Add caesars-cipher.js

Javascript/caesars-cipher.js

@@ -0,0 +1,18 @@
+function rot13(str) {
+  var result = "";
+  const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+  const alphaShift = alphabet.substring(13) + alphabet.substring(0,13);
+
+  for (var char = 0; char < str.length; char++) {
+    if (alphabet.indexOf(str[char]) != -1) {
+      var getIndex = alphaShift.indexOf(str[char]);
+      result += alphabet[getIndex];
+    } else {
+      result += str[char];
+    }
+  }
+
+  return result;
+}
+
+rot13("SERR PBQR PNZC");