dataType : "JSON",
is wrong. jQuery option values are case-sensitive.
It should be:
dataType : "json",
Or you can leave it out, and you will need to make sure you set a proper JSON header. (But jQuery also tries "intelligent guess").
BTW:
method: "POST",
is correct. Always check the documentation for exact spelling and capitalization of options.
As Jake suggests, you should learn to use your browser's inspection and debug tools for examining the data that was sent by the server.
I always urge developers to test their server functionality independently of their web pages. Use a command-line tool like cUrl to send requests to your server, and then either show results on the console or save results in a file for examination.