JavaScript的加载顺序
在HTML中，JavaScript可以写到程序的任何地方：<HTML>标签前， <BODY>标签前，<BODY>标签里面, <BODY>标签后, <HTML> 标签后.
下面，我们通过一个HTML中嵌套Javascript的小程序，来演示一下代码执行过程
1. 程序代码
2. 执行结果
<!DOCTYPE html><script> alert("script-before Html"); document.write("<br>"+"script-before Html"+"<br>"); document.write("<h2 id =‘pp1‘> This is an example about HTML load process procedure </h2><h2 id = ‘pp2‘> This is an example about HTML load process procedure </h2><h2 id = ‘pp3‘> This is an example about HTML load process procedure </h2>");</script><html lang="en"><title> Test </title><head><script> alert("script-head"); document.write("script-head"+"<br>");</script></head><body onload="alert(‘html-tag‘);"><script> alert("script-body-begin"); document.write("script-body begin"+"<br>");</script><h1> This is a HTML file Process Progress test file </h1><script> alert("script-body-end"); document.write("script-body end"+"<br>");</script></body><script> alert("script-body-outer"); document.write("script-body-outer"+"<br>"); document.getElementById("pp2").innerHTML = "modified by script-body-outer";</script></html><script> alert("script-html-outer"+"<br>"); document.write("script-html-outer"+"<br>"); document.getElementById("pp3").innerHTML = "modified by script-htlm-outer";</script>2. 执行结果
下图是执行完<Body>的输出
下面是执行完最终的输出：
3 代码解释
1. 这个程序文件是顺序执行的，先执行<HTML>标签前==》<head>...</head> ==》<body>...</body> ==》<\body>后==><\html>后
2. 在<HTML执行之前，调用JavaScript 创建了3个段落：
<h2 id =‘pp1‘> This is an example about HTML load process procedure </h2><h2 id = ‘pp2‘> This is an example about HTML load process procedure </h2><h2 id = ‘pp3‘> This is an example about HTML load process procedure </h2>");3. 在<\Body>和<\html> 执行后，修改<h2>标签id为"pp2"和"pp3"的内容
</body><script> alert("script-body-outer"); document.write("script-body-outer"+"<br>"); document.getElementById("pp2").innerHTML = "modified by script-body-outer";</script></html><script> alert("script-html-outer"+"<br>"); document.write("script-html-outer"+"<br>"); document.getElementById("pp3").innerHTML = "modified by script-htlm-outer";</script>4. 在<Html>标签后的Java语句，类似使用了Ajax机制，在文档加载完成，对部分内容进行了更改。