how to parse json in javascript

Most browsers support JSON.parse(), which is defined in ECMA-262 5th Edition (the specification that JS is based on). Its usage is simple:

var json = '{"result":true,"count":1}',
    obj = JSON.parse(json);

alert(obj.count);

For the browsers that don't you can implement it using json2.js.

As noted in the comments, if you're already using jQuery, there is a $.parseJSON function that maps to JSON.parse if available or a form of eval in older browsers. However, this performs additional, unnecessary checks that are also performed by JSON.parse, so for the best all round performance I'd recommend using it like so:

var json = '{"result":true,"count":1}',
    obj = JSON && JSON.parse(json) || $.parseJSON(json);

This will ensure you use native JSON.parse immediately, rather than having jQuery perform sanity checks on the string before passing it to the native parsing function.

Without using a library you can use eval - the only time you should use. It's safer to use a library though.

eg...

var response = '{"result":true , "count":1}';

var parsedJSON = eval('('+response+')');

var result=parsedJSON.result;
var count=parsedJSON.count;

alert('result:'+result+' count:'+count);

First of all, you have to make sure that the JSON code is valid. After that, I would recommend using a JavaScript library such as jQuery or Prototype if you can because these things are handled well in those libraries. On the other hand, if you don't want to use a library and you can vouch for the validity of the JSON object, I would simply wrap string in an anonymous function and use the eval function. This is not recommended if you are getting the JSON object from another source that isn't absolutely trusted because the eval function allows for renegade code if you will. Here is an example of using the eval function:

var strJSON = '{"result":true,"count":1}';
var objJSON = eval("(function(){return " + strJSON + ";})()");
alert(objJSON.result);
alert(objJSON.count);

If you control what browser is being used or you are not worried people with older browser, you can always use the JSON.parse method. This is really the ideal solution for the future.