Jest and Vitest, mocking modules, build and bundling choices, and monorepo workflows.
3 questions in Node.js & TypeScript.
Substitute at the network layer or behind your own client interface, not at the HTTP library's function. Stubbing the library couples the test to how you make the call rather than to what you send, so the test keeps passing through refactors that change the request and breaks on refactors that change nothing.
Drive business logic through injected dependencies rather than module mocks, intercept at the process boundary for outbound HTTP and time, and use a real database in a container for anything where the database is the logic — SQL, constraints, transactions and migrations.
The failing test is a victim, not the cause: something earlier mutated state it shares, usually a module evaluated once, a global that was patched and not restored, or rows left in a database. Bisect the ordering to find the pair, then remove the sharing.