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React

567The Classic useEffect Fetch Pattern
568Why Fetch Happens in useEffect
569Managing Loading States
570Handling Fetch Errors
571The Empty Dependency Array Pattern
572Fetching Based on Props or State
573Preventing Fetch on Every Render
574The Stale Closure Problem
575Setting State After Unmount Warning
576Basic Cleanup with Boolean Flags
577Why This Pattern is Problematic at Scale
578What Race Conditions Are in Data Fetching
579The Fast-Click Search Box Problem
580Why useEffect Doesn't Cancel Previous Fetches
581The Ignore Flag Pattern
582Implementing the Ignore Flag Pattern
583When Ignore Flags Aren't Enough
584Visualizing Race Condition Scenarios
585Race Conditions with Multiple Dependencies
586Testing for Race Conditions
587Why Modern Libraries Solve This Automatically
588Why Race Conditions Happen with Fetch in Effects
589The AbortController API Basics
590Wiring AbortController into useEffect Cleanup
591Handling AbortError Without Noise
592The Complete Abort Pattern in One Example
593When AbortController Isn't Enough
594The Ignore Flag Pattern as Fallback
595AbortController with POST/PUT Requests
596Why Libraries Handle This Better
597What StrictMode Does: Intentional Double-Mounting
598Why Your Effect Runs Twice on Mount
599The Double-Fetch Network Waterfall
600This Only Happens in Development
601Why React Does This: Finding Missing Cleanup
602Using AbortController to Make Fetches StrictMode-Safe
603Seeing Double Fetches as a Feature, Not a Bug
604Ignoring the Second Request's Result with a Flag
605The Problem with Manual useEffect Data Fetching
606TanStack Query's Automatic Caching and Deduplication
607Background Refetching and Stale-While-Revalidate
608Built-In Loading, Error, and Success States
609Optimistic Updates and Mutations Made Simple
610Pagination and Infinite Scrolling Patterns
611When useEffect Data Fetching Still Makes Sense

React

567The Classic useEffect Fetch Pattern
568Why Fetch Happens in useEffect
569Managing Loading States
570Handling Fetch Errors
571The Empty Dependency Array Pattern
572Fetching Based on Props or State
573Preventing Fetch on Every Render
574The Stale Closure Problem
575Setting State After Unmount Warning
576Basic Cleanup with Boolean Flags
577Why This Pattern is Problematic at Scale
578What Race Conditions Are in Data Fetching
579The Fast-Click Search Box Problem
580Why useEffect Doesn't Cancel Previous Fetches
581The Ignore Flag Pattern
582Implementing the Ignore Flag Pattern
583When Ignore Flags Aren't Enough
584Visualizing Race Condition Scenarios
585Race Conditions with Multiple Dependencies
586Testing for Race Conditions
587Why Modern Libraries Solve This Automatically
588Why Race Conditions Happen with Fetch in Effects
589The AbortController API Basics
590Wiring AbortController into useEffect Cleanup
591Handling AbortError Without Noise
592The Complete Abort Pattern in One Example
593When AbortController Isn't Enough
594The Ignore Flag Pattern as Fallback
595AbortController with POST/PUT Requests
596Why Libraries Handle This Better
597What StrictMode Does: Intentional Double-Mounting
598Why Your Effect Runs Twice on Mount
599The Double-Fetch Network Waterfall
600This Only Happens in Development
601Why React Does This: Finding Missing Cleanup
602Using AbortController to Make Fetches StrictMode-Safe
603Seeing Double Fetches as a Feature, Not a Bug
604Ignoring the Second Request's Result with a Flag
605The Problem with Manual useEffect Data Fetching
606TanStack Query's Automatic Caching and Deduplication
607Background Refetching and Stale-While-Revalidate
608Built-In Loading, Error, and Success States
609Optimistic Updates and Mutations Made Simple
610Pagination and Infinite Scrolling Patterns
611When useEffect Data Fetching Still Makes Sense

← React

Lesson 598 of 2,396·16. useEffect for Data FetchingPro lesson

Why Your Effect Runs Twice on Mount

In StrictMode, effects run, cleanup, then run again—simulating a component unmount/remount cycle to test robustness.

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