refactoring-typescript

To separate business logic from I/O in TypeScript, identify system boundaries, define a conventional interface contract, implement a thin wrapper for external dependencies, and refactor logic to depend on the interface via dependency injection.

The best way to refactor TypeScript code for testing is to isolate external calls like HTTP and file systems into thin system wrappers, create mock implementations under a mocks directory, and configure tooling to exclude wrappers from coverage.

Dependency injection improves TypeScript architecture by allowing business logic to depend on interface contracts rather than direct system interactions, enabling safe migrations and easier mock generation for testing.

Yes, you can use interfaces to mock file system and HTTP calls by defining a conventional interface contract, implementing a thin wrapper that delegates to external dependencies, and creating a corresponding mock implementation for your tests.

You need to create system wrappers in TypeScript when you have tangled business logic mixed with system interactions like I/O, HTTP, or database calls, allowing you to isolate external dependencies and achieve observable, testable code.