JS异步笔试题

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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