JavaScript的加载顺序

tianqi 2020-06-21

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

 JavaScript的加载顺序

下面是执行完最终的输出:

 JavaScript的加载顺序

 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。

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