canvas-toBlob.js | A canvas.toBlob implementation | Canvas library

i have visited a github code to increase effort for saving a file from javascript/html. We have some problem with to use it, because it mae for node.js. I'v found some demo.js code to save the file from demo.html itself. But, it is useless because i unfortunatly can't edit, because i wonder it just for demo.html class, and it will become crash if i make a reference to another html file.

This is the code File Saver.js

I have a div which the content is filled dynamically from the server and plot some bar chart, after which I am try to get this same div and append it to form data so to save it as an image on the server. The first part work perfectly (plotting the chart). The second part which I've tried with the code below only upload an empty PNG file with no content of chart.