Update caesar.js

caesar/caesar.js

@@ -7,6 +7,9 @@ const caesar = function(string, shift) {
 
 const codeSet = code => (code < 97 ? 65 : 97);
 
+# this function is just a fancy way of doing % so that it works with negative numbers
+# see this link for details:
+# https://stackoverflow.com/questions/4467539/javascript-modulo-gives-a-negative-result-for-negative-numbers
 const mod = (n, m) => (n % m + m) % m;
 
 const shiftChar = (char, shift) => {