react18-lifecycle-patterns

# React 18 Lifecycle Patterns Reference for migrating the three unsafe class component lifecycle methods to React 18.3.1 compliant patterns. ## Quick Decision Guide Before migrating any lifecycle method, identify the **semantic category** of what the method does. Wrong category = wrong migration. The table below routes you to the correct reference file. ### componentWillMount - what does it do? | What it does | Correct migration | Reference | |---|---|---| | Sets initial state (`this.setState(...)`) | Move to `constructor` | [→ componentWillMount.md](references/componentWillMount.md#case-a) | | Runs a side effect (fetch, subscription, DOM) | Move to `componentDidMount` | [→ componentWillMount.md](references/componentWillMount.md#case-b) | | Derives initial state from props | Move to `constructor` with props | [→ componentWillMount.md](references/componentWillMount.md#case-c) | ### componentWillReceiveProps - what does it do? | What it does | Correct migration | Reference | |---|---|---| | Async side effect triggered by prop change (fetch, cancel) | `componentDidUpdate` | [→ componentWillReceiveProps.md](references/componentWillReceiveProps.md#case-a) | | Pure state derivation from new props (no side effects) | `getDerivedStateFromProps` | [→ componentWillReceiveProps.md](references/componentWillReceiveProps.md#case-b) | ### componentWillUpdate - what does it do? | What it does | Correct migration | Reference | |---|---|---| | Reads the DOM before update (scroll, size, position) | `getSnapshotBeforeUpdate` | [→ componentWillUpdate.md](references/componentWillUpdate.md#case-a) | | Cancels requests / runs effects before update | `componentDidUpdate` with prev comparison | [→ componentWillUpdate.md](references/componentWillUpdate.md#case-b) | --- ## The UNSAFE_ Prefix Rule **Never use `UNSAFE_componentWillMount`, `UNSAFE_componentWillReceiveProps`, or `UNSAFE_componentWillUpdate` as a permanent fix.** Prefixing suppresses the React 18.3.1 warning but does NOT: - Fix concurrent mode safety issues - Prepare the codebase for React 19 (where these are removed, with or without the prefix) - Fix the underlying semantic problem the migration is meant to address The UNSAFE_ prefix is only appropriate as a temporary hold while scheduling the real migration sprint. Mark any UNSAFE_ prefix additions with: ```jsx // TODO: React 19 will remove this. Migrate before React 19 upgrade. // UNSAFE_ prefix added temporarily - replace with componentDidMount / getDerivedStateFromProps / etc. ``` --- ## Reference Files Read the full reference file for the lifecycle method you are migrating: - **`references/componentWillMount.md`** - 3 cases with full before/after code - **`references/componentWillReceiveProps.md`** - getDerivedStateFromProps trap warnings, full examples - **`references/componentWillUpdate.md`** - getSnapshotBeforeUpdate + componentDidUpdate pairing Read the relevant file before writing any migration code.