Dependency injection in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is dependency injection in Express?

Dependency injection is a way to provide parts (dependencies) that a piece of code needs, instead of creating them inside the code. In Express, it means passing services or objects to routes or middleware instead of hardcoding them.

beginner

Why use dependency injection in Express apps?

It helps keep code clean and easy to test. You can swap parts without changing the main code, making your app more flexible and easier to maintain.

intermediate

How can you inject a database connection into an Express route?

You can create the database connection outside the route and pass it as a parameter or attach it to the request object using middleware, so the route can use it without creating it again.

intermediate

Example: What does this code do?

const app = express();
const service = new Service();
app.use((req, res, next) => {
  req.service = service;
  next();
});

This middleware adds a service instance to every request as req.service. Routes can then use req.service to access the service without creating it themselves.

intermediate

What is a common pattern to inject dependencies in Express without global variables?

Use middleware to attach dependencies to the request object or use factory functions that create routes with dependencies passed as arguments.

What is the main goal of dependency injection in Express?

A To pass dependencies to routes instead of creating them inside

B To make routes slower

C To avoid using middleware

D To hardcode all dependencies inside routes

Dependency injection means passing dependencies to routes or middleware instead of creating them inside, making code cleaner and easier to test.

How can you provide a database connection to all routes in Express?

A Use global variables only

B Create a new connection inside each route

C Attach the connection to req object using middleware

D Ignore the connection and fetch data directly

Attaching the database connection to the req object via middleware allows all routes to access it without recreating it.

Which of these is NOT a benefit of dependency injection?

A More flexible code

B Harder to maintain code

C Easier testing

D Better separation of concerns

Dependency injection makes code easier to maintain, not harder.

What does this middleware do? app.use((req, res, next) => { req.logger = logger; next(); });

A Creates a new logger for each request

B Stops the request from continuing

C Removes the logger from requests

D Attaches a logger instance to each request

This middleware adds a logger instance to the req object so routes can use it.

Which pattern helps avoid global variables when injecting dependencies?

A Using factory functions to create routes with dependencies

B Creating dependencies inside routes

C Ignoring dependencies

D Using global variables everywhere

Factory functions allow passing dependencies as arguments, avoiding global variables.

Explain how dependency injection improves testing in Express applications.

Think about how you can swap parts when testing.

Describe two ways to inject dependencies into Express routes.

One way involves middleware, the other involves functions that return routes.

Practice

1. What is the main purpose of dependency injection in an Express app?

easy

A. To provide parts of the app with the resources they need without hardcoding them

B. To make the app run faster by caching data

C. To automatically generate routes based on file names

D. To handle errors globally in the app

Solution

1. Step 1: Understand dependency injection concept

   Dependency injection means giving parts of your app what they need instead of making them find it themselves.

2. Step 2: Identify the purpose in Express

   In Express, this helps keep code organized and flexible by passing dependencies explicitly.

3. Final Answer:

   To provide parts of the app with the resources they need without hardcoding them -> Option A

4. Quick Check:

   Dependency injection = Provide needed resources [OK]

Hint: Think: "Give what is needed, don't make it find" [OK]

Common Mistakes:

• Confusing dependency injection with caching
• Thinking it auto-generates routes
• Mixing it with error handling

2. Which of the following is the correct way to inject a service into an Express router?

easy

A. const router = express.Router(); router.use(service);

B. const router = express.Router(service);

C. function createRouter(service) { const router = express.Router(); router.service = service; return router; }

D. const router = express.Router(); router.service = service();

Solution

1. Step 1: Check how to pass dependencies in Express

   Express.Router() does not accept parameters directly, so passing service as argument is invalid.

2. Step 2: Use a factory function to inject service

   Creating a function that takes service and returns a router with service attached is the correct pattern.

3. Final Answer:

   function createRouter(service) { const router = express.Router(); router.service = service; return router; } -> Option C

4. Quick Check:

   Inject by factory function = function createRouter(service) { const router = express.Router(); router.service = service; return router; } [OK]

Hint: Use a function to pass dependencies, not Router constructor [OK]

Common Mistakes:

• Passing service directly to express.Router()
• Calling service as a middleware incorrectly
• Assigning service without a factory function

3. Consider this code snippet:

function createService() {
  return { getName: () => 'MyService' };
}

function createRouter(service) {
  const router = require('express').Router();
  router.get('/name', (req, res) => {
    res.send(service.getName());
  });
  return router;
}

const service = createService();
const router = createRouter(service);

// What happens when a GET request is made to '/name'?

medium

A. The server will crash due to missing middleware

B. The response will be undefined

C. There will be a runtime error because service is not defined

D. The response will be 'MyService'

Solution

1. Step 1: Analyze service creation

   The createService function returns an object with getName method returning 'MyService'.

2. Step 2: Analyze router handler

   The router uses the injected service and calls getName to send the response on '/name' route.

3. Final Answer:

   The response will be 'MyService' -> Option D

4. Quick Check:

   Injected service method returns 'MyService' [OK]

Hint: Injected service methods run as expected in route handlers [OK]

Common Mistakes:

• Assuming service is undefined inside handler
• Thinking service causes runtime error
• Confusing middleware with service injection

4. You have this code snippet:

function createRouter(service) {
  const router = require('express').Router();
  router.get('/data', (req, res) => {
    res.send(service.getData());
  });
  return router;
}

const router = createRouter();

What is the main issue here?

medium

A. The router is missing the service argument, causing service to be undefined

B. The router should be created with express() instead of express.Router()

C. The getData method is not defined on the router

D. The route path '/data' is invalid in Express

Solution

1. Step 1: Check function call for createRouter

   The createRouter function expects a service argument, but none is passed, so service is undefined.

2. Step 2: Understand impact on route handler

   Calling service.getData() will cause a runtime error because service is undefined.

3. Final Answer:

   The router is missing the service argument, causing service to be undefined -> Option A

4. Quick Check:

   Missing dependency argument = undefined service [OK]

Hint: Always pass dependencies when calling factory functions [OK]

Common Mistakes:

• Thinking express() should replace express.Router() here
• Confusing method definition with dependency injection
• Assuming route path is invalid

5. You want to inject a database client into multiple routers in your Express app. Which approach best supports this using dependency injection?

hard

A. Use a global variable for the database client and import it in each router file

B. Create a factory function for each router that accepts the database client and returns a configured router

C. Attach the database client directly to the Express app instance and access it via req.app

D. Create middleware that initializes the database client on every request

Solution

1. Step 1: Understand dependency injection best practice

   Injecting dependencies via factory functions keeps routers independent and testable.

2. Step 2: Evaluate options for injecting database client

   Factory functions allow passing the client explicitly to each router, avoiding globals and repeated initialization.

3. Final Answer:

   Create a factory function for each router that accepts the database client and returns a configured router -> Option B

4. Quick Check:

   Factory functions for injection = Create a factory function for each router that accepts the database client and returns a configured router [OK]

Hint: Use factory functions to pass shared resources cleanly [OK]

Common Mistakes:

• Using global variables which reduce testability
• Attaching client to app instance instead of injecting
• Initializing client on every request causing overhead