var _canvas = document.getElementById("chessBoard");

function drawChessBoard(canvas) {
  let ctx = canvas.getContext("2d");
  let w = canvas.width;
  let h = canvas.height;
  ctx.fillStyle = '#c0c0c0';
  for (let r = 0; r < 8; r++) {
    for (let c = 0; c < 8; c++) {
      if ((r + c) % 2) ctx.fillRect(w/8*r, h/8*c, w/8, h/8);
    }
  }
}

drawChessBoard(_canvas);