Lissajous Curve

<!DOCTYPE html>
<meta charset="utf-8">
<style>
  body {margin:0;}
  .controls {list-style: none; z-index: 99;}
  canvas {position: absolute; left: 300px; top: 0px;}
  ul {padding: 10px;}
</style>
<ul class="controls">
  <li>
    <input  type="range" 
            class="a" 
            min="0" 
            max="10" 
            step=".1" 
            value=".4">
    a: <text class="a"></text>
  </li>
  <li>
    <input  type="range" 
            class="b" 
            min="0" 
            max="10" 
            step=".1" 
            value=".6">
    b: <text class="b"></text>
  </li>
  <li>
    Phase: <input  type="checkbox" 
                    class="checkbox" 
                    checked>
  </li>
</ul>

const scale = Math.min(width, height)/3;

const canvas = document.querySelector('canvas');
      canvas.width = width;
      canvas.height = height;

const canvasCtx = canvas.getContext('2d');
      canvasCtx.fillStyle = 'rgb(255, 255, 255)';
      canvasCtx.strokeStyle = 'rgb(37, 37, 109)';
      canvasCtx.lineWidth = .01;

let offset = 0;

draw();

function draw() {
  const [a, b, rotate] = getVals();
  if (rotate) offset += .005;

document.querySelector("text.a").innerHTML = a;
  document.querySelector("text.b").innerHTML = b;

canvasCtx.fillRect(0, 0, width, height);
  canvasCtx.beginPath();

let i = 10000;

canvasCtx.moveTo(...calcPoint(a, b, i--));
  while (i--) canvasCtx.lineTo(...calcPoint(a, b, i));
  canvasCtx.stroke();

window.requestAnimationFrame(draw);
}

function getVals() {
  const a = +document.querySelector("input.a").value,
        b = +document.querySelector("input.b").value,
        rotate = document.querySelector(".checkbox").checked;  
  return [a, b, rotate];  
}

function calcPoint(a, b, i) {
  const x = Math.sin(a*i + offset) * scale + width/3;
  const y = Math.sin(b*i) * scale + height/2;
  return [x, y];
}
</script>