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

Confirm the detector match and collect the required evidence before deciding whether an edit is warranted.

Fires only when a reducer is wired to React's own useReducer and one control-flow path both mutates the original state parameter or a reachable value and returns that same top-level reference. Mutations include assignments, updates, deletes, mutating array or collection methods, Object.assign/Reflect.set, and recognized mutating lodash calls. Known detector limitation: nested mutation followed by a new top-level wrapper, such as state.user.name = value; return { ...state }, is not reported, but it still mutates previous React state and is not a valid immutable update. Suppress only when type information proves a persistent collection method returns a new value rather than mutating its receiver.

Evidence boundary

The diagnostic proves only that the detector’s modeled source pattern matched. It does not prove runtime impact, product intent, rendered failure, or that one remediation is correct.

Establish the environment, repository policy, exceptions, and required rendered or runtime evidence before deciding the occurrence.

Record one outcome:

• Confirmed failure: The required evidence establishes the violation.
• Rejected: A documented exception or false-positive predicate applies.
• Needs evidence: Named evidence can still be collected.
• Unavailable: Required evidence cannot be collected in this run.
• Waived with evidence: An authorized, scoped exception applies to an established failure.
• Observation: The review records an optional tradeoff without claiming a defect.

A waiver records its scope, authority, evidence, and review condition. It is not a pass or false positive.

Default severity is registry metadata. Use the occurrence’s JSON severity after repository configuration when ordering real findings.

Fix prompt

Apply this candidate correction only after the required evidence confirms the risk.

Stop mutating the incoming state on every changed path. Copy each changed level, not only the top-level wrapper: use return { ...state, user: { ...state.user, name: value } } for nested objects and immutable array transforms or a copied array for collections. Copy Map and Set before mutating the copy. Replace Object.assign(state, patch) and mutating path helpers with immutable equivalents. For a true no-op action, return state without mutating anything reachable from it. See https://react.dev/learn/updating-objects-in-state

Repository-wide copy prompt

Use this repository-wide prompt only after validating each occurrence. For one occurrence, use the guidance above.