Context

When adding a mobile feature in an existing codebase, strong test design matters as much as the implementation. Interviewers want to see that you can add confidence quickly without over-testing low-value paths.

Core Question

Explain how you would think about writing tests for a new mobile feature while pairing in an existing codebase.

Address these points:

1. How do you decide what to test first when the codebase already has patterns and constraints?
2. How do you choose between unit, integration, and UI/end-to-end tests for the feature?
3. How do you handle dependencies such as network calls, local storage, feature flags, and asynchronous behavior?

Scope Guidance

Focus on a practical engineering approach rather than a specific test framework. The interviewer expects a structured explanation of risk-based coverage, test boundaries, maintainability, and how you would learn existing conventions before adding new tests.

Risk-Based Test Selection

Start by identifying the highest-risk behavior: user-visible flows, business rules, and failure paths. This helps you add confidence quickly instead of writing many low-value tests.

Test Pyramid

Most coverage should come from fast unit tests, with fewer integration tests and a small number of end-to-end tests. This balances speed, confidence, and maintenance cost.

Dependency Isolation

External dependencies such as APIs, storage, clocks, and feature flags should be controlled in tests. Isolation makes tests deterministic and easier to debug.

api = FakeApi(response={"status": "ok"})
storage = InMemoryStore()
feature = build_feature(api=api, storage=storage)

Existing Codebase Conventions

In a mature codebase, consistency matters. Reusing established test helpers, naming, and fixture patterns reduces friction for reviewers and future maintainers.

Behavior Over Implementation

Good tests validate observable outcomes rather than internal method calls whenever possible. This keeps tests stable when refactoring changes internals but not behavior.

result = reducer(state, action)
assert result.error_message == "Offline mode"