# `react-doctor/no-side-effect-in-state-updater-function`

Side effect inside a state updater function

- **Category:** Bugs
- **Severity:** warn
- **Source:** oxlint-plugin-react-doctor
- **Framework:** global
- **Enabled when:** react

## Validate the diagnostic

Confirm the reported code matches this rule before you edit it.

Confirm the reported code matches `react-doctor/no-side-effect-in-state-updater-function`. Compare it with the Before example and verify the same API, framework, and execution context. Treat it as a false positive only when one of those conditions differs.

## Compare the code

The corrected pattern shows a focused way to address the diagnostic.

### Reported pattern

```tsx
const List = ({ nextItem }) => {
  const [items, setItems] = useState([]);
  const addItem = () => {
    setItems((currentItems) => {
      analytics.track("added");
      return [...currentItems, nextItem];
    });
  };
  return <button onClick={addItem}>{items.length}</button>;
};
```

### Corrected pattern

```tsx
const List = ({ nextItem }) => {
  const [items, setItems] = useState([]);
  const addItem = () => {
    analytics.track("added");
    setItems((currentItems) => [...currentItems, nextItem]);
  };
  return <button onClick={addItem}>{items.length}</button>;
};
```

## How to fix

Use the corrected pattern as a reference. Preserve behavior that the rule does not require you to change.

### Copyable fix prompt

Copy this self-contained prompt into your coding agent after you confirm the diagnostic.

````text
Fix every confirmed `react-doctor/no-side-effect-in-state-updater-function` diagnostic in the current repository.

Required change:
- React may replay a state updater, so callbacks, analytics, and persistence inside it can run more than once. Compute state purely, then perform the side effect outside the setter.

Reference transformation:

Before:
```tsx
const List = ({ nextItem }) => {
  const [items, setItems] = useState([]);
  const addItem = () => {
    setItems((currentItems) => {
      analytics.track("added");
      return [...currentItems, nextItem];
    });
  };
  return <button onClick={addItem}>{items.length}</button>;
};
```

After:
```tsx
const List = ({ nextItem }) => {
  const [items, setItems] = useState([]);
  const addItem = () => {
    analytics.track("added");
    setItems((currentItems) => [...currentItems, nextItem]);
  };
  return <button onClick={addItem}>{items.length}</button>;
};
```

Constraints:
- Confirm the reported code matches the Before pattern.
- Make the smallest change that fixes the root cause.
- Preserve behavior and interfaces unrelated to this diagnostic.
- Reuse existing project components, utilities, and conventions.
- Do not introduce render-phase side effects, render-phase state updates, or Hooks rule violations.
- Adapt identifiers and framework details instead of copying blindly.
- Do not disable the rule or suppress matching code.
- Confirm this rule is enabled for the project: `react`.

Verification:
- Run focused tests for the changed behavior.
- Run React Doctor and confirm this diagnostic no longer appears.
- Report the files changed and any checks you could not run.
````