browser.js

'use strict';

var crypto = global.crypto || global.msCrypto
if(crypto && crypto.getRandomValues) {
  module.exports = randomBytes;
} else {
  module.exports = oldBrowser;
}
function randomBytes(size, cb) {
  var bytes = new Buffer(size); //in browserify, this is an extended Uint8Array
    /* This will not work in older browsers.
     * See https://developer.mozilla.org/en-US/docs/Web/API/window.crypto.getRandomValues
     */

  crypto.getRandomValues(bytes);
  if (typeof cb === 'function') {
    return process.nextTick(function () {
      cb(null, bytes);
    });
  }
  return bytes;
}
function oldBrowser() {
  throw new Error(
      'secure random number generation not supported by this browser\n'+
      'use chrome, FireFox or Internet Explorer 11'
    )
}