Append JSON Strings

This will do:

// Input:
var json_string = '{"name":"John"}';

// Convert JSON array to JavaScript object
var json_obj = JSON.parse( json_string );

// Add new key value pair "myData": "Helo World" to object
json_obj.myData = 'Hello World';

// Another more dynamic way to add new key/value pair
json_obj['myAnotherData'] = 'FooBar';

// Convert back to JSON string
json_string = JSON.stringify( json_obj );

// Log to console:
console.log( json_string );

Output:

{
    "name": "John",
    "myData": "Hello World",
    "myAnotherData": "FooBar"
}

jQuery JSON parser:

If you want to use jQuery parser or need support for IE7 you can replace

var json_obj = JSON.parse( json_string );

with

var json_obj = $.parseJSON( json_string );

Unfortunately, jQuery cannot convert json_object back to JSON string, you will need another plugin or library for that.

More on topic:

MDN documentation

Browser support