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663What is Derived State?
664The useEffect Anti-pattern for Derived State
665Computing Derived Values Directly in Render
666Performance: Render Computation is Fast
667Example: Filtering a List
668Example: Calculating Totals and Aggregates
669When Derived State Depends on Props
670The Stale State Problem with useEffect
671Expensive Computations: When to Use useMemo
672Testing Render vs Effect Approaches
673Refactoring Effect-Based Derived State
674The Event Handler Pattern
675Why useEffect is Wrong for Click Handlers
676The Common Mistake: Effect Watching State Set by Click
677Refactor: Moving Logic into the Handler
678When Events Need State: Reading Current State
679Sequential Actions After Events
680Event Handlers with Async Operations
681The Mental Model: Cause and Effect
682When Effects Are Actually Needed After Events
683The Props-Change-Triggers-Effect Antipattern
684Derived State: Compute During Render Instead
685The Key Prop: React's Built-In Reset Mechanism
686When Key Prop Solves the Problem Better
687Resetting All State vs Selective Updates
688Avoiding useState + useEffect for Prop Mirroring
689Controlled vs Uncontrolled: Let Parent Own State
690The getDerivedStateFromProps Mental Model
691Combining Prop and Local State Correctly
692Refactoring Effect-Based Syncing to Key Prop
693When Computation Happens: Render vs Effect Timing
694The Effect Antipattern: Computing Derived Values
695Expensive Computation Without Memoization
696useMemo for Expensive Computations
697The Dependency Array for Computation
698Extracting Computation Outside the Component
699When to Extract vs Inline with useMemo
700Avoiding the useEffect Computation Trap

663What is Derived State?
664The useEffect Anti-pattern for Derived State
665Computing Derived Values Directly in Render
666Performance: Render Computation is Fast
667Example: Filtering a List
668Example: Calculating Totals and Aggregates
669When Derived State Depends on Props
670The Stale State Problem with useEffect
671Expensive Computations: When to Use useMemo
672Testing Render vs Effect Approaches
673Refactoring Effect-Based Derived State
674The Event Handler Pattern
675Why useEffect is Wrong for Click Handlers
676The Common Mistake: Effect Watching State Set by Click
677Refactor: Moving Logic into the Handler
678When Events Need State: Reading Current State
679Sequential Actions After Events
680Event Handlers with Async Operations
681The Mental Model: Cause and Effect
682When Effects Are Actually Needed After Events
683The Props-Change-Triggers-Effect Antipattern
684Derived State: Compute During Render Instead
685The Key Prop: React's Built-In Reset Mechanism
686When Key Prop Solves the Problem Better
687Resetting All State vs Selective Updates
688Avoiding useState + useEffect for Prop Mirroring
689Controlled vs Uncontrolled: Let Parent Own State
690The getDerivedStateFromProps Mental Model
691Combining Prop and Local State Correctly
692Refactoring Effect-Based Syncing to Key Prop
693When Computation Happens: Render vs Effect Timing
694The Effect Antipattern: Computing Derived Values
695Expensive Computation Without Memoization
696useMemo for Expensive Computations
697The Dependency Array for Computation
698Extracting Computation Outside the Component
699When to Extract vs Inline with useMemo
700Avoiding the useEffect Computation Trap

Lesson 696 of 2,396·18. When NOT to Use useEffectPro lesson

useMemo for Expensive Computations

When profiling confirms slowness, wrap expensive computation in useMemo with proper dependencies to skip re-computation on unchanged inputs.

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