How do you implement a custom API versioning with backward compatibility?

Question

Accepted Answer

**API versioning** — support multiple API versions.

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

class APIVersioning {
    constructor() {
        this.versions = new Map();
    }

register(version, router) {
        this.versions.set(version, router);
    }

middleware() {
        return (req, res, next) => {
            const version = req.headers["api-version"] ||
                req.query.version ||
                this.extractFromPath(req.path);

const router = this.versions.get(version);
            if (!router) {
                return res.status(400).json({
                    error: `API version ${version} not supported`,
                    supported: Array.from(this.versions.keys()),
                });
            }

req.apiVersion = version;
            router(req, res, next);
        };
    }

extractFromPath(path) {
        const match = path.match(/\/v(\d+)\//);
        return match ? `v${match[1]}` : "v1";
    }
}

// Version-specific routers
const v1Router = express.Router();
v1Router.get("/users", (req, res) => {
    res.json({ users: [], version: "v1" });
});

const v2Router = express.Router();
v2Router.get("/users", (req, res) => {
    res.json({ data: [], meta: {}, version: "v2" });
});

// Setup
const api = new APIVersioning();
api.register("v1", v1Router);
api.register("v2", v2Router);

app.use("/api", api.middleware());
```

Node.js: How do you implement a custom API versioning with backward compatibility?