async testResult() {
                    let first = await this.fn1();
                    let second = await this.fn2();
                    let third = await this.fn3();
                    console.log('我是最后打印d'); //6s后打印出
                }

       this.testResult()

浏览器打印：

 大概的基础用法就这样