1、示例
<!DOCTYPE html>
<html lang="zh">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta http-equiv="X-UA-Compatible" content="ie=edge" />
<title>Promise和setTimeout执行顺序 面试题</title>
</head>
<body>
<script type="text/javascript"> setTimeout(function() { console.log(1) }, 0); new Promise(function(a, b) { console.log(2); for(var i = 0; i < 10; i++) { i == 9 && a(); } console.log(3); }).then(function() { console.log(4) }); console.log(5) </script>
</body>
</html>
2、解释
最需要 解释的是：then和settimeout执行顺序，即setTimeout(fn, 0)在下一轮“事件循环”开始时执行，Promise.then()在本轮“事件循环”结束时执行。因此then 函数先输出，settimeout后输出。
3、自测题：
<!DOCTYPE html>
<html lang="zh">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta http-equiv="X-UA-Compatible" content="ie=edge" />
<title>js 执行顺序</title>
</head>
<body>
<script src="https://cdn.bootcss.com/lodash.js/4.17.10/lodash.min.js"></script>
<script type="text/javascript"> console.log(1) setTimeout(function(){ console.log(2); let promise = new Promise(function(resolve, reject) { console.log(7); resolve() }).then(function(){ console.log(8) }); },1000); setTimeout(function(){ console.log(10); let promise = new Promise(function(resolve, reject) { console.log(11); resolve() }).then(function(){ console.log(12) }); },0); let promise = new Promise(function(resolve, reject) { console.log(3); resolve() }).then(function(){ console.log(4) }).then(function(){ console.log(9) }); console.log(5) </script>
</body>
</html>