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Tree Shaking: Shipping Only the Code You Use
Tree shaking is a build step that eliminates unused code from your final bundle. Bundlers like webpack use it to shrink JavaScript files by removing functions you import but never call.
How do you unit test a custom React hook like useCounter?
This tests whether you know hooks must run inside a component and that renderHook provides that scaffold. A strong answer names renderHook and act, explains result.current access, and warns against calling hooks directly.
Measure Component Render Costs with <Profiler>
React's <Profiler> is a stopwatch for your components, measuring render times programmatically. Wrap any UI tree to log detailed performance metrics on every update, helping you pinpoint slow renders. The footgun: It's disabled in production by default.
Testing a component that consumes React Context
Render inside the Context.Provider with a value, or omit it to exercise the default; use a custom render wrapper for reuse.
ISR: Static Speed with Dynamic Freshness
ISR offers the speed of a static site with the freshness of a server-rendered one. It serves a cached page, then regenerates it in the background after a set time, making it ideal for blogs or e-commerce sites.
Difference between jest.fn, jest.spyOn, and jest.mock in React
Tests whether you distinguish Jest's three mocking scopes. Good answers: jest.fn for standalone callbacks; jest.spyOn to observe existing methods; jest.mock to swap entire modules. Red flag: using them interchangeably or saying spyOn replaces modules.
Single Source of Truth: Centralized App State
A Single Source of Truth consolidates all application state into one central object, or 'store.' This makes state predictable and easier to debug, especially in libraries like Redux, by creating one place to look for data.
How do you balance unit, integration, and end-to-end tests in Next.js?
This tests allocation of test types across Next.js boundaries. Propose 70 percent unit tests for utilities, 20 percent integration tests for data fetching, and 10 percent end-to-end tests for critical flows, weighing cost and confidence.
Redux: Actions are Events, Reducers are Event Handlers
Redux Actions are events describing *what* happened (e.g., 'task added'), while Reducers are functions that specify *how* state changes in response. This pattern is central to managing global state. The biggest footgun is mutating state directly in a reducer.
Cypress auth strategies: UI login vs programmatic session
Tests your grasp of Cypress test isolation and speed tradeoffs. A strong answer contrasts slow UI login (ideal for the auth flow itself) against programmatic auth via cy.request or cy.session (fast setup for protected routes).
MSW vs mocking fetch or axios directly in Jest tests
MSW intercepts HTTP via Service Workers and Node, staying client-agnostic; direct fetch or axios mocks couple tests to specific libraries.
Redux Toolkit: `createSlice` Bundles Your Redux Logic
createSlice bundles a piece of state, its reducers, and action creators into one unit, killing Redux boilerplate. Use it to organize your store by feature. The footgun: its "mutating" syntax only works here due to Immer, not in plain Redux.
How do you create a /dashboard route in Next.js App Router?
Tests App Router file-system routing and the page.js convention. Good answer: create app/dashboard/page.js with a default export, note that only page.js creates an addressable route, and layouts do not.
Jotai Derived Atoms: Computed State from Other Atoms
Think of a derived atom like a spreadsheet formula: its value is computed from other atoms. Use it for reactive values like a cart total from item prices. The footgun: the get in your read function is reactive, but the get in your write function.
What is layout.js and how does it differ from shared headers?
Layout.js persists across navigations without remounting; a shared header in each page.js remounts and loses state.
State Normalization: Treat Your Store Like a Database
Treat your Redux state like a database. Instead of nesting related data, flatten it into separate 'tables' keyed by ID. This simplifies updates and prevents unnecessary re-renders. The footgun is storing API data as-is, creating update bugs.
How do you create a /blog/[slug] route and access the slug value?
Use a [slug] directory with page.js; in App Router read the params prop in server components or useParams in client components.
SSR State Hydration: Syncing Server and Client State
SSR hydration syncs server and client state to prevent UI errors. The server renders the page with initial data, then sends that data to the client to "hydrate" its own state store. The footgun is using a global store on the server, which leaks data.
How do loading.js and error.js integrate with Suspense and Error Boundaries?
Tests grasp of App Router conventions wrapping React primitives. loading.js suspends its segment for instant navigation feedback; error.js adds a client Error Boundary for child segment crashes. Wrong: assuming they are server-only or replace manual usage.
RTK Query: Your Redux Data Fetching Layer
RTK Query treats server data as a cache, not global state, handling fetching and loading states for you. Use it to replace manual fetch logic and keep data fresh across components. Forgetting to call setupListeners disables automatic refetching.