D3js click function to save SVG as dataurl in IMG tag, load into CANVAS and save as PNG dataurl, and auto download the actual PNG file.

svg-image2.js

d3.select("#save").on("click", function(){
  var html = d3.select("svg")
        .attr("version", 1.1)
        .attr("xmlns", "http://www.w3.org/2000/svg")
        .node().parentNode.innerHTML;

  //console.log(html);
  var imgsrc = 'data:image/svg+xml;base64,'+ btoa(html);
  var img = '<img src="'+imgsrc+'">'; 
  d3.select("#svgdataurl").html(img);


  var canvas = document.querySelector("canvas"),
	  context = canvas.getContext("2d");

  var image = new Image;
  image.src = imgsrc;
  image.onload = function() {
	  context.drawImage(image, 0, 0);

	  var canvasdata = canvas.toDataURL("image/png");

	  var pngimg = '<img src="'+canvasdata+'">'; 
  	  d3.select("#pngdataurl").html(pngimg);

	  var a = document.createElement("a");
	  a.download = "sample.png";
	  a.href = canvasdata;
	  a.click();
  };

});

@ppierce01

http://lmgtfy.com/?q=http%3A%2F%2Fg.zeos.in%2F%3Fq%3Die9%2520btoa%2520is%2520undefined