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.