Example of GrowthBook in JS routes

example-routes.js

import { GrowthBook } from "@growthbook/growthbook";

var express = require('express');
var router = express.Router();
...
// grab the Mixpanel factory
var Mixpanel = require('mixpanel');

// create an instance of the mixpanel client
var mixpanel = Mixpanel.init('<YOUR_TOKEN>');
...

...
// Home page route.
router.get('/', function (req, res) {
  // Define the experimental context
  const growthbook = new GrowthBook({
    // The attributes used to assign variations
    user: { id: req.userId }, // could use sessionId here if they're no userid.
    // Called when a user is put into an experiment
    trackingCallback: (experiment, result) => {
      mixpanel.track("Experiment Viewed", {
        distinct_id: req.sessionId,
        userId: req?.userId || null,
        sessionId: req.sessionId,
        experimentId: experiment.key,
        variationId: result.variationId,
      });
    },
  });
  
  // define the experiment:
  const { value } = growthbook.run({
    key: "my-experiment",
    variations: ["Control", "New Page"],
  });

  if(value === 1) {
    res.send('New Page');
  } else {
    res.send('Control');
  }
  
})