async, await运用

1.async 函数声明及调用方式：

// async 函数声明,调用
const a1 = async() => {
    console.log("this is an async function1")
};

async function a2() {
    console.log("this is an async function2")
}

a1();
a2();
;(async ()=>{console.log("this is an async function3")})()

2.async 函数返回值：async 函数会返回一个Promise对象，所以处理返回值的方式和Promise一毛一样

// async 函数返回值
const b1 = async (arg) => {
    return arg
}
var res = b1('hello');
res.then(resolve=>{console.log(resolve)});

3.直接获取async 函数返回值： await

// 直接获取 async函数返回值: await需要与 async 函数搭配
var ret = async (x) => {
    return x
}
var solve = async () => {
    var some_ret = await ret('hello,world');
    console.log(some_ret);
}
solve();

// 也可以直接获取 Promise 返回值
var solvePromise = async() => {
    var some_ret = await new Promise(resolve=>{
        resolve('hello, world')
    })
    console.log(some_ret);
}
solvePromise();

await的作用是 中断当前函数体中语句的执行, 等待await 后 Promise 执行完毕;

4.async 函数串行执行

// async 函数串行执行方式
const a = async() => {
    const b = await (async () => {})()
    const c = await (async () => {})()
}
a();

5.async 函数并行执行

// async 函数并行执行方式
const a = async () => {
    const b = (async () => {})()
    const c = (async () => {})()
    const b1 = await b;
    const c1 = await c;
}
a();
// b,c 会分别先执行, 在b1 和 c1 变成串行