How do you implement A/B testing in a Node.js application?

Question

Accepted Answer

**A/B testing** — route users to different variants.

```javascript
const crypto = require("crypto");

class ABTest {
    constructor() {
        this.experiments = new Map();
    }

addExperiment(name, variants) {
        this.experiments.set(name, variants);
    }

getVariant(experimentName, userId) {
        const variants = this.experiments.get(experimentName);
        if (!variants) return null;

// Deterministic hash
        const hash = crypto
            .createHash("md5")
            .update(`${experimentName}:${userId}`)
            .digest("hex");
        const index = parseInt(hash.slice(0, 8), 16) % variants.length;
        return variants[index];
    }
}

// Middleware
const abTest = new ABTest();
abTest.addExperiment("checkout-flow", ["control", "simplified"]);

app.use((req, res, next) => {
    req.experiments = {};
    for (const [name] of abTest.experiments) {
        req.experiments[name] = abTest.getVariant(name, req.user.id);
    }
    next();
});

app.get("/checkout", (req, res) => {
    if (req.experiments["checkout-flow"] === "simplified") {
        return res.render("checkout-simplified");
    }
    res.render("checkout-control");
});
```

**Key points:**
- Deterministic assignment (hash).
- Track conversions per variant.
- Statistical significance before deciding.

Node.js: How do you implement A/B testing in a Node.js application?