Dependency injection: the big idea

Where did this.walkers come from?

The controller used this.walkers.findOne(id) — but it never wrote new WalkersService(). That's dependency injection (DI), and it's the single idea that makes Nest worth using.

You declare what you need as a constructor parameter, and Nest's container provides it. The controller says I need a WalkersService; Nest builds one (once) and passes it in. The class never reaches out for its dependencies — they're handed to it. That inversion is the whole trick.

constructor(private readonly walkers: WalkersService) {}