20191115-异步
<h5>1.异步笔试题</h5>
<pre><code class="language-javascript">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');
//script start => // 同步
//async1 start => //异步1-第一步
//async2 => //异步2-第一步
//=>promise1 => //异步3-第一步
//=>script end => //同步
//=>async1 end => //异步1-第二步
//=>promise2 => //异步2-第二步
//=>setTimeout => //setTimeOut 回调
</code></pre>
<h5>2.异步第二题</h5>
<pre><code class="language-javascript">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');
//1. 'script start'
//2. 'async1 start'
//3. 'promise1'
//4. 'promise3'
//5. 'script end'
//6. 'promise2'
//7. 'async1 end'
//8. 'promise4'
//9. 'script end'</code></pre>
<h5>3.异步第三题</h5>
<pre><code class="language-javascript">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');
//1.'script start'
//2.'async1 start'
//3.'promise1'
//4.'script end'
//5.'promise2'
//6.'setTimeout3'
//7.'setTimeout2'
//8.'setTimeout1'
//提示：在输出为promise2之后，接下来会按照加入setTimeout队列的顺序来依次输出，通过代码我们可以看到加入顺序为3 2 1，所以会按3，2，1的顺序来输出。</code></pre>
<h5>4.异步第四题</h5>
<pre><code class="language-javascript">async function a1 () {
console.log('a1 start')
await a2()
console.log('a1 end') //执行结束
}
async function a2 () {
console.log('a2')
}
console.log('script start')
setTimeout(() => {
console.log('setTimeout')
}, 0)
Promise.resolve().then(() => {
console.log('promise1') //异步直接执行第一步
})
a1()
let promise2 = new Promise((resolve) => {
resolve('promise2.then')
console.log('promise2') //直接执行
})
promise2.then((res) => {
console.log(res)
Promise.resolve().then(() => {
console.log('promise3')
})
})
console.log('script end')
//1.'script start'
//2.'a1 start'
//3.'a2'
//4.'promise2'
//5.'script end'
//6.'promise1'
//7.'a1 end'
//8.'promise2.then'
//9.'promise3'
//10.'setTimeout'</code></pre>
<h4></h4>