Chess checker

chess_ex.js

function encode(p, p2) {
  let t = 0;
  t = (t << 6) | p;
  t = (t << 6) | p2;
  return t;
}
function decode(s) {
  return [s & (2 ** 6 - 1), s >> 6];
}

function check(s) {
  let k = -1,
    q,
    t,
    r,
    n;
  for (let i = 0; i < 64; i++) {
    if (s[i] === "K") k = i;
    if (s[i] === "Q") q = i;
    if (s[i] === "T") t = ((t || 0) << 6) | i;
    if (s[i] === "R") r = ((r || 0) << 6) | i;
    if (s[i] === "N") n = ((n || 0) << 6) | i;
  }
  function checkLine(piece) {
    let low, high;
    if (k % 8 === piece % 8) {
      [low, high] = [Math.min(k, piece), Math.max(k, piece)];
      isInCheck = true;
      for (let i = low + 8; i < high - 8; i += 8) {
        isInCheck = isInCheck && s[i] && s[i] === ".";
        if (!isInCheck) return false;
      }
      return true;
    }
    if (Math.floor(k / 8) === Math.floor(piece / 8)) {
      isInCheck = true;
      for (let i = low; i < high; i++) {
        isInCheck = isInCheck && s[i] && s[i] === ".";
        if (!isInCheck) return false;
      }
      return true;
    }
    return false;
  }
  function checkDiagonal(piece) {
    let low, high;
    if (Math.abs(k - piece) % 9 === 0) {
      [low, high] = [Math.min(k, piece), Math.max(k, piece)];
      isInCheck = true;
      for (let i = low + 9; i < high - 9; i += 9) {
        isInCheck = isInCheck && s[i] && s[i] === ".";
        if (!isInCheck) return false;
      }
      return true;
    }
    if (Math.abs(k - piece) % 7 === 0) {
      isInCheck = true;
      for (let i = low + 7; i < high - 7; i += 7) {
        isInCheck = isInCheck && s[i] && s[i] === ".";
        if (!isInCheck) return false;
      }
      return true;
    }
    return false;
  }
  //check for valid queen, tower and rook
  if (checkLine(q) || checkDiagonal(q)) return true;
  if (t !== undefined) {
    let tower = t >> 6;
    if (checkLine(tower)) return true;
    tower = t & (2 ** 6 - 1);
    if (checkLine(tower)) return true;
  }
  if (r !== undefined) {
    let rook = r >> 6;
    if (checkDiagonal(rook)) return true;
    rook = r & (2 ** 6 - 1);
    if (checkDiagonal(rook)) return true;
  }

  // check knights
  if (n !== undefined) {
    let knight = n >> 6;
    let diff = Math.abs(k - knight);
    if (diff === 10 || diff === 6 || diff === 15 || diff === 17) return true;
    knight = n & (2 ** 6 - 1);
    diff = Math.abs(k - knight);
    if (diff === 10 || diff === 6 || diff === 15 || diff === 17) return true;
  }

  // check pawns
  if (s[k + 7] === "P" || s[k + 9] === "P") return true;

  return false;
}