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

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.