react-keyed-flatten-children/README.md

Flattens React children and fragments to an array with predictable and stable keys

# react-keyed-flatten-children

Similar to [React's built-in `Children.toArray` method](https://reactjs.org/docs/react-api.html#reactchildrentoarray), this utility takes children and returns them as an array for introspection or filtering. Different from `Children.toArray`, it will flatten nested arrays and `React.Fragment`s into a regular, one-dimensional array while ensuring element and fragment keys are preserved, unique, and stable between renders.

## getting started

This library requires a version of `react-is` matching your React version be installed alongside this library as a peer dependency. So, if you're using React 19:

```
npm install react-keyed-flatten-children react-is@19
```

If you're using React 18 or lower:

```
npm install react-keyed-flatten-children react-is@18
```

## why?

From the documentation of Children.toArray:

> [toArray] returns the children opaque data structure as a flat array with keys assigned to each child. Useful if you want to manipulate collections of children in your render methods, especially if you want to reorder or slice this.props.children before passing it down.

Unfortunately it has some thorny edges:

- [Children.toArray does not traverse into fragments](https://github.com/facebook/react/issues/6889), which limits flexibility of its use.
- Existing solutions exist, but they do not preserve the keys of the children and fragments, which throws away valuable performance optimisations provided through React keys.
- You're might be doing something a little wild, so you want the concept of "children" to as predictable as possible for you, and for the consumers of your library or component, [to avoid issues like this down the line](https://github.com/ReactTraining/react-router/issues/5785#issuecomment-351067856).

Some have proposed, soon after Fragments were introduced, that [a built-in `React.Children.toFlatArray` would be useful](https://github.com/reactjs/rfcs/pull/61), but it was decided against since "`React.Children` is in maintenance mode" until a better solution is devised.

[View the codesandbox here](https://codesandbox.io/s/react-keyed-flatten-children-example-yghsp) to get hands-on with how and when to utilise this module.

### for using this in your app

I've written a more application-focussed explanation in my article ["Addressing Children.toArray's thorny edges"](https://tommckenzie.dev/posts/react-keyed-flatten-children.html).

### for library authors

In most cases `react-keyed-flatten-children` is a drop-in replacement for `Children.toArray`.

```jsx
import flattenChildren from "react-keyed-flatten-children";

const MenuList = ({ children }) => {
  const [selectedKey, setSelectedKey] = useState(null);

  return (
    <div role="menu">
      {flattenChildren(props.children).map(child => {
        if (child.type === MenuItem) {
          return React.cloneElement(child, {
            selected: child.key === selectedKey,
            onClick: () => setSelectedKey(child.key)
          });
        }
        return child;
      })}
    </div>
  );
};
```

Now consumers can use arrays, fragments, or conditionally render items and your library will continue to work predictably.

```jsx
<MenuList>
  <h2>Animals</h2>
  <MenuItem>Dogs</MenuItem>
  <MenuItem>Cats</MenuItem>

  <h2>Cars</h2>
  {CARS_ARRAY.map(car => (
    <MenuItem>{car}</MenuItem>
  ))}

  {isLoggedIn() && (
    <>
      <h2>User</h2>
      <MenuItem>You!</MenuItem>
      <MenuItem>Someone else!</MenuItem>
    </>
  )
</MenuList>
```

### for everyone else

Work around libraries which don't support fragments passed into children.

```jsx
import flattenChildren from "react-keyed-flatten-children";
import { Switch, Route } from "react-router";

// A <Switch> looks through its children <Routes>, but won't match <Routes> within fragments.
// <FlexibleSwitch> will flatten out its children so <Switch> is able to see all children.
const FlexibleSwitch = ({ children }) => (
  <Switch>{flattenChildren(children)}</Switch>
);

const AppRoutes = ({ user }) => (
  <Router>
    <GlobalNavigation user={user} />
    <FlexibleSwitch>
      <Route path="/about">
        <About />
      </Route>
      {user && (
        <>
          <Route path="/users">
            <Users />
          </Route>
          <Route path="/settings">
            <Settings />
          </Route>
        </>
      )}
      <Route path="/">
        <Home />
      </Route>
    </FlexibleSwitch>
  </Router>
);
```

## license

MIT