How do I add a subfolder to Cpath?

I have this code and I want to add subfolder to the code so the output files will go in a subfolder named pdf. How can I do this??

/* Extract Pages to Folder */

    var re = /.*\/|\.pdf$/ig;

    var filename = this.path.replace(re,"");

    {

        for ( var i = 0;  i < this.numPages; i++ )

        this.extractPages

         ({

            nStart: i,

            cPath : Number(filename) + (i+1) + ".pdf"

        });

    };