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Why add brackets to eval in JS_javascript tips

WBOYOriginal: 2016-05-16 15:05:401601browse

eval introduction

---eval is a method of the global object prototype in the ECMA definition;

---The parameter accepted by eval is js code in string format. This string will be executed by the execution engine (remember what I said in 'Advanced Programming', create a new execution engine at this time), and then return the result to The location where eval is called.

<!DOCTYPE html>
<html>
<head>
<title>eval学习</title>
<script type="text/javascript">
/*
eval("表达式");执行表达式语句
eval("("+javascript类型+")");转为javascript对象
*/
var jsonObj={"name":"ljl","data":123};//json,是javascript的对象
var jsonString='{"name":"ljl","data":123}';//javascript的string类型，字符串内容符合json格式的样式
var objType=eval("("+jsonString+")");//通过eval函数将json字符转为javascript对象
alert( typeof jsonString);//string
alert( typeof objType);//obj
alert(eval(123));//123
alert(typeof eval("("+123+")"));//number
var x=2;
var y=eval('x+1');//执行 2+1 表达式
alert('y= '+y);//3
</script>
</head> 
<body>
</body>
</html>

Additional: Summary

eval is one of the dynamic features of js. It can directly execute js programs and return results. A common use is to restore json data to js objects;

However, because it can dynamically change the context object at runtime, it brings the risk of injection attacks;

When using it, pay attention to how eval understands the syntax of strings. A common problem is the 'braces' problem

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