Save svg image rendered by a javascript to local disk as .png file

I am new to SVG and not an advanced user of JavaScript. I have a webpage with svg content dynamically rendered by javascript. In Internet Explorer when I right click on the svg content, I get option "Save Picture As" and I am able to save the content as png or svg.

How do I programatically do it by having a button and allow user to save the content in png on to their machine.