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執行順序 面試題</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、解釋
最需要 解釋的是：then和settimeout執行順序，即setTimeout(fn, 0)在下一輪“事件循環”開始時執行，Promise.then()在本輪“事件循環”結束時執行。因此then 函數先輸出，settimeout后輸出。
3、自測題：
<!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 執行順序</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>