Q:

why using eval and parsonJson together?

I think jquery $.parseJSON can convert jsons string to JavaScript object, why someone still use eval($.parseJSON) together?

A:

Seems like redundant at all. 
jQuery parseJSON function uses native JSON in browsers that support it. According to Douglas Crockford (the author of original JSON specification), "the parse method uses the eval method to do the parsing, guarding it with several regular expressions to defend against accidental code execution hazards." 
If the browser does not support JSON natively, jQuery uses new Function constructor to return parsed object, which is eval() equivalent with some scoping differences.

