async/await与Promise

async/await是语法糖，可以让我们以同步代码的写法执行异步逻辑。

async/await语法

需要注意的是await后面的代码执行逻辑，几个示例

demo1:

async function async1() {
    console.log("async1 start");
    await async2();
    console.log("async1 end");
}
async function async2() {
    console.log("async2 end");
}

console.log("script start");
async1();
console.log("script end");

/**
 * Result:
 * script start
    async1 start
    async2 end
    script end
    async1 end
*/

demo2: 为了演示await async1的效果，外面又套了一层async方法

(async function () {
    async function async1() {
        console.log("async1 start");
        await async2();
        console.log("async1 end");
    }
    async function async2() {
        console.log("async2 end");
    }

    console.log("script start");
    await async1();
    console.log("script end");
})();

/**
 * Result:
 * script start
    async1 start
    async2 end
    async1 end  *这里因为await提前了*
    script end
*/

与Promise的关系

关于捕获异常

try…catch无法捕获Promise的reject。而await则可以

try {
    Promise.reject("promise error");
} catch {
    // 控制台打出 Uncaught (in promise) promise error；说明未被捕获
    console.log("get promise error");
}

(async function () {
    try {
        await Promise.reject("error");
    } catch {
        // 输出get async error
        console.log("get async error");
    }
})();