diff --git a/caesar/caesar.js b/caesar/caesar.js
index 5839278..47a07bf 100644
--- a/caesar/caesar.js
+++ b/caesar/caesar.js
@@ -5,6 +5,10 @@ const caesar = function(string, shiftValue) {
     .join("");
 };
 
+const isLetter = code => {
+  return (code >= 65 && code <= 90) || (code >= 97 && code <= 122);
+}
+
 // This function implements a version of the modulo operator
 // that returns the smallest positive remainder even for negative inputs.
 // See this link for details:
@@ -14,7 +18,7 @@ const mod = (n, m) => (n % m + m) % m;
 const shift = (char, shiftValue) => {
   const code = char.charCodeAt();
 
-  if ((code >= 65 && code <= 90) || (code >= 97 && code <= 122)) {
+  if (isLetter(code)) {
     const base = code < 97 ? 65 : 97;
     const shiftedCode = mod(code + shiftValue - base, 26) + base;
     return String.fromCharCode(shiftedCode);