有的网页为了防止 tampermonkey 这类插件读取页面中的 js 变量，把变量写到了匿名自调用函数 self-invocation 中，例如

<script type="text/javascript">
(function() {
  var a = 1;
  console.log(a);
})();
</script>
而我为了某些不可告人的目的必须读取到变量 a 的值。

一种有效的方式是，使用 ajax 获取当前页面的源代码，然后正则匹配中某个指定变量的值。

正则匹配
var s = ‘var name = "hahahaha"‘;
var s = ‘var name = "hahahaha";‘;

s.match(/var \w+ = (.+);/);
0: "var name = "hahahaha";"
1: ""hahahaha""

var s = ‘var name = "hahahaha"; var age = 10;‘;
s.match(/var \w+ = (.+);/);
0: "var name = "hahahaha"; var age = 10;"
1: ""hahahaha"; var age = 10"

// 避免贪婪匹配
s.match(/var \w+ = (.+?);/);
0: "var name = "hahahaha";"
1: ""hahahaha""

s.match(/var age = (.+?);/);
0: "var age = 10;"
1: "10"
避免贪婪匹配
*  - zero or more
*? - zero or more (non-greedy)
+  - one or more
+? - one or more (non-greedy)
?  - zero or one
?? - zero or one (non-greedy)