Convert base64 string to ArrayBuffer

I need to convert a base64 encode string into an ArrayBuffer. The base64 strings are user input, they will be copy and pasted from an email, so they're not there when the page is loaded. I would like to do this in javascript without making an ajax call to the server if possible.

I found those links interesting, but they didt'n help me:

ArrayBuffer to base64 encoded string

this is about the opposite conversion, from ArrayBuffer to base64, not the other way round

http://jsperf.com/json-vs-base64/2

this looks good but i can't figure out how to use the code.

Is there an easy (maybe native) way to do the conversion? thanks