The atob() and btoa() methods allow authors to transform content to and from the base64 encoding.
In these APIs, for mnemonic purposes, the “b” can be considered to
stand for “binary”, and the “a” for “ASCII”. In practice, though, for
primarily historical reasons, both the input and output of these
functions are Unicode strings.
From : http://www.w3.org/TR/html/webappapis.html#atob