1function encode(str, rails) {
2 const fence = Array(rails).fill().map(() => []);
3 let rail = 0, dir = 1;
4 for (const ch of str) {
5 fence[rail].push(ch);
6 rail += dir;
7 if (rail === 0 || rail === rails - 1) dir *= -1;
8 }
9 return fence.flat().join("");
10}