Skills ZICQ category：Writing & Research react18-lifecycle-patterns

React18 Lifecycle Patterns

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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What this skill does

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Per Agent Skills progressive disclosure: name and description load at startup (~100 tokens); the full SKILL.md body loads when the skill activates; scripts/, references/, and assets/ load only as needed. This file's sections: React 18 Lifecycle Patterns; Quick Decision Guide; componentWillMount - what does it do?; componentWillReceiveProps - what does it do?; componentWillUpdate - what does it do?; The UNSAFE_ Prefix Rule.

File analysis

File analysis: besides SKILL.md, the body references references/componentWillMount.md, references/componentWillReceiveProps.md, references/componentWillUpdate.md. Those resources load on demand.

React 18 Lifecycle PatternsQuick Decision GuidecomponentWillMount - what does it do?componentWillReceiveProps - what does it do?componentWillUpdate - what does it do?The UNSAFE_ Prefix RuleReference Files

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SKILL.md & Agent activation

Official spec ↗

name: react18-lifecycle-patterns
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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Files referenced by the instructions · 3

references/componentWillMount.md
references/componentWillReceiveProps.md
references/componentWillUpdate.md

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Install the agent skill "react18-lifecycle-patterns" into my project. The full SKILL.md and official description are at https://zicq.com/en/skills/skl-91cd14d5cf1be606-React18-Lifecycle-Patterns.html
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SKILL.md source Copy freely; the source file is unchanged

---
name: react18-lifecycle-patterns
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.'
---

# 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.

Readable layout

name: react18-lifecycle-patterns 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.'

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 | | Runs a side effect (fetch, subscription, DOM) | Move to componentDidMount | → componentWillMount.md | | Derives initial state from props | Move to constructor with props | → componentWillMount.md |

componentWillReceiveProps - what does it do?

| What it does | Correct migration | Reference | |---|---|---| | Async side effect triggered by prop change (fetch, cancel) | componentDidUpdate | → componentWillReceiveProps.md | | Pure state derivation from new props (no side effects) | getDerivedStateFromProps | → componentWillReceiveProps.md |

componentWillUpdate - what does it do?

| What it does | Correct migration | Reference | |---|---|---| | Reads the DOM before update (scroll, size, position) | getSnapshotBeforeUpdate | → componentWillUpdate.md | | Cancels requests / runs effects before update | componentDidUpdate with prev comparison | → componentWillUpdate.md |

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:

// TODO: React 19 will remove this. Migrate before React 19 upgrade.
// UNSAFE_ prefix added temporarily - replace with componentDidMount / getDerivedStateFromProps / etc.

Reference Files