How Can I Improve My Node.js App with Dependency Injection?

4 min readMar 14, 2025

When building a Node.js application, one of the biggest challenges is managing dependencies in a way that keeps the code maintainable, testable, and scalable. If you’ve ever found yourself struggling with tight coupling between modules, difficulty in testing, or messy code filled with require statements, dependency injection (DI) might be the solution you need.

What is Dependency Injection?

Dependency injection is a design pattern that helps manage dependencies in a structured way. Instead of a module creating its own dependencies, they are provided from the outside. This decouples components, making them easier to test, maintain, and scale.

Consider a typical example without dependency injection:

class UserService {
  constructor() {
    this.db = new Database();
  }

  getUser(id) {
    return this.db.findUser(id);
  }
}

Here, UserService directly depends on Database, meaning that if we ever need to change how Database works (e.g., switch from MongoDB to PostgreSQL), we have to modify UserService. This tight coupling makes our code harder to maintain and test.

Now, let’s see how dependency injection helps.

Benefits of Dependency Injection in Node.js

Easier Testing — Dependencies can be replaced with mocks or stubs, making unit testing more effective.
Better Code Maintainability — Since dependencies are injected, updating or swapping components requires fewer changes.
Enhanced Modularity — Components become reusable and interchangeable.
Improved Scalability — Large applications benefit from dependency management, especially in microservices or modular architectures.

Implementing Dependency Injection in Node.js

There are multiple ways to implement DI in Node.js. Let’s go through them one by one.

1. Constructor Injection

A simple way to inject dependencies is through the constructor.

class UserService {
  constructor(database) {
    this.db = database;
  }

  getUser(id) {
    return this.db.findUser(id);
  }
}

// Creating an instance of the Database class
class Database {
  findUser(id) {
    return { id, name: "John Doe" };
  }
}

// Injecting the dependency
const dbInstance = new Database();
const userService = new UserService(dbInstance);

console.log(userService.getUser(1));

Here, UserService no longer creates its own Database instance. Instead, it receives it from the outside, making it easy to swap with a mock during testing.

2. Factory Function Injection

Instead of using classes, we can use factory functions to inject dependencies.

const createUserService = (database) => ({
  getUser: (id) => database.findUser(id),
});

const database = {
  findUser: (id) => ({ id, name: "Jane Doe" }),
};

const userService = createUserService(database);
console.log(userService.getUser(2));

This is a clean and functional approach, making dependency management even more flexible.

3. Using a Dependency Injection Container

For larger applications, managing dependencies manually can become overwhelming. A dependency injection container like Awilix or InversifyJS can automate this process.

Using Awilix

First, install the package:

npm install awilix

Then, define your dependencies and register them in a container:

const { createContainer, asClass } = require("awilix");

class Database {
  findUser(id) {
    return { id, name: "Alice" };
  }
}

class UserService {
  constructor({ database }) {
    this.db = database;
  }

  getUser(id) {
    return this.db.findUser(id);
  }
}

// Create a container and register dependencies
const container = createContainer();
container.register({
  database: asClass(Database).singleton(),
  userService: asClass(UserService).singleton(),
});

// Resolve dependencies
const userService = container.resolve("userService");
console.log(userService.getUser(3));

With a DI container, we centralize dependency management, making it easier to maintain and scale.

When Should You Use Dependency Injection?

When building large applications where managing dependencies manually becomes complex.
When writing unit tests and needing to mock dependencies easily.
When following clean architecture principles or domain-driven design (DDD).
When working with microservices that require loose coupling between components.

When Should You Avoid It?

If your application is small and does not have complex dependencies.
If adding a DI container adds unnecessary complexity.
If your dependencies are simple and do not require frequent swapping or mocking.

Conclusion

Dependency injection is a powerful technique for managing dependencies in Node.js applications. You achieve better modularity, maintainability, and testability by injecting dependencies instead of creating them inside modules. Whether you choose constructor injection, factory functions, or a DI container, applying DI principles can greatly improve your Node.js app’s structure.

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