JS异步笔试题

摘要：分析题目先执行微任务后执行宏任务因此结果为按照这个思路做以下题目做出如下更改运行结果为更改如下更改如下运行结果为运行结果为运行结果为参考自

参考自https://github.com/Advanced-F...

async function async1() {
    console.log("async1 start");
    await async2();
    console.log("async1 end");
}
async function async2() {
    console.log("async2");
}
console.log("script start");
setTimeout(function() {
    console.log("setTimeout");
}, 0)
async1();
new Promise(function(resolve) {
    console.log("promise1");
    resolve();
}).then(function() {
    console.log("promise2");
});
console.log("script end");

运行结果为:

script start
async1 start
async2
promise1
script end
async1 end
promise2
setTimeout

这道题涉及到事件循环中函数执行顺序的问题,其中包括async,await,setTimeout,Promise函数

1.event loop
JS主线程不断的循环往复的从任务队列中读取任务,执行任务,这种运行机制为事件循环.

2.Microtasks, Macrotasks
Microtasks(微任务)和Macrotasks(宏任务)是异步任务的一种类型,Microtasks的优先级要高于Macrotasks,下面是它们所包含的api:

microtasks

process.nextTicks
promise
Object.observe
MutationObserver



macrotasks

setTimeout
setImmerdiate
I/O
UI渲染

1. 每一个 event loop 都有一个 microtask queue
2. 每个 event loop 会有一个或多个macrotaks queue ( 也可以称为task queue )
3. 一个任务 task 可以放入 macrotask queue 也可以放入 microtask queue中
4. 每一次event loop，会首先执行 microtask queue， 执行完成后，会提取 macrotask queue 的一个任务加入 microtask queue， 接着继续执行microtask queue，依次执行下去直至所有任务执行结束。

分析题目:

先执行微任务后执行宏任务,因此结果为

script start
async1 start
async2
promise1
script end
async1 end
promise2
setTimeout

按照这个思路,做以下题目:

async function async1() {
    console.log("async1 start");
    await async2();
    console.log("async1 end");
}
async function async2() {
    //async2做出如下更改：
    new Promise(function(resolve) {
    console.log("promise1");
    resolve();
}).then(function() {
    console.log("promise2");
    });
}
console.log("script start");

setTimeout(function() {
    console.log("setTimeout");
}, 0)
async1();

new Promise(function(resolve) {
    console.log("promise3");
    resolve();
}).then(function() {
    console.log("promise4");
});

console.log("script end");

运行结果为:

script start
async1 start
promise1
promise3
script end
promise2
async1 end
promise4
setTimeout

async function async1() {
    console.log("async1 start");
    await async2();
    //更改如下：
    setTimeout(function() {
        console.log("setTimeout1")
    },0)
}
async function async2() {
    //更改如下：
    setTimeout(function() {
        console.log("setTimeout2")
    },0)
}
console.log("script start");

setTimeout(function() {
    console.log("setTimeout3");
}, 0)
async1();

new Promise(function(resolve) {
    console.log("promise1");
    resolve();
}).then(function() {
    console.log("promise2");
});
console.log("script end");

运行结果为

script start
async1 start
promise1
script end
promise2
setTimeout3
setTimeout2
setTimeout1

async function a1 () {
    console.log("a1 start")
    await a2()
    console.log("a1 end")
}
async function a2 () {
    console.log("a2")
}

console.log("script start")

setTimeout(() => {
    console.log("setTimeout")
}, 0)

Promise.resolve().then(() => {
    console.log("promise1")
})

a1()

let promise2 = new Promise((resolve) => {
    resolve("promise2.then")
    console.log("promise2")
})

promise2.then((res) => {
    console.log(res)
    Promise.resolve().then(() => {
        console.log("promise3")
    })
})
console.log("script end")

运行结果为

script start
a1 start
a2
promise2
script end
promise1
a1 end
promise2.then
promise3
setTimeout

async function async1() {
    console.log("async1 start")
    await async2()
    console.log("async1 end")
}
async function async2() {
    console.log("async2")
}
console.log("script start")
setTimeout(function () {
    console.log("settimeout")
})
async1()
new Promise(function (resolve) {
    console.log("promise1")
    resolve()
}).then(function () {
    console.log("promise2")
})
console.log("script end")

运行结果为

script start
async1 start
async2
promise1
script end
async1 end
promise2
settimeout

参考自https://github.com/Advanced-F...