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React & Next.js Frontend Foundations

React Concurrency, Forms & Performance

Transitions, deferred rendering, React 19 Actions, optimistic UI, external stores, profiling, code splitting, and large-list behavior.

10 drillsPractice one at a timeOrdered pathArchitecture scenarioDebugging scenarioConcept questionComparison question

Recommended start

React memoization judgment

Treats memoization as measured performance work and accounts for prop identity, context, readability, and React Compiler.

Hard~12 minStep 1 of 10

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1React memoization judgmentRecommended startTreats memoization as measured performance work and accounts for prop identity, context, readability, and React Compiler.Architecture scenarioHard~12 minReactTypeScriptPractice 2Profile React rerendersMoves from a reproducible user interaction to render evidence, root cause, proportional repair, and field validation.Debugging scenarioHard~12 minReactTypeScriptPractice 3React TransitionsKeeps input and direct feedback urgent while rendering expensive navigation or result updates as interruptible work.Concept questionHard~10 minReactTypeScriptPractice 4Deferred value vs debounceSeparates interruptible background rendering from time-based event or request suppression.Comparison questionHard~10 minReactTypeScriptPractice 5External store subscriptionsDefines a coherent subscription and snapshot contract for state owned outside React, including server rendering.Concept questionHard~10 minReactTypeScriptPractice 6Form Actions with useActionStateModels pending, validation, success, retry, and authoritative server outcomes around a React 19 form Action.Architecture scenarioHard~12 minReactTypeScriptPractice 7Concurrent optimistic UIDesigns optimistic identity, pending state, reconciliation, rollback, and overlapping mutation behavior.Architecture scenarioHard~12 minReactTypeScriptPractice 8Lazy loading and SuspenseSplits meaningful code, keeps stable UI visible, preloads likely paths, and pairs loading with error recovery.Architecture scenarioHard~12 minReactTypeScriptPractice 9Large-list virtualizationRenders a measured window while preserving identity, scrolling, accessibility, dynamic sizes, and interaction state.Architecture scenarioHard~12 minReactTypeScriptPractice 10Test concurrent React UITests user-observable urgent, pending, success, failure, interruption, and out-of-order behavior without scheduler coupling.Architecture scenarioHard~12 minReactTypeScriptPractice

Practice order

10 drillsOne at a time

1React memoization judgmentUp next / Hard / ~12 min
2Profile React rerendersHard / ~12 min
3React TransitionsHard / ~10 min
4Deferred value vs debounceHard / ~10 min
5External store subscriptionsHard / ~10 min
6Form Actions with useActionStateHard / ~12 min
7Concurrent optimistic UIHard / ~12 min
8Lazy loading and SuspenseHard / ~12 min

+2 more drills in this section