You can find a Base64 encode/decode method here.

[Link: scotch.io]

Look at the 2nd section for browsers that don't support the encoding natively. There is a single-line object defined that you can use in Pronto to encode the HTTP Request Header if you want to build your own http client as I did in the first example.

Authentication: Basic [base64 encoded username:password string]

Add the HTTP Request header with this...

socket.write("Authentication: Basic base64string\r\n");