async await 理解

转载

mb5ff5930cde1cd 2021-09-12 16:02:00

参考 https://segmentfault.com/a/1190000007535316

案例1：

async function testAsync() {
    return "hello async";
}

const result = testAsync();
console.log(result);

结果是：

Promise { 'hello async' }

返回的是一个 promise 对象

案例2：

function getSomething() {
    return "something";
}

async function testAsync() {
    return Promise.resolve("hello async");
}

async function test() {
    const v1 = await getSomething();
    const v2 = await testAsync();
    console.log(v1, v2);
}

test();

结果是：

something hello async

await 不仅用于等待promise对象，还可以用于任何其他值，是一样的

案例3：

function getSomething() {

setTimeout(() => {

return "something";

}, 3000)

}

async function testAsync() {

return Promise.resolve("hello async");

}

async function test() {

const v1 = await getSomething();

const v2 = await testAsync();

console.log(v1, v2);

}

test();

结果是：

undefined 'hello async'

对于setTimeout也是一样，没有特殊待遇，会直接输出结果，如果在setTimeout前面增加return,则会将setTimeout函数返回

Timeout {
  _called: false,
  _idleTimeout: 1,
  _idlePrev: [TimersList],
  _idleNext: [Timeout],
  _idleStart: 85,
  _onTimeout: [Function],
  _timerArgs: undefined,
  _repeat: null,
  _destroyed: false,
  [Symbol(unrefed)]: false,
  [Symbol(asyncId)]: 8,
  [Symbol(triggerId)]: 1 }  'hello async'

案例4：

function getSomething() {
    return new Promise(() => {
        setTimeout(() => {
            return "something";
        }, 3000)
    })
}

async function testAsync() {
    return Promise.resolve("hello async");
}

async function test() {
    const v1 = await getSomething();
    const v2 = await testAsync();
    console.log(v1, v2);
}

test();

结果是：

没有输出值

案例5：

function getSomething() {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve("something");
        }, 3000)
    })
}

async function testAsync() {
    return Promise.resolve("hello async");
}

async function test() {
    const v1 = await getSomething();
    const v2 = await testAsync();
    console.log(v1, v2);
}

test();

结果是：

3s之后输出  something hello async

案例6：

function getSomething() {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve("something");
            return 111111;
        }, 3000)
    })
}

async function testAsync() {
    return Promise.resolve("hello async");
}

async function test() {
    const v1 = await getSomething();
    const v2 = await testAsync();
    console.log(v1, v2);
}

test();