Comparasion

function getFileExtension(i) {

    // i will be a string, but it may not have a file extension.
    // return the file extension (with no period) if it has one, otherwise false
   if(i.indexOf(".") > 0) {
       return i.slice(i.indexOf(".") + 1, i.length);
   } else {
       return false;
   }
}

// here we go! Given a filename in a string (like 'test.jpg'),

getFileExtension('pictureofme.pdf');