📘The game grid

Every Tetris rests on a grid: an array of rows, each row an array of cells (0 = empty, 1 = filled).

Classic trap: every row must be a FRESH array. If all rows share the same array, filling one cell fills them all!

function makeGrid(rows, cols) {
  var grid = [];
  for (var r = 0; r < rows; r++) {
    var row = [];
    // fill row with 0s, then grid.push(row)
  }
  return grid;
}