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

btoa

Uses Buffer to emulate the exact functionality of the browser's btoa (except that it supports unicode and the browser may not).

It turns binary data to base64-encoded ascii.

(function () {
  "use strict";
  
  var btoa = require('btoa')
    , bin = "Hello, 世界"
    , b64 = btoa(bin)
    ;

  console.log(b64); // "SGVsbG8sIBZM"
}());

Note: Unicode may or may not be handled incorrectly.