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window.onload = function() {
  const canvas = document.getElementById('mycanvas');
  const ctx = canvas.getContext('2d');
  
  strokeRoundedRect(ctx, 10, 10, 100, 80, 20)

  function strokeRoundedRect(ctx, x, y, width, height, radius) {
    ctx.beginPath();
    ctx.moveTo(x+radius, y);
    ctx.arcTo(x+width, y, x+width, y+height, radius);
    ctx.arcTo(x+width, y+height, x, y+height, radius);
    ctx.arcTo(x, y+height, x, y, radius);
    ctx.arcTo(x, y, x+width, y, radius);
    ctx.stroke();
  }
}

canvas#mycanvas(width='640' height='400')