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524What is a Side Effect?
525Pure Functions vs Side Effects
526Common Side Effects in React
527Why Side Effects Can't Run During Render
528The useEffect Hook Signature
529When useEffect Runs: After Commit
530Effects Run After Every Render (By Default)
531Reading Props and State in Effects
532Multiple Effects in One Component
533Effects vs Event Handlers
534The Mental Model: Synchronization
535Why Effects are Often Overused
536What is the Dependency Array?
537Empty Array: Run Once on Mount
538No Array: Run on Every Render
539Array with Values: Run When Dependencies Change
540Primitive Dependencies: Equality by Value
541Object and Array Dependencies: Equality by Reference
542Multiple Dependencies
543The ESLint Rule: exhaustive-deps
544Why Missing Dependencies Cause Bugs
545Fixing Missing Dependencies: Add Them
546Fixing Missing Dependencies: Remove the Need
547Function Dependencies: The Problem
548Function Dependencies: useCallback Solution
549When to Disable the ESLint Rule
550What Cleanup Functions Are
551The Cleanup Function Signature
552When Cleanup Runs: Before Re-run
553When Cleanup Runs: On Unmount
554Cleanup for Event Listeners
555Cleanup for Timers
556Cleanup for Subscriptions
557Cleanup for AbortController
558What Happens Without Cleanup
559Cleanup in StrictMode: Double Invocation
560The Render-Commit-Paint-Effect Sequence
561Why Effects Run After Paint
562useLayoutEffect: The Synchronous Alternative
563Visual Flicker and Layout Effect Use Cases
564The Performance Cost of useLayoutEffect
565SSR Warnings with useLayoutEffect
566Choosing Between useEffect and useLayoutEffect

524What is a Side Effect?
525Pure Functions vs Side Effects
526Common Side Effects in React
527Why Side Effects Can't Run During Render
528The useEffect Hook Signature
529When useEffect Runs: After Commit
530Effects Run After Every Render (By Default)
531Reading Props and State in Effects
532Multiple Effects in One Component
533Effects vs Event Handlers
534The Mental Model: Synchronization
535Why Effects are Often Overused
536What is the Dependency Array?
537Empty Array: Run Once on Mount
538No Array: Run on Every Render
539Array with Values: Run When Dependencies Change
540Primitive Dependencies: Equality by Value
541Object and Array Dependencies: Equality by Reference
542Multiple Dependencies
543The ESLint Rule: exhaustive-deps
544Why Missing Dependencies Cause Bugs
545Fixing Missing Dependencies: Add Them
546Fixing Missing Dependencies: Remove the Need
547Function Dependencies: The Problem
548Function Dependencies: useCallback Solution
549When to Disable the ESLint Rule
550What Cleanup Functions Are
551The Cleanup Function Signature
552When Cleanup Runs: Before Re-run
553When Cleanup Runs: On Unmount
554Cleanup for Event Listeners
555Cleanup for Timers
556Cleanup for Subscriptions
557Cleanup for AbortController
558What Happens Without Cleanup
559Cleanup in StrictMode: Double Invocation
560The Render-Commit-Paint-Effect Sequence
561Why Effects Run After Paint
562useLayoutEffect: The Synchronous Alternative
563Visual Flicker and Layout Effect Use Cases
564The Performance Cost of useLayoutEffect
565SSR Warnings with useLayoutEffect
566Choosing Between useEffect and useLayoutEffect

Lesson 559 of 2,396·15. useEffect FundamentalsPro lesson

Cleanup in StrictMode: Double Invocation

Learning that StrictMode runs setup→cleanup→setup in development to surface missing cleanup bugs before production.

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