标签：javascript
1. script with async = "async" and without defer: the browser load the outter-js and DOM in async mode, they are loaded at the same time.3. script without defer and without async: when browser meet a outter-js, it begin load the js file, when load action finished, when load the rest of DOM.
test1. async without defer VS without async and defer
<html>
<head>
<title>
async without defer
</title>
</head>
<body>
</body>
<script>
var beginTime = new Date();
console.log("Begin Time:"+beginTime.getTime());
</script>
<script async="async" src="1scriptElement_async.js">
</script>
<script>
var endTime = new Date();
console.log("End Time:"+endTime.getTime());
</script>
</html>
End Time:1439903376425
<html>
<head>
<title>
without defer and async
</title>
</head>
<body>
</body>
<script>
var beginTime = new Date();
console.log("Begin Time:"+beginTime.getTime());
</script>
<script src="1scriptElement_async.js">
</script>
<script>
var endTime = new Date();
console.log("End Time:"+endTime.getTime());
</script>
</html>End Time:1439903528115
test2. defer without async VS without async and defer
<html>
<head>
<title>
without async and defer
</title>
</head>
<script>
var beginTime = new Date();
console.log("Begin Time:"+beginTime.getTime());
</script>
<script src="1scriptElement_async.js">
</script>
<script>
hello();
var endTime = new Date();
console.log("End Time:"+endTime.getTime());
</script>
<body>
<img src="IMG1.JPG">
</body>
</html>hello,world!
End Time:1439904005804
<html>
<head>
<title>
defer without async
</title>
</head>
<script>
var beginTime = new Date();
console.log("Begin Time:"+beginTime.getTime());
</script>
<script defer="defer" src="1scriptElement_async.js">
</script>
<script>
hello();
var endTime = new Date();
console.log("End Time:"+endTime.getTime());
</script>
<body>
<img src="IMG1.JPG">
</body>
</html>Uncaught ReferenceError: hello is not defined
因为设置defer="defer"后，要等图像加载完成才能下载js，运行hello()时还没有下载js，所以chrome控制台显示错误。
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标签：javascript
原文地址：http://blog.csdn.net/tsinggao/article/details/47759841
