Zooming stage relative to pointer position

Instructions: Mousewheel over canvas.
Konva Zoom Image on Hover Demoview raw
<!DOCTYPE html>
<html>
  <head>
    <script src="https://unpkg.com/[email protected]/konva.min.js"></script>
    <meta charset="utf-8" />
    <title>Konva Zoom Relative to Stage Demo</title>
    <style>
      body {
        margin: 0;
        padding: 0;
        overflow: hidden;
        background-color: #f0f0f0;
      }
    </style>
  </head>

  <body>
    <div id="container"></div>
    <script>
      var width = window.innerWidth;
      var height = window.innerHeight;

      var stage = new Konva.Stage({
        container: 'container',
        width: width,
        height: height,
      });

      var layer = new Konva.Layer();
      stage.add(layer);

      var circle = new Konva.Circle({
        x: stage.width() / 2,
        y: stage.height() / 2,
        radius: 50,
        fill: 'green',
      });
      layer.add(circle);

      var scaleBy = 1.01;
      stage.on('wheel', (e) => {
        // stop default scrolling
        e.evt.preventDefault();

        var oldScale = stage.scaleX();
        var pointer = stage.getPointerPosition();

        var mousePointTo = {
          x: (pointer.x - stage.x()) / oldScale,
          y: (pointer.y - stage.y()) / oldScale,
        };

        // how to scale? Zoom in? Or zoom out?
        let direction = e.evt.deltaY > 0 ? 1 : -1;

        // when we zoom on trackpad, e.evt.ctrlKey is true
        // in that case lets revert direction
        if (e.evt.ctrlKey) {
          direction = -direction;
        }

        var newScale = direction > 0 ? oldScale * scaleBy : oldScale / scaleBy;

        stage.scale({ x: newScale, y: newScale });

        var newPos = {
          x: pointer.x - mousePointTo.x * newScale,
          y: pointer.y - mousePointTo.y * newScale,
        };
        stage.position(newPos);
      });
    </script>
  </body>
</html>
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