Adding the Caesar's Cipher algorithm

Ciphers/caesarsCipher.js

@@ -0,0 +1,43 @@
+/**
+ * Caesar's Cipher - also known as the ROT13 Cipher is when
+ * a letter is replaced by the one that is 13 spaces away 
+ * from it in the alphabet. If the letter is in the first half
+ * of the alphabet we add 13, if it's in the latter half we
+ * subtract 13 from the character code value.
+ */
+
+/**
+ * Decrypt a ROT13 cipher
+ * @param {String} str - string to be decrypted
+ * @return {String} decrypted string
+ */
+function rot13(str) {
+  let response = [];
+  let strLength = str.length;
+
+  for (let i =0; i < strLength; i++) {
+    const char = str.charCodeAt(i);
+
+    switch(true) {
+      // Check for non-letter characters
+      case char < 65 || (char > 90 && char < 97) || char > 122:
+        response.push(str.charAt(i));
+        break;
+      // Letters from the second half of the alphabet
+      case (char > 77 && char <= 90 ) || (char > 109 && char <= 122):
+        response.push(String.fromCharCode(str.charCodeAt(i) - 13));
+        break;
+      // Letters from the first half of the alphabet
+      default:
+        response.push(String.fromCharCode(str.charCodeAt(i) + 13));
+    }
+  }
+  return response.join('');
+}
+
+
+// Caesars Cipher Example
+const encryptedString = 'Uryyb Jbeyq';
+const decryptedString = rot13(encryptedString);
+
+console.log(decryptedString); // Hello World