Children

Learn how to use Flow to strictly type the children of your React components

React elements can have zero, one, or many children. Being able to type these children with Flow allows you to build expressive APIs with React children.

Generally, the type you should try first when adding a type for the children of your React component is React.Node.

Like this:

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import * as React from 'react';

type Props = {
  children?: React.Node,
};

function MyComponent(props: Props) {
  return <div>{props.children}</div>;
}

Note: You need to use import * as React from 'react' here instead of import React from 'react' to get access to the React.Node type. We explain why that is in the React Type Reference.

However, if you want to do anything more powerful with the React children API then you will need a strong intuition of how React handles children. Let us look at a couple of cases before continuing to help build that intuition.

If you already have a strong intuition about how React children work then feel free to skip to our examples demonstrating how to type various children patterns that commonly show up in React components.

Our first case is an element with no children:

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<MyComponent />;

// which is the same as...
React.createElement(MyComponent, {});

If you pass in no children when creating an element of MyComponent then props.children will not be set. If you try to access props.children, it will be undefined.

What happens when you have a single child?

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<MyComponent>{42}</MyComponent>;

// which is the same as...
React.createElement(MyComponent, {}, 42);

If you pass in a single value then props.children will be exactly that single value. Here props.children will be the number 42. Importantly, props.children will not be an array! It will be exactly the number 42.

What happens when you have multiple children?

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<MyComponent>{1}{2}</MyComponent>;

// which is the same as...
React.createElement(MyComponent, {}, 1, 2);

Now, if you pass two values then props.children will be an array. Specifically in this case props.children will be [1, 2].

Multiple children may also look like:

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<MyComponent>{'hello'} world</MyComponent>;

// which is the same as...
React.createElement(MyComponent, {}, 'hello', ' world');

In which props.children would be the array ['hello', ' world'].

or:

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<MyComponent>{'hello'} <strong>world</strong></MyComponent>;

// which is the same as...
React.createElement(
  MyComponent,
  {},
  'hello',
  ' ',
  React.createElement('strong', {}, 'world'),
);

In which props.children would be the array ['hello', ' ', <strong>world</strong>].

Moving on to the next case. What happens if we have a single child, but that child is an array?

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<MyComponent>{[1, 2]}</MyComponent>;

// which is the same as...
React.createElement(MyComponent, {}, [1, 2]);

This follows the same rule that when you pass in a single child then props.children will be exactly that value. Even though [1, 2] is an array it is a single value and so props.children will be exactly that value. That is to say props.children will be the array [1, 2] and not an array of arrays.

This case happens often when you use array.map() such as in:

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<MyComponent>
  {messages.map(message => <strong>{message}</strong>)}
</MyComponent>

// which is the same as...
React.createElement(
  MyComponent,
  {},
  messages.map(message => React.createElement('strong', {}, message)),
);

So a single array child is left alone, but what happens if we have multiple children that are arrays?

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<MyComponent>{[1, 2]}{[3, 4]}</MyComponent>;

// which is the same as...
React.createElement(MyComponent, {}, [1, 2], [3, 4]);

Here props.children will be an array of arrays. Specifically props.children will be [[1, 2], [3, 4]].

The rule to remember with React children is that if you have no children then props.children will not be set, if you have one single child then props.children will be set to exactly that value, and if you have two or more children then props.children will be a new array of those values.

Note: Watch out for whitespace! Take the following:

<MyComponent>{42}  </MyComponent>

This compiles to: React.createElement(MyComponent, {}, 42, ' '). (With the spaces!) See how the spaces show up as part of the children? In this case props.children would be [42, ' '] and not the number 42. However, the following is fine:

<MyComponent>
  {42}
</MyComponent>

It will compile to what you would expect: React.createElement(MyComponent, {}, 42).

Newlines and indentation after newlines are stripped, but watch out for whitespace when using a component with strict types around what children may be.

Note: Watch out for comments! Take the following:

<MyComponent>
  // some comment...
  {42}
</MyComponent>

This compiles to: React.createElement(MyComponent, {}, '// some comment...', 42). See how the comment is included in the element’s children? In this case props.children would be ['// some comment...', 42] which includes the comment. To write comments in JSX use the following syntax:

<MyComponent>
  {/* some comment... */}
  {42}
</MyComponent>

Now let's see how you would take this intuition and type the children of various React components.

Only allowing a specific element type as children.

Sometimes you only want a specific component as the children to your React component. This often happens when are building a table component which needs specific column children components, or a tab bar which needs specific configuration for each tab. One such tab bar component that uses this pattern is React Native’s <TabBarIOS> component.

React Native’s <TabBarIOS> component only allows React element children and those elements must have a component type of <TabBarIOS.Item>. You are expected to use <TabBarIOS> like:

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<TabBarIOS>
  <TabBarIOS.Item>{/* ... */}</TabBarIOS.Item>
  <TabBarIOS.Item>{/* ... */}</TabBarIOS.Item>
  <TabBarIOS.Item>{/* ... */}</TabBarIOS.Item>
</TabBarIOS>

You are not allowed to do the following when using <TabBarIOS>:

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<TabBarIOS>
  <TabBarIOS.Item>{/* ... */}</TabBarIOS.Item>
  <TabBarIOS.Item>{/* ... */}</TabBarIOS.Item>
  <View>{/* ... */}</View>
  <SomeOtherComponent>{/* ... */}</SomeOtherComponent>
  <TabBarIOS.Item>{/* ... */}</TabBarIOS.Item>
</TabBarIOS>

See how we added <View> and <SomeOtherComponent> as children to <TabBarIOS>? This is not allowed and <TabBarIOS> will throw an error. How do we make sure Flow does not allow this pattern?

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import * as React from 'react';

class TabBarIOSItem extends React.Component<{}> {
  // implementation...
}

type Props = {
  children: React.ChildrenArray<React.Element<typeof TabBarIOSItem>>,
};

class TabBarIOS extends React.Component<Props> {
  static Item = TabBarIOSItem;
  // implementation...
}

<TabBarIOS>
  <TabBarIOS.Item>{/* ... */}</TabBarIOS.Item>
  <TabBarIOS.Item>{/* ... */}</TabBarIOS.Item>
  <TabBarIOS.Item>{/* ... */}</TabBarIOS.Item>
</TabBarIOS>;

We set the type of props to React.ChildrenArray<React.Element<typeof TabBarIOSItem>> which will guarantee that <TabBarIOS> must only have children that are TabBarIOS.Item React elements.

Our types reference has more information about both React.ChildrenArray<T> and React.Element<typeof Component>.

Note: If you want methods like map() and forEach() or to handle a React.ChildrenArray<T> as a normal JavaScript array then React provides the React.Children API to do just this. It has functions like React.Children.toArray(props.children) that you can use to treat your React.ChildrenArray<T> as a flat array.

Enforcing that a component only gets a single child.

Sometimes you want to enforce that your component will only receive a single child. You could use React.Children.only() function to enforce this constraint, but you could also enforce this in Flow. To do this you would not wrap the type for your children in React.ChildrenArray<T>. Like so:

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import * as React from 'react';

type Props = {
  children: React.Element<any>,
};

function MyComponent(props: Props) {
  // implementation...
}

// Not allowed! You must have children.
<MyComponent />;

// Not ok! We have multiple element children.
<MyComponent>
  <div />
  <div />
  <div />
</MyComponent>;

// This is ok. We have a single element child.
<MyComponent>
  <div />
</MyComponent>;

Typing function children or other exotic children types.

React allows you to pass any value as the children of a React component. There are some creative uses of this capability such as using a function for children which could look like this:

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<MyComponent>
  {data => (
    <div>{data.foo}</div>
  )}
</MyComponent>

react-router version 4 asks for a function as the children to its <Route> component. You would provide a function as the children to react-router like this:

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<Route path={to}>
  {({ match }) => (
    <li className={match ? 'active' : ''}>
      <Link to={to} {...rest}/>
    </li>
  )}
</Route>

(Example adapted from the react-router documentation.)

Here is how you would type the <Route> component in Flow:

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import * as React from 'react';

type Props = {
  children: (data: { match: boolean }) => React.Node,
  path: string,
  // other props...
};

class Route extends React.Component<Props> {
  // implementation...
}

<Route path={to}>
  {({ match }) => (
    <li className={match ? 'active' : ''}>
      <Link to={to} {...rest}/>
    </li>
  )}
</Route>;

The type for children is a function that takes in some object type and returns a React.Node which is the type for any value that can be rendered by React. A children function does not need to return React.Node. It could return any type, but in this case react-router wants to render the result returned by the children function.

This pattern is also not limited to function children. You could also pass in arbitrary object or class types.

Using React.Node but without some primitive types like strings.

React.Node is the general type for children, but sometimes you might want to use React.Node while excluding some primitives like strings and numbers. The React Native <View> component does this, for example.

The React Native <View> component will allow any primitive value or any React element as its children. However, <View> does not allow strings or numbers as children! You could use React.Node as the children type for <View>, however React.Node includes strings which we don’t want for <View>. So we need to create our own type.

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import * as React from 'react';

type ReactNodeWithoutStrings = React.ChildrenArray<
  | void
  | null
  | boolean
  | React.Element<any>
>;

type Props = {
  children?: ReactNodeWithoutStrings,
  // other props...
};

class View extends React.Component<Props> {
  // implementation...
}

React.ChildrenArray<T> is a type that models React nested array data structure for children. ReactNodeWithoutStrings uses React.ChildrenArray<T> to be an arbitrarily nested array of null, boolean, or React elements.

React.Element<typeof Component> is the type of a React element like <div/> or <MyComponent/>. Notably elements are not the same as components!

Note: If you want methods like map() and forEach() or to handle a React.ChildrenArray<T> as a normal JavaScript array then React provides the React.Children API to do just this. It has functions like React.Children.toArray(props.children) that you can use to treat your React.ChildrenArray<T> as a flat array.

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