Testing and faking Angular dependencies

A practical guide to testing and faking Angular dependencies using dependency injection tokens and the Angular testing utilities. Covers how to replace tree-shakable providers with fake values using TestBed and the inject function, including gotchas for each approach. Demonstrates testing value factories that depend on platform-specific APIs (Navigator, Location), building a fake user agent enum, and writing component tests for an IE11 deprecation banner with a custom test harness that handles setup, event simulation, and assertions.