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React

743Complex State with Multiple Sub-Values
744State Transitions That Depend on Previous State
745The Cognitive Load of Many useState Calls
746Avoiding Callback Hell in Event Handlers
747When State Logic Becomes Testable Business Logic
748Co-locating Related Updates
749Encapsulating Validation and Constraints
750Passing Dispatch Instead of Multiple Setters
751When You'd Reach for Redux but It's Overkill
752Modeling State Machines
753The Readability Tipping Point
754Reducer Function: (state, action) => newState
755State Parameter: Current Snapshot
756Action Parameter: Instruction Object
757Return Value: Next State
758Pure Function Requirements
759Immutable Updates: Creating New Objects
760Switch Statement Pattern
761Default Case: Returning Current State
762TypeScript: Discriminated Unions for Actions
763Action Objects: type and payload
764String Literal Types for Action Types
765Action Creators: Functions That Return Actions
766Discriminated Unions with TypeScript
767Naming Action Types: SCREAMING_SNAKE_CASE
768When to Use Multiple Action Types vs Single with Payload
769Meta Fields: Adding Timestamps, IDs, or Metadata
770Error Actions: Standardizing Failure Cases
771The Problem: Expensive Initial State Computation
772The init Function Parameter
773init Function Signature and Arguments
774When Lazy Initialization Matters
775Comparing useState Lazy Init vs useReducer
776Common Pattern: Reusable Init Functions
777The Global State Problem That useReducer + Context Solves
778Creating the Context Pair: State and Dispatch
779The Provider Component Pattern
780Custom Hooks for Consuming State and Dispatch
781Why Splitting Contexts Matters for Performance
782Action Creators for Better Developer Experience
783Organizing Reducer, Actions, and Types in One Module
784Testing Components That Use Context + Reducer
785When This Pattern Beats External State Libraries
786Migration Path: From Prop Drilling to useReducer + Context

React

743Complex State with Multiple Sub-Values
744State Transitions That Depend on Previous State
745The Cognitive Load of Many useState Calls
746Avoiding Callback Hell in Event Handlers
747When State Logic Becomes Testable Business Logic
748Co-locating Related Updates
749Encapsulating Validation and Constraints
750Passing Dispatch Instead of Multiple Setters
751When You'd Reach for Redux but It's Overkill
752Modeling State Machines
753The Readability Tipping Point
754Reducer Function: (state, action) => newState
755State Parameter: Current Snapshot
756Action Parameter: Instruction Object
757Return Value: Next State
758Pure Function Requirements
759Immutable Updates: Creating New Objects
760Switch Statement Pattern
761Default Case: Returning Current State
762TypeScript: Discriminated Unions for Actions
763Action Objects: type and payload
764String Literal Types for Action Types
765Action Creators: Functions That Return Actions
766Discriminated Unions with TypeScript
767Naming Action Types: SCREAMING_SNAKE_CASE
768When to Use Multiple Action Types vs Single with Payload
769Meta Fields: Adding Timestamps, IDs, or Metadata
770Error Actions: Standardizing Failure Cases
771The Problem: Expensive Initial State Computation
772The init Function Parameter
773init Function Signature and Arguments
774When Lazy Initialization Matters
775Comparing useState Lazy Init vs useReducer
776Common Pattern: Reusable Init Functions
777The Global State Problem That useReducer + Context Solves
778Creating the Context Pair: State and Dispatch
779The Provider Component Pattern
780Custom Hooks for Consuming State and Dispatch
781Why Splitting Contexts Matters for Performance
782Action Creators for Better Developer Experience
783Organizing Reducer, Actions, and Types in One Module
784Testing Components That Use Context + Reducer
785When This Pattern Beats External State Libraries
786Migration Path: From Prop Drilling to useReducer + Context

← React

Lesson 758 of 2,396·20. useReducerPro lesson

Pure Function Requirements

Reducers must be pure: same inputs always produce same output, no side effects, no mutations.

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