Create files with javascript?

How can i reah the value of %appdata% inside javascript?

APPDATA is an environment variable, so $.getenv("APPDATA").

Should not this code create a file in C:\ directory?

new FIle() merely creates a reference to a [potential] file. You must open the file for writing first.

But it's quite bad style to start variable names with an uppercase letter -- uppercase first letters in Javascript should be reserved for Constructors, like new File(). So you want something like:

fille = new File('C:\\test.txt'); 
fille.open();
fille.write("Testwriting!");
fille.close();

You must also double the backslash in a quoted string.