---
source: https://iinventedlight.com/library/notes/react-usereducer.md
title: "Managing complex state with useReducer"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "CC BY 4.0 (reactjs/react.dev LICENSE-DOCS); prose restates cited source lines in own words"
license_family: react-docs-ccby
attribution: "Summary by iinventedlight (AI-assisted). Sources: React docs (react.dev), useReducer – React, CC BY 4.0 (reactjs/react.dev LICENSE-DOCS); React docs (react.dev), Sharing State Between Components – React, CC BY 4.0 (reactjs/react.dev LICENSE-DOCS)."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-06
status: published
checked_on: 2026-11-06
as_of: 2026-11-06
confidence: medium
untested: "No React code was run for this note; only the fetched documentation pages were read."
category: coding-references
topic_area: "code-web: useReducer"
tags: [react, usereducer, state]
client: Librarian
sources:
  - id: S1
    title: "useReducer – React"
    publisher: React docs (react.dev)
    url: https://react.dev/reference/react/useReducer
    accessed: 2026-11-06
    license: "CC BY 4.0 (reactjs/react.dev LICENSE-DOCS)"
  - id: S2
    title: "Sharing State Between Components – React"
    publisher: React docs (react.dev)
    url: https://react.dev/learn/sharing-state-between-components
    accessed: 2026-11-06
    license: "CC BY 4.0 (reactjs/react.dev LICENSE-DOCS)"
---
# Managing complex state with useReducer

useReducer is a React Hook that lets you add a reducer to your component. [S1]

## Reducer updates

The reducer function specifies how the state gets updated from the previous state and an action. [S1]

State and action can be of any types. [S1]

You can pass an initializer function that should return the initial state. [S1]

## When shared transitions grow

Sometimes, you want the state of two components to always change together. [S2]

Add state to the common parent and pass it down together with the event handlers. [S2]

## Claims

- id: C1
  claim: useReducer adds a reducer to a component.
  source: S1
  evidence: "that lets you add a reducer to your component"

- id: C2
  claim: The reducer specifies how state updates.
  source: S1
  evidence: "specifies how the state gets updated"

- id: C3
  claim: State and action may be any types.
  source: S1
  evidence: "State and action can be of any types"

- id: C4
  claim: Shared updates can live in a common parent.
  source: S2
  evidence: "Add state to the common parent"

## Sources

- [S1] useReducer – React. React docs (react.dev). https://react.dev/reference/react/useReducer . License: CC BY 4.0 (reactjs/react.dev LICENSE-DOCS). Accessed 2026-11-06.

- [S2] Sharing State Between Components – React. React docs (react.dev). https://react.dev/learn/sharing-state-between-components . License: CC BY 4.0 (reactjs/react.dev LICENSE-DOCS). Accessed 2026-11-06.


License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).