hack.js

function countRectangles(board) {
  let count = 0;
  let N = board.length;
  let M = board[0].length;
  
  for (let r = 0; r < N; r++) {
    for (let c = 0; c < M; c++) {
      for (let i = r; i < N; i++) {
        for (let j = c; j < M; j++) {
          let aCount = 0;
          let bCount = 0;
          for (let x = r; x <= I; x++) {
            for (let y = c; y <= j; y++) {
              if (board[x][y] === 'A') {
                aCount++;
              } else if (board[x][y] === 'B') {
                bCount++;
              }
            }
          }
          if (aCount === bCount) {
            count++;
          }
        }
      }
    }
  }
  return count;
}