Add Rail Fence cipher implementation in JavaScript

Ciphers/RailFenceCipher.js

@@ -0,0 +1,20 @@
+function railFenceCipher(text, rails) {
+  if (rails <= 1) return text;
+
+  const fence = Array.from({ length: rails }, () => []);
+  let rail = 0;
+  let direction = 1;
+
+  for (const char of text) {
+    fence[rail].push(char);
+    rail += direction;
+
+    if (rail === 0 || rail === rails - 1) {
+      direction *= -1;
+    }
+  }
+
+  return fence.flat().join('');
+}
+
+module.exports = railFenceCipher;