When handling `null`/`undefined`, be explicit about intent: - **Choose the correct fallback source**: if a variable is `undefined`, ensure `??` falls back to the intended “truth” (not a stale previous value). - **Avoid silent dropping**: don’t filter out non-null `ReactNode` variants in a way that changes semantics between render modes (inline vs overflow). If separators/text are part of the host contract, preserve them intentionally. - **Prefer exhaustiveness over implicit `null`**: for union/enum switches, use an exhaustive pattern so new members don’t silently fall through to `null`. - **Match tests to the real nullability contract**: if a value is always a boolean, assert `false` (not `undefined`). Example (routing fallback): ```ts const reportedWorkspaceCwd = connection.sessionId ? connection.workspaceCwd : (selectedWorkspaceCwd ?? primaryWorkspaceCwd); // If selectedWorkspaceCwd is undefined, fall back to primaryWorkspaceCwd—not a stale connection.workspaceCwd. ``` Example (exhaustive switch): ```ts switch (checks) { case 'passing': return ...; case 'failing': return ...; case 'none': return null; default: { const _exhaustive: never = checks; return null; } } ``` Example (test nullability): ```ts expect(latestChatEditorProps.disabled).toBe(false); // not undefined ```