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

function drawChessBoard(canvas) {
  let ctx = canvas.getContext('2d');

  let indexMargin = (canvas.width + canvas.height) / 2 / 16;
  let w = canvas.width - indexMargin;
  let h = canvas.height - indexMargin;
  let bw = w / 8;
  let bh = h / 8;

  // Draw squares
  ctx.fillStyle = '#c0c0c0';
  for (let r = 0; r < 8; r++) {
    for (let c = 0; c < 8; c++) {
      if ((r + c) % 2) ctx.fillRect(bw*c+indexMargin, bh*r+indexMargin, bw, bh);
    }
  }

  // Draw letters
  let indexLetters = 'abcdefgh';
  ctx.fillStyle = 'black';
  let fontsize = indexMargin / 2;
  ctx.font = fontsize.toString() + 'px Monospace';
  for (let idx = 0; idx < 8; idx++) {
    ctx.fillText(indexLetters[idx],
                 bw*idx + bw/2 + indexMargin - fontsize / 4,
                 indexMargin / 2);
    ctx.fillText((8-idx).toString(),
                 indexMargin / 2 - fontsize / 4,
                 bh*idx + bh/2 + indexMargin + fontsize / 4);
  }
}

drawChessBoard(_canvas);