Alias auth0 users in mixpanel, expects `MIXPANEL_TOKEN` to be set in auth0 and `mpDistinctID` to be sent in `state` as base64 encoded JSON

auth0-mixpanel-alias.js

function (user, context, callback) {
  // https://github.com/auth0/rules/blob/master/rules/signup.md
  user.app_metadata = user.app_metadata || {};
  var state64 = context.request.query.state || context.request.body.state;
  // https://github.com/node-browser-compat/atob/blob/master/node-atob.js
  var state = new Buffer(state64, 'base64').toString('binary');
  var statep = JSON.parse(state);
  
  // short-circuit if the user has already been aliased.
  if (user.app_metadata.mp_aliased) return callback(null, user, context);
  
  // https://github.com/auth0/rules/blob/master/rules/mixpanel-track-event.md
  // https://mixpanel.com/help/reference/http#distinct-id-alias
  var mpEvent = {
    "event": "$create_alias",
    "properties": {
        "distinct_id": statep.mpDistinctID,
        "alias": user.user_id,
        "token": configuration.MIXPANEL_TOKEN
    }
  };

  var base64Event = new Buffer(JSON.stringify(mpEvent)).toString('base64');

  request.get({
    url: 'http://api.mixpanel.com/track/',
    qs: {
      data: base64Event
    }
  }, function (e, r, b){
    if (!e && r.statusCode === 200) {
      // https://github.com/auth0/rules/blob/master/rules/signup.md
      user.app_metadata.mp_aliased = true;
      auth0.users.updateAppMetadata(user.user_id, user.app_metadata)
      .then(function() {
        console.log('requested an alias for user ' + user.user_id + ' to ' + statep.mpDistinctID + ' with mixpanel');
        callback(null, user, context);
      })
      .catch(function(e) {
        console.log('error updating auth0 metadata for mixpanel aliasing');
        callback(e);
      });
    }
    else {
      console.log('error aliasing user with mixpanel');
      callback(e);
    }
  });
}