[JavaScript] Interesting eval

String calculation

1. let num = eval("1+1") => 2
2. let num = eval("x=10;y=20;x*y") => 200

JSON conversion

• Function defined as a string requires "(" and ")" as prefix and suffix
• Example:

var fctStr1 = 'function a() {}'
var fctStr2 = '(function a() {})'
var fct1 = eval(fctStr1)  // 返回undefined
var fct2 = eval(fctStr2)  // 返回一个函数

• Example:
  let num = eval("("+ "{'name':'zhangsan', age:20}" +")")=> {name:'zhangsan', age: 20}
  
  
  

but! Just have fun! Never use eval in a formal environment!

eval() is a dangerous function, it executes code with the same permissions as the caller. If the string code you run with eval() is modified by a malicious party (innocent person), you may end up running malicious code on the user's computer under the authority of your webpage/extension. More importantly, third-party code can see the scope of a certain eval() when it is called, which may also lead to some attacks in different ways