react18-batching-patterns
Provides exact patterns for diagnosing and fixing automatic batching regressions in React 18 class components. Use this skill whenever a class component has multiple setState calls in an async method, inside setTimeout, inside a Promise .then() or .catch(), or in a native event handler. Use it before writing any flushSync call - the decision tree here prevents unnecessary flushSync overuse. Also use this skill when fixing test failures caused by intermediate state assertions that break after React 18 upgrade.
What this skill does
# React 18 Automatic Batching Patterns
Reference for diagnosing and fixing the most dangerous silent breaking change in React 18 for class-component codebases.
## The Core Change
| Location of setState | React 17 | React 18 |
|---|---|---|
| React event handler | Batched | Batched (same) |
| setTimeout | **Immediate re-render** | **Batched** |
| Promise .then() / .catch() | **Immediate re-render** | **Batched** |
| async/await | **Immediate re-render** | **Batched** |
| Native addEventListener callback | **Immediate re-render** | **Batched** |
**Batched** means: all setState calls within that execution context flush together in a single re-render at the end. No intermediate renders occur.
## Quick Diagnosis
Read every async class method. Ask: does any code after an `await` read `this.state` to make a decision?
```
Code reads this.state after await?
YES → Category A (silent state-read bug)
NO, but intermediate render must be visible to user?
YES → Category C (flushSync needed)
NO → Category B (refactor, no flushSync)
```
For the full pattern for each category, read:
- **`references/batching-categories.md`** - Category A, B, C with full before/after code
- **`references/flushSync-guide.md`** - when to use flushSync, when NOT to, import syntax
## The flushSync Rule
**Use `flushSync` sparingly.** It forces a synchronous re-render, bypassing React 18's concurrent scheduler. Overusing it negates the performance benefits of React 18.
Only use `flushSync` when:
- The user must see an intermediate UI state before an async operation begins
- A spinner/loading state must render before a fetch starts
- Sequential UI steps have distinct visible states (progress wizard, multi-step flow)
In most cases, the fix is a **refactor** - restructuring the code to not read `this.state` after `await`. Read `references/batching-categories.md` for the correct approach per category.
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