Dependency injection in Express - Mini Project: Build & Apply

Dependency Injection in Express

📖 Scenario: You are building a simple Express server that needs to use a service for greeting users. To keep your code clean and testable, you will use dependency injection to provide the greeting service to your route handler.

🎯 Goal: Create an Express app that uses dependency injection to pass a greeting service to a route handler. The app will respond with a greeting message when a user visits the /greet endpoint.

📋 What You'll Learn

Create a greeting service function that returns a greeting string.

Create a configuration variable for the greeting message prefix.

Inject the greeting service into the Express route handler.

Set up the Express app to listen on port 3000.

💡 Why This Matters

🌍 Real World

Dependency injection helps keep Express apps modular and testable by separating service logic from route handlers.

💼 Career

Understanding dependency injection is important for writing clean, maintainable backend code in Node.js and Express, a common skill in web development jobs.

1

Create the greeting service function

Create a function called greetingService that takes a name parameter and returns the string `Hello, ${name}!`.

Express

// Your code here

Need a hint?

Define a function that uses a template string to return a greeting.

2

Create a greeting prefix configuration variable

Create a constant called greetingPrefix and set it to the string 'Hello'.

Express

function greetingService(name) {
  return `Hello, ${name}!`;
}

// Your code here

Need a hint?

Create a constant variable to hold the greeting prefix string.

3

Inject the greeting service into the Express route handler

Create an Express app by requiring express and calling express(). Then create a route handler for /greet that uses the injected greetingService to send a greeting for the query parameter name. Use req.query.name to get the name.

Express

function greetingService(name) {
  return `Hello, ${name}!`;
}

const greetingPrefix = 'Hello';

// Your code here

Need a hint?

Use express() to create the app and define a GET route for /greet. Use the greeting service inside the route.

4

Start the Express server on port 3000

Add code to make the Express app listen on port 3000 using app.listen.

Express

const express = require('express');

function greetingService(name) {
  return `Hello, ${name}!`;
}

const greetingPrefix = 'Hello';

const app = express();

app.get('/greet', (req, res) => {
  const name = req.query.name || 'Guest';
  const message = greetingService(name);
  res.send(message);
});

// Your code here

Need a hint?

Use app.listen(3000) to start the server on port 3000.