Dependency injection is a design pattern that allows one part of your application to use another without creating it manually.

Instead of creating objects yourself, you simply declare that your class needs them. Nest.js creates those objects and provides them automatically.

For example, a controller can use a service like this:

constructor(private usersService: UsersService) {}

Here is what is happening:

• UsersService is a dependency;
• The controller requests it through its constructor;
• Nest.js creates the service and injects it automatically.

Without dependency injection, you would need to create the service yourself:

const usersService = new UsersService();

As applications grow, manually creating and managing objects becomes difficult. Dependency injection keeps your code easier to maintain, test, and reuse by allowing Nest.js to manage object creation for you.