Most Node.js developers import modules directly, which creates hidden dependencies that make testing painful and swapping implementations difficult. This post explains the difference between Dependency Injection (a technique) and Dependency Inversion (a SOLID principle), then shows how to implement manual DI in TypeScript using factory functions and a composition root — no frameworks required. It compares module mocking (brittle, path-coupled, type-unsafe) against plain object injection (type-safe, portable, self-contained tests). For larger apps with complex dependency graphs, tsyringe is briefly introduced as a lightweight container option. Key takeaway: DI is just passing arguments to functions, and it makes unit testing trivial without any mocking framework.