rail_fence_cipher_2.js

function railFenceCipher(operation, string, height = 3) {
  function encrypt(string, height, fence) {
    let currentRail = 0;
    let moveUp = true;
    let moveDown = false;
  
    for (let i = 0; i < fence[0].length; i += 1) {
      fence[currentRail].splice(i, 1, string[i]);
  
      if ((moveUp && currentRail + 1 > height - 1) ||
          (moveDown && currentRail - 1 < 0)) {
        if (moveUp) currentRail -= 1;
        if (moveDown) currentRail += 1;
        moveUp = !moveUp;
        moveDown = !moveDown;
      } else if (moveUp) {
        currentRail += 1;
      } else if (moveDown) {
        currentRail -= 1;
      }
    }

    return fence.reduce((str, rail) => str += rail.filter(char => /\w/.test(char)).join(''), '');
  }

  function decrypt(string, fence) {
    string = string.split('');
    fence = fence.map(rail => rail.map(char => char === 'placeholder' ? string.shift() : char));
    return fence.reduce((fence, rail) => fence += rail.join(' ') + '\n', '');
  }

  function addPlaceholders(fence, numberOfRails) {
    let cycle = numberOfRails * 2 - 2;

    fence.forEach((rail, index) => {
      const processRail = (rail, railIndex) => {
        while (railIndex < rail.length ) {
          rail.splice(railIndex, 1, 'placeholder');
          railIndex += cycle;
        }
      }

      let railIndex = index;
      let railNumber = index + 1;

      processRail(rail, railIndex);

      if (railNumber > 1 && railNumber < fence.length) {
        railIndex = cycle - railNumber + 1;
        processRail(rail, railIndex);
      }
    });

    return fence;
  }

  if (operation === 'encrypt' && height === 1) {
    return 'Height must be 2 or more.';
  } 

  string = string.replace(/\W/g, '');
  fence = [...Array(height)].map((_, i) => i)
              .map(rail => [...Array(string.length)].map((_, i) => i))
              .map(rail => rail.map(element => '.'));

  if (operation === 'encrypt') {
    return encrypt(string, height, fence);
  } else if (operation === 'decrypt') {
    return decrypt(string, addPlaceholders(fence, height));
  }
}

cipherText = 'WECRLTEERDSOEEFEAOCAIVDEN';
console.log(railFenceCipher('decrypt', cipherText, 3));

cipherText = '[IA_EZS]_ELYLK_UZERLIPL';
console.log(railFenceCipher('decrypt', cipherText, 3));

cipherText = 'INLLUCOOAHOVLSHEC';
console.log(railFenceCipher('decrypt', cipherText, 5));

message = 'WE ARE DISCOVERED FLEE AT ONCE';
console.log(railFenceCipher('encrypt', message, 3));

message = 'I LOVE LAUNCH SCHOOL!';
console.log(railFenceCipher('encrypt', message, 5));