react-doctor/no-mutating-reducer-state

Return a new reducer state object/array/collection instead of mutating the current state and returning the same top-level reference.

Validation prompt

Use this to decide whether a fired diagnostic is real or a false positive.

Fires only when a reducer is wired to React's own useReducer — resolved via isCallToImportedReactUseReducer, which matches import { useReducer } from "react" (including useReducer as useReactReducer) and React.useReducer/React-namespace-or-default calls — AND one control-flow path both (a) mutates the original state param or a value reachable from it, then (b) returns that same top-level reference. Mutations counted: member-target assignment/update/delete (state.count++, state.x = y, delete state.x), receiver-mutating array/collection methods on a state-derived receiver (state.todos.push, state.set, state.add), Object.assign(state,...)/Reflect.set(state,...), mutating-lodash _.set/set/merge(state,...), and logical assignments (??=). Identity is tracked per path: const next = state and single-level destructures (const { items } = state, const [first] = state.items) stay state-reachable; same-reference returns include return state, return alias, return state.sort(...), return Object.assign(state, patch), and either branch of return cond ? state : {...state}. The rule will NOT fire on (no reviewer action needed, all verified valid fixtures): clone-first code that returns the clone (const next = {...state} / [...state.items] / new Map(state) / state.items.slice(), mutate the clone, return next); nested mutation under a NEW top-level wrapper (state.a.b = x; return {...state}); Array.reduce callbacks; locally-shadowed or non-React useReducer; Immer produce/useImmerReducer wrappers; lodash/fp set or a custom non-lodash set import; and Object.assign/Reflect.set where Object/Reflect is shadowed by a local binding. The ONE genuine residual false positive a reviewer MUST suppress by hand: a state value backed by a persistent/immutable collection (Immutable.js Map, Mori) whose .set/.add/.delete return a NEW container — the rule cannot see types, so it flags this discard-the-result pattern as a same-reference mutation even though no real mutation occurs.

Fix prompt

Use this once validation confirms the diagnostic is real.

Stop mutating the incoming state on the path that returns it. Clone first, then mutate and return the clone: object writes become const next = { ...state }; next.count += 1; return next;; array mutators become return { ...state, items: [...state.items, item] } or copy with state.items.slice() / Array.from(state.items) before push/splice/sort; Map/Set become const next = new Map(state); next.set(k, v); return next;; replace Object.assign(state, patch) with return { ...state, ...patch } and _.set(state, path, v) with an immutable update. For a true no-op action return state unchanged WITHOUT mutating it first. See https://react.dev/learn/updating-objects-in-state