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> ==》<ody>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。