Javascripteval () usage _ javascript skills

Some eval usage methods and techniques for instance analysis. To put it simply, you can convert a string into an expression and return the execution result.
Eval Method
See
String object
Applied to: Global Objects
Requirements
Version 1
Check and execute the JScript code.
Eval (codeString)
The required codestring parameter is a string value that contains valid JScript code. This string will be analyzed and executed by the JScript analyzer.
Description
The eval function allows dynamic execution of JScript source code. For example, the following code creates a new variable named mydate that contains the Date object:
Eval ("var mydate = new Date ();");
The context when the Code passed to the eval method is executed is the same as that when the eval method is called.

<Script language = "JavaScript"> <! -- Var test = 'alert ("HI! ") '; Eval (test) // --> script
[Ctrl + A select all Note: If you need to introduce external Js, You need to refresh it to execute]

Let's look at this case:

<P> hide the following text </p> <p id = "aa1"> text 1 </p> <p id = "aa2"> text 2 </p> <p id = "aa3"> Text 3 </p> <p id = "aa4"> text 4 </p> remove the comment and run it to understand <script language =" javascript "> function hidep (str) {for (I = 1; I <= 4; I ++) {// (str + I ). style. display = "none"; // incorrect syntax, cannot hide the layer // eval (str + I ). style. display = "none"; // method 1 return expression str + I to the javascript value alert (str + I + ". style. display = 'none' "); // The eval (str + I + ". style. display = 'none' "); // or use eval to directly execute a string (string It must comply with the javascript syntax). The hidden text // string will be analyzed and executed by the JScript analyzer. // The expression represented by the string "str + I" executed by the eval () function, provided that the string must be an understandable javascript expression, eval () the function will execute and return the value of this string expression // aa1, aa2, aa3, aa4 // separate str + I // document. getElementById (str + I ). style. display = "none"; // equivalent to using this method} script
[Ctrl + A select all Note: If you need to introduce external Js, You need to refresh it to execute]