Base64 encoding in Sproutcore

I need to invoke restful webservices in Sproutcore which requires authentication header for http request. I write a code like in Sproutcore:

authHeader: function () {
     var userAndPass = "username:password";
     var auth = "Basic " + Base64.encode(userAndPass);
     return auth;
 },
 

However, when I run it, sadi Base64 is not defined.

Anybody knows how to fix it or do it in sproutcore. Thanks

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