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64 lines
3.6 KiB
Markdown
64 lines
3.6 KiB
Markdown
---
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name: react18-lifecycle-patterns
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description: 'Provides exact before/after migration patterns for the three unsafe class component lifecycle methods - componentWillMount, componentWillReceiveProps, and componentWillUpdate - targeting React 18.3.1. Use this skill whenever a class component needs its lifecycle methods migrated, when deciding between getDerivedStateFromProps vs componentDidUpdate, when adding getSnapshotBeforeUpdate, or when fixing React 18 UNSAFE_ lifecycle warnings. Always use this skill before writing any lifecycle migration code - do not guess the pattern from memory, the decision trees here prevent the most common migration mistakes.'
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---
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# React 18 Lifecycle Patterns
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Reference for migrating the three unsafe class component lifecycle methods to React 18.3.1 compliant patterns.
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## Quick Decision Guide
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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.
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### componentWillMount - what does it do?
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| What it does | Correct migration | Reference |
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|---|---|---|
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| Sets initial state (`this.setState(...)`) | Move to `constructor` | [→ componentWillMount.md](references/componentWillMount.md#case-a) |
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| Runs a side effect (fetch, subscription, DOM) | Move to `componentDidMount` | [→ componentWillMount.md](references/componentWillMount.md#case-b) |
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| Derives initial state from props | Move to `constructor` with props | [→ componentWillMount.md](references/componentWillMount.md#case-c) |
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### componentWillReceiveProps - what does it do?
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| What it does | Correct migration | Reference |
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|---|---|---|
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| Async side effect triggered by prop change (fetch, cancel) | `componentDidUpdate` | [→ componentWillReceiveProps.md](references/componentWillReceiveProps.md#case-a) |
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| Pure state derivation from new props (no side effects) | `getDerivedStateFromProps` | [→ componentWillReceiveProps.md](references/componentWillReceiveProps.md#case-b) |
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### componentWillUpdate - what does it do?
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| What it does | Correct migration | Reference |
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| Reads the DOM before update (scroll, size, position) | `getSnapshotBeforeUpdate` | [→ componentWillUpdate.md](references/componentWillUpdate.md#case-a) |
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| Cancels requests / runs effects before update | `componentDidUpdate` with prev comparison | [→ componentWillUpdate.md](references/componentWillUpdate.md#case-b) |
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---
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## The UNSAFE_ Prefix Rule
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**Never use `UNSAFE_componentWillMount`, `UNSAFE_componentWillReceiveProps`, or `UNSAFE_componentWillUpdate` as a permanent fix.**
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Prefixing suppresses the React 18.3.1 warning but does NOT:
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- Fix concurrent mode safety issues
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- Prepare the codebase for React 19 (where these are removed, with or without the prefix)
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- Fix the underlying semantic problem the migration is meant to address
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The UNSAFE_ prefix is only appropriate as a temporary hold while scheduling the real migration sprint. Mark any UNSAFE_ prefix additions with:
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```jsx
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// TODO: React 19 will remove this. Migrate before React 19 upgrade.
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// UNSAFE_ prefix added temporarily - replace with componentDidMount / getDerivedStateFromProps / etc.
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```
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---
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## Reference Files
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Read the full reference file for the lifecycle method you are migrating:
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- **`references/componentWillMount.md`** - 3 cases with full before/after code
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- **`references/componentWillReceiveProps.md`** - getDerivedStateFromProps trap warnings, full examples
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- **`references/componentWillUpdate.md`** - getSnapshotBeforeUpdate + componentDidUpdate pairing
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Read the relevant file before writing any migration code.
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