页面动态加入<script>标签并执行代码

https://www.cnblogs.com/lvdabao/p/4253704.html

1、document.write() 

1）使用此方法插入的script标签，会被紧跟着放到承载document.write()方法的script标签后面，

DOCTYPE html>
<html lang="en">
<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>Documenttitle>
    <script>
        var a = "