SignaturePad undo feature

by InferOn

HTML

<script src="https://szimek.github.io/signature_pad/js/signature_pad.js"></script>
<div class="wrapper">
  <canvas id="signature-pad" class="signature-pad" width=400 height=200></canvas>
</div>

<button id="save-png">Save as PNG</button>
<button id="save-jpeg">Save as JPEG</button>
<button id="save-svg">Save as SVG</button>
<button id="undo">Undo</button>
<button id="clear">Clear</button>

CSS

.wrapper {
  position: relative;
  width: 400px;
  height: 200px;
  -moz-user-select: none;
  -webkit-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

.signature-pad {
  position: absolute;
  left: 0;
  top: 0;
  width:400px;
  height:200px;
  background-color: white;
}

JavaScript

var canvas = document.getElementById('signature-pad');

// Adjust canvas coordinate space taking into account pixel ratio,
// to make it look crisp on mobile devices.
// This also causes canvas to be cleared.
function resizeCanvas() {
    // When zoomed out to less than 100%, for some very strange reason,
    // some browsers report devicePixelRatio as less than 1
    // and only part of the canvas is cleared then.
    var ratio =  Math.max(window.devicePixelRatio || 1, 1);
    canvas.width = canvas.offsetWidth * ratio;
    canvas.height = canvas.offsetHeight * ratio;
    canvas.getContext("2d").scale(ratio, ratio);
}

window.onresize = resizeCanvas;
resizeCanvas();

var signaturePad = new SignaturePad(canvas, {
  backgroundColor: 'rgb(255, 255, 255)' // necessary for saving image as JPEG; can be removed is only saving as PNG or SVG
});

document.getElementById('save-png').addEventListener('click', function () {
  if (signaturePad.isEmpty()) {
    return alert("Please provide a signature first.");
  }
  
  var data = signaturePad.toDataURL('image/png');
  console.log(data);
  window.open(data);
});

document.getElementById('save-jpeg').addEventListener('click', function () {
  if (signaturePad.isEmpty()) {
    return alert("Please provide a signature first.");
  }

  var data = signaturePad.toDataURL('image/jpeg');
  console.log(data);
  window.open(data);
});

document.getElementById('save-svg').addEventListener('click', function () {
  if (signaturePad.isEmpty()) {
    return alert("Please provide a signature first.");
  }

  var data = signaturePad.toDataURL('image/svg+xml');
  console.log(data);
  console.log(atob(data.split(',')[1]));
  window.open(data);
});

document.getElementById('clear').addEventListener('click', function () {
  signaturePad.clear();
});

document.getElementById('undo').addEventListener('click', function () {
	var data = signaturePad.toData();
  if (data) {
    data.pop(); // remove the last dot or line
    signaturePad.fromData(data);
...