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add note about eslint:no-label #1009

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add note about eslint:no-label #1009

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12 changes: 12 additions & 0 deletions content/docs/faq-state.md
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Expand Up @@ -76,6 +76,18 @@ handleSomething() {
}
```

One common pitfall when writing arrow functions is accidentally relying on labels and implicit returns:
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I would call this out as a 'rarely-used language feature called "labels"'

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@swyxio swyxio Jun 28, 2018 •
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I don't have strong feelings on this, whoever is merging or reviewing please feel free to change it or decide.


```js
// good
this.setState(({ bool }) => ({ bool: !bool }));

// bad
this.setState(({ bool }) => { bool: !bool }); // doesn't actually update state
```

This is because `bool:` in the second example is interpreted as a [label](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label) instead of an object key, so the arrow function in the second example actually returns `undefined`. This may cause very confusing bugs, so we recommend using [eslint:no-labels](https://eslint.org/docs/rules/no-labels) to warn when this happen. This rule does come with [eslint-config-react-app](https://github.com/facebook/create-react-app/blob/d36603979591f37eed6f7a3efcba136e7547e75d/packages/eslint-config-react-app/index.js#L81) so if you use `create-react-app` you don't need to worry, but take note if you have your own custom ESLint setup.

[Learn more about setState](/docs/react-component.html#setstate)

### When is `setState` asynchronous?
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