console 命令行工具 X clear

console

var img = new Image();
img.src = 'https://pic.zuhaowan.com/images/gg_img/2019-07-26/5d3a58f93503b.jpg';
img.onload = function() {
  draw(this);
};

function draw(img) {
  var canvas = document.getElementById('canvas');
  var ctx = canvas.getContext('2d');
  ctx.drawImage(img, 0, 0);
  img.style.display = 'none';
  var zoomctx = document.getElementById('zoom').getContext('2d');
 
  var smoothbtn = document.getElementById('smoothbtn');
  var toggleSmoothing = function(event) {
    zoomctx.imageSmoothingEnabled = this.checked;
    zoomctx.mozImageSmoothingEnabled = this.checked;
    zoomctx.webkitImageSmoothingEnabled = this.checked;
    zoomctx.msImageSmoothingEnabled = this.checked;
  };
  smoothbtn.addEventListener('change', toggleSmoothing);

  var zoom = function(event) {
    var x = event.layerX;
    var y = event.layerY;
    zoomctx.drawImage(canvas,
                      Math.abs(x - 5),
                      Math.abs(y - 5),
                      15, 15,
                      0, 0,
                      200, 200);
  };

  canvas.addEventListener('mousemove', zoom);
}

<html>

<body>
	<canvas id="canvas" width="300" height="227"></canvas>
<canvas id="zoom" width="300" height="227"></canvas>
<div>
<label for="smoothbtn">
  <input type="checkbox" name="smoothbtn" checked="checked" id="smoothbtn">
  平滑
</label>
</div>
</body>

</html>