# Source: https://react.dev/reference/eslint-plugin-react-hooks/lints/purity

---
title: purity
---

<Intro>

Validates that [components/hooks are pure](/reference/rules/components-and-hooks-must-be-pure) by checking that they do not call known-impure functions.

</Intro>

## Rule Details {/*rule-details*/}

React components must be pure functions - given the same props, they should always return the same JSX. When components use functions like `Math.random()` or `Date.now()` during render, they produce different output each time, breaking React's assumptions and causing bugs like hydration mismatches, incorrect memoization, and unpredictable behavior.

## Common Violations {/*common-violations*/}

In general, any API that returns a different value for the same inputs violates this rule. Usual examples include:

- `Math.random()`
- `Date.now()` / `new Date()`
- `crypto.randomUUID()`
- `performance.now()`

### Invalid {/*invalid*/}

Examples of incorrect code for this rule:

```js
// ❌ Math.random() in render
function Component() {
  const id = Math.random(); // Different every render
  return <div key={id}>Content</div>;
}

// ❌ Date.now() for values
function Component() {
  const timestamp = Date.now(); // Changes every render
  return <div>Created at: {timestamp}</div>;
}
```

### Valid {/*valid*/}

Examples of correct code for this rule:

```js
// ✅ Stable IDs from initial state
function Component() {
  const [id] = useState(() => crypto.randomUUID());
  return <div key={id}>Content</div>;
}
```

## Troubleshooting {/*troubleshooting*/}

### I need to show the current time {/*current-time*/}

Calling `Date.now()` during render makes your component impure:

```js {expectedErrors: {'react-compiler': [3]}}
// ❌ Wrong: Time changes every render
function Clock() {
  return <div>Current time: {Date.now()}</div>;
}
```

Instead, [move the impure function outside of render](/reference/rules/components-and-hooks-must-be-pure#components-and-hooks-must-be-idempotent):

```js
function Clock() {
  const [time, setTime] = useState(() => Date.now());

  useEffect(() => {
    const interval = setInterval(() => {
      setTime(Date.now());
    }, 1000);

    return () => clearInterval(interval);
  }, []);

  return <div>Current time: {time}</div>;
}
```