Introduction to points to note when parsing JSON with jquery eval_jquery

There are generally two ways to parse JSON strings into JSON data format in JS:

1. One is to use the eval() function.

2. Use Function object to perform return analysis.

Use the eval function to parse, and use jquery's each method to traverse

Use jquery to parse JSON data, as the transmission object of jquery asynchronous request, jquery request The result returned is a json object. What is considered here is the string returned by the server in JSON form. For JSON objects encapsulated by plug-ins such as JSONObject, it is similar to this and will not be explained here.
The JSON string set is first given here. The string set is as follows:

The code is as follows:

Copy code The code is as follows:

var data="
{
root:
[
{name:'1',value:'0'},
{name:'6101',value:'Beijing'},
{name:'6102',value:'Tianjin'},
{name:'6103',value:'Shanghai' },
{name:'6104',value:'Chongqing City'},
{name:'6105',value:'Weinan City'},
{name:'6106',value: 'Yan'an City'},
{name:'6107',value:'Hanzhong City'},
{name:'6108',value:'Yulin City'},
{name:'6109 ',value:'Ankang City'},
{name:'6110',value:'Shangluo City'}
]
}";

Here is jquery asynchronous Based on the obtained data type - json object and string, the processing methods of the results obtained in the two ways are introduced respectively.

1. For the JSON string returned by the server, if the jquery asynchronous request does not specify the type, or accepts it in the form of a string, then it needs to be objectified. The method is not too troublesome, that is, convert the string Put it in eval() and execute it once. This method is also suitable for obtaining json objects in the ordinary javascipt method. The following example illustrates:

var dataObj=eval("(" data ")");//Convert to json object
Why do you need to eval here? What about adding "("(" data ")");//"?

The reason is: the problem of eval itself. Since json starts and ends with "{}", it will be processed as a statement block in JS, so it must be forced to be converted into an expression.

The purpose of adding parentheses is to force the eval function to convert the expression in the parentheses into an object when processing JavaScript code, rather than executing it as a statement. For example, take the object literal {}. If no outer brackets are added, then eval will recognize the braces as the beginning and end marks of the JavaScript code block, and {} will be considered to execute an empty statement. So the following two execution results are different:

Copy code The code is as follows:

alert( eval("{}"); // return undefined
alert(eval("({})");// return object[Object]

For this writing method, in JS , you can see it everywhere

such as: (function()) {}(); When doing closure operations, etc. -------------------------------------------------- ---------------

Copy code The code is as follows:

alert(dataObj.root.length);//Output the number of child objects of root
$.each(dataObj.root,fucntion(idx,item){
if(idx==0){
return true;
}
//Output the name and value of each root sub-object
alert("name:" item.name ",value:" item.value);
} )

Note: For general js to generate json objects, you only need to replace the $.each() method with a for statement, and the others remain unchanged.

2. For the server. The returned JSON string, if the jquery asynchronous request sets the type (usually this configuration attribute) to "json", or uses the $.getJSON() method to obtain the server return, then the eval() method is not needed, because at this time The result obtained is already a json object. You only need to call the object directly. Here, the $.getJSON method is used as an example to illustrate the data processing method:

Copy code The code is as follows:

$.getJSON("http://www.phpzixue.cn/",{param:"gaoyusi"},function(data){
//The data returned here is already a json object
//The following other operations are the same as the first case
$.each(data.root,function(idx,item){
if(idx ==0){
return true;//Same as countinue, return false the same as break
}
alert("name:" item.name ",value:" item.value);
} );
});


What needs special attention here is that the eval() method in method 1 dynamically executes the string (possibly a js script), which can easily cause system security issues. Therefore, you can use some third-party client script libraries that circumvent eval(). For example, JSON in JavaScript provides a script library of no more than 3k.

The second method of parsing is to use the Function object. Its typical application is the parsing of the returned data such as success under the AJAX method in JQUERY

Copy code The code is as follows:

var json='{"name":"CJ","age":18}';
data =(new Function("","return " json))();

The data at this time is a json object that will be parsed.