# `react-doctor/no-legacy-class-lifecycles`

Migrate legacy pre-commit class lifecycles to concurrent-safe patterns; their UNSAFE_ aliases remain available but deprecated in React 19

- **Status:** Active
- **Category:** Correctness
- **Assessment:** Evidence-required risk
- **Required evidence:** source code
- **Default configuration:** Enabled
- **Default severity:** error
- **Scope:** All supported frameworks
- **Active when:** always
- **Requirements:** react
- **Tags:** migration-hint
- **Priority:** 66 (P2)
- **Source:** oxlint-plugin-react-doctor
- **Rule set:** oxlint-plugin-react-doctor 0.9.3 (prompt schema 2)

## Validation prompt

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

Fires when a class body (`ClassBody`) contains a `MethodDefinition` or `PropertyDefinition` whose key Identifier is exactly `componentWillMount`, `componentWillReceiveProps`, or `componentWillUpdate`, OR their `UNSAFE_`-prefixed aliases: a pure name match, class-body-scoped (object-literal methods, standalone functions, or these names used as variables never fire). Confirm the enclosing class is a mounted React component and review the lifecycle's intent. Both the legacy names and their UNSAFE_ aliases are deprecated because they are unsafe with concurrent rendering; the UNSAFE_ forms remain documented and executable in React 19. Suppress a class that is not a React component but coincidentally defines the same method name.

## 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, explicit repository policy, relevant exceptions, and required rendered or runtime evidence before deciding the occurrence. This page’s Assessment and Required evidence fields define the review contract.

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

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 or expiry condition. It is not a pass or false positive.

## Fix prompt

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

Map each legacy hook to the replacement that matches its intent rather than merely renaming it. Move componentWillMount side effects and subscriptions to componentDidMount, and put pure initial field or state setup in the constructor. Prefer deriving values during render; when state truly must adjust to prop changes, use the narrowest documented pattern, and move prop-driven side effects to a guarded componentDidUpdate. Move pre-update DOM reads to getSnapshotBeforeUpdate and consume the snapshot in componentDidUpdate. Keeping an UNSAFE_ alias can be a temporary compatibility step, but it does not make the lifecycle concurrent-safe. See https://react.dev/reference/react/Component#unsafe_componentwillreceiveprops

## Repository-wide copy prompt

Use this only for a repository-wide pass after validating each occurrence. For one occurrence, use the occurrence-level guidance above.

````text
Fix every confirmed `react-doctor/no-legacy-class-lifecycles` diagnostic in the current repository.

Required change:
- Map each legacy hook to the replacement that matches its intent rather than merely renaming it. Move componentWillMount side effects and subscriptions to componentDidMount, and put pure initial field or state setup in the constructor. Prefer deriving values during render; when state truly must adjust to prop changes, use the narrowest documented pattern, and move prop-driven side effects to a guarded componentDidUpdate. Move pre-update DOM reads to getSnapshotBeforeUpdate and consume the snapshot in componentDidUpdate. Keeping an UNSAFE_ alias can be a temporary compatibility step, but it does not make the lifecycle concurrent-safe. See https://react.dev/reference/react/Component#unsafe_componentwillreceiveprops.

Validation before editing:
Fires when a class body (`ClassBody`) contains a `MethodDefinition` or `PropertyDefinition` whose key Identifier is exactly `componentWillMount`, `componentWillReceiveProps`, or `componentWillUpdate`, OR their `UNSAFE_`-prefixed aliases: a pure name match, class-body-scoped (object-literal methods, standalone functions, or these names used as variables never fire). Confirm the enclosing class is a mounted React component and review the lifecycle's intent. Both the legacy names and their UNSAFE_ aliases are deprecated because they are unsafe with concurrent rendering; the UNSAFE_ forms remain documented and executable in React 19. Suppress a class that is not a React component but coincidentally defines the same method name.

Constraints:
- Make the smallest change that fixes the root cause.
- Preserve behavior and interfaces unrelated to this diagnostic.
- Reuse existing project components, utilities, and conventions.
- Do not introduce render-phase side effects, render-phase state updates, or Hooks rule violations.
- Adapt identifiers and framework details instead of copying blindly.
- Do not disable the rule or suppress matching code.

Assessment:
- Record detector evidence, applicability facts, assumptions, missing evidence, and the rule class for this occurrence.
- Return one outcome: Confirmed failure, Rejected, Needs evidence, Unavailable, Waived with evidence, or Observation.
- A waiver records the established failure, scope, authority, evidence, and review or expiry condition. It is not a pass or false positive.

Verification:
- Run focused tests for the changed behavior.
- Run React Doctor and confirm this diagnostic no longer appears from changed code.
- Run an unfiltered scan of the affected scope before claiming no cross-category regression.
- Report the files changed and any checks you could not run.
````