node-browser-compat.js/atob/README.md

atob

Uses Buffer to emulate the exact functionality of the browser's atob.

Note: Unicode may be handled incorrectly (like the browser).

It turns base64-encoded ascii data back to binary.

(function () {
  "use strict";
  
  var atob = require('atob')
    , b64 = "SGVsbG8gV29ybGQ="
    , bin = atob(b64)
    ;

  console.log(bin); // "Hello World"
}());