Download canvas

window.onload = function () {
  let canvas = document.getElementById("canvas");
  let ctx = canvas.getContext("2d");

  let margin = 20;
  let rect = {
    x: margin,
    y: margin,
    width: canvas.width - (margin * 2),
    height: canvas.height - (margin * 2)
  }

  // Fill in canvas
  ctx.beginPath();
  ctx.rect(rect.x, rect.y, rect.width, rect.height, 20);
  ctx.stroke();

  let elem = window.document.createElement('a');
  elem.href = canvas.toDataURL("image/png");
  elem.download = 'img.png';
  elem.innerText = "download"
  document.body.appendChild(elem);
  // auto click
  //elem.click();
  //document.body.removeChild(elem);
}