Passing Objects to API Request Handlers with ExpressJS

Loggers, DALs, and more

Modular, maintainable, extensible pattern for accessing objects without import.

The Problem

You have a need to access an object in multiple API request handlers, but don’t want to create a singleton (especially in JS) and import it everywhere.

The Solution

The solution is a pattern I’m calling Request Injection Middleware.

// <root>/index.ts
const localNeo4JDriver: Neo4JDriver = new Neo4JDriver(
    NEO4J_URI,
    NEO4J_UNAME,
    NEO4J_PW
);
localNeo4JDriver.initializeDriver();
...
const app: Express = express();
...
app.use((req, _, next) => {
    try {
        req.n4jDriver = localNeo4JDriver.getDriver()!;
    } catch (e) {
        log.error(e);
        process.exit(1);
    }
    next();
});

The Neo4JDriver class is used to instantiate a driver when the server starts for use during request handling. Since request handlers don’t seem to have proper access to app-level objects (they’re supposed to, but app.set() seems to only store strings properly and not objects), I decided to just inject my objects into the request object using a middleware lambda. They can then be accessed in the request handler.

// Register/index.ts
// router object for the register endpoint
router
    .post("/", async (req: Request, res: Response) => {
        const neo4jDriver: Driver = req.n4jDriver;

This way I don’t have to mess with JS singletons. I’m going to want a bunch of Neo4JDrivers spun up dynamically for federation purposes, so creating a singleton is not ideal.

To alter the Request object to permit the new parameters, add this to the environment.d.ts:

import { Driver } from 'neo4j-driver'
declare global {
	namespace Express {
		export interface Request {
			n4jDriver: Driver
		}
	}
}
export {}

Happy coding!

Neo4JDriver class

For reference, here is the Neo4JDriver class.

export class Neo4JDriver {
	private driver: Driver | null = null

	constructor(
		readonly uri: string,
		readonly uname: string,
		private pw: string
	) {}

	async initializeDriver(): Promise<boolean> {
		try {
			this.driver = neo4j.driver(this.uri, neo4j.auth.basic(this.uname, this.pw))
			const serverInfo = await this.driver.getServerInfo()
			log.info('Neo4J connection established')
			log.info(serverInfo)
		} catch (err) {
			if (typeof err === 'string') {
				log.error(`Neo4J connection error: ${err}`)
			} else if (err instanceof Neo4jError) {
				log.error(`Neo4J connection error: ${err}\nCause:${err?.cause}`)
			} else {
				log.error(`Neo4J connection error: ${JSON.stringify(err)}`)
			}
			return false
		}
		return true
	}

	getDriver(): Driver | null {
		return this.driver
	}
}