Express.js

Set up Featurevisor SDK instance in an Express.js application using a custom middleware, including TypeScript integration for evaluating feature flags.

Hello World application

Before going into Featurevisor integration, let's create a simple Hello World Express.js application.

We start by installing the package:

$ npm install --save express

Then we create a file index.js with the following content:

// index.js
const express = require("express");

const PORT = 3000;

const app = express();

app.get("/", (req, res) => {
  res.send("Hello World!");
});

app.listen(PORT, () => {
  console.log(`Example app listening on port ${PORT}`);
});

We can start the server with this command:

$ node index.js
Example app listening on port 3000

Featurevisor integration

We install the Featurevisor SDK first:

$ npm install --save @featurevisor/sdk

We can now create an instance of the SDK and use it in our application:

// index.js
const express = require("express");
const { createInstance } = require("@featurevisor/sdk");

const PORT = 3000;
const DATAFILE_URL = "https://cdn.yoursite.com/datafile.json";
const REFRESH_INTERVAL = 60 * 5; // every 5 minutes

const app = express();

const f = createInstance({
  datafileUrl: DATAFILE_URL,

  // optionally refresh the datafile every 5 minutes,
  // without having to restart the server
  refreshInterval: REFRESH_INTERVAL,
});

app.get("/", (req, res) => {
  const featureKey = "myFeature";
  const context = { userId: "user-123" };

  const isEnabled = f.isEnabled(featureKey, context);

  if (isEnabled) {
    res.send("Hello World!");
  } else {
    res.send("Not enabled yet!");
  }
});

app.listen(PORT, () => {
  console.log(`Example app listening on port ${PORT}`);
});

Middleware

If is very unlikely that we will have all our routes defined in the same index.js file, making it difficult for us to use the same Featurevisor SDK instance in all of them.

To solve this problem, we can create a custom middleware that will set the Featurevisor SDK instance to the req object, so that we can use the same instance in all our routes throughout the lifecycle of this application.

// index.js

// ...

const f = createInstance({
  datafileUrl: DATAFILE_URL,
  refreshInterval: REFRESH_INTERVAL,
});

app.use((req, res, next) => {
  req.f = f;

  next();
});

// ...

Now from anywhere in our application (either in index.js or some other module), we can access the Featurevisor SDK instance via req.f:

app.get("/my-route", (req, res) => {
  const { f } = req;

  const featureKey = "myFeature";
  const context = { userId: "user-123" };

  const isEnabled = f.isEnabled(featureKey, context);

  if (isEnabled) {
    res.send("Hello World!");
  } else {
    res.send("Not enabled yet!");
  }
});

TypeScript usage

If you are using TypeScript, you can extend the Request interface to add the f property for Featurevisor SDK's instance.

Create a new custom.d.ts file and make sure to add it in tsconfig.json's files section:

import { FeaturevisorInstance } from "@featurevisor/sdk";

declare namespace Express {
  export interface Request {
    f: FeaturevisorInstance;
  }
}

Working repository

You can find a fully functional example of this integration on GitHub: https://github.com/featurevisor/featurevisor-example-expressjs.