Save modified property value #20

Hi,
i'm a javascript newbi and i don't understand how to save a modified property value in a properties file. Can someone help me?

I have a properties file, that i want to edit in a GUI. The properties file comes in as a string, gets parsed, so that i have an array with properties. After editing the properties values i want to change them in the file and send it back as string. Everything else in this file, like the comments, should not be touched and should stay in the file-string.

I have tried to do this via reviver, but it didn't work. Have someone an example?