1. props.children (default slot projection)

Let’s build a simple Material design like Card component, which should be used like this:

const App = () => {

return (

<div>

<Card>A simple card</Card>

</div>

)

}

So we will leverage children props to render anything that will be provided between <Card>{PROJECTED CONTENT}</Card> tags.

Implementation is following:

export class Card extends Component {

render() {

return <div className="card">{this.props.children}</div>

}

}

Which renders our card:

Simple Card component with children

All good! But what if user forgets to provide children ?

const App = () => {

return (

<div>

<Card></Card>

</div>

)

}

Well, we’ll get no compile nor runtime errors and our app will render with empty Card …

empty Card because no content was provided

This is definitely not what we wanted. So how to prevent this? We can definitely write some runtime validation logic or we can leverage TypeScript and explicitly define Card component props API constraints. In this case, we need to define explicitly children prop, to make it required.

Curious reader may ask, where did we get this.props.children in our Card component implementation when we haven't defined any props ? props.children are baked within @types/react.d.ts definition and are marked as optional which mirrors React API behaviour.

children? definition within react.d.ts

With that said here is our implementation:

simple Card component with default children projection

Now if consumers of our component forgets to define children, they will get compile time error 💪:

Proper type checking of Component children API

2. props.children - children as an object map (named slots projection)

Now let’s extends our Card API, by providing additional API like:

header

media

content

actions

Your initial thoughts about this API might look like following:

<Card>

<CardHeader title="Shiba Inu">...</CardHeader>

<CardMedia src="examples/shiba2.jpg"/>

<CardContent>

<p>

The Shiba Inu is the smallest of the six original ...

</p>

</CardContent>

<CardActions>

<Button>Like</Button>

<Button>Share</Button>

<CardActions>

</Card>

Which is old good compound components pattern, which of course works, but in my opinion this is too much churn which is nicely solved with so called named slots projection API provided natively within browser via Shadow DOM (WebComponents). We can achieve similar pattern in React by passing an Object map as children prop ( That's the beauty of React, it's mostly "just JavaScript" ).

So instead our API for Card could look like following:

<Card>

{{

header: 'Shiba Inu',

media: <img src="examples/shiba2.jpg" />,

content: (

<p>The Shiba Inu is the smallest of the six original ...</p>

),

actions: (

<>

<button>Like</button>

<button>Share</button>

</>

),

}}

</Card>

With Card implementation:

Card component with children as an object map ( named slots )

We’re constraining children prop to be required and to have shape of an object map. Also TypeScript will give us nice intellisense and compile time errors if we missed something, without need to browse docs of Card component ❤️.

This is great indeed, but the API isn’t very flexible as we have to always provide all props defined on children object map.

Fix is quite easy, we can just make our children map to be optional with only content being required ( we don't wanna get empty cards within our UI ).

type Props = {

children: {

content: ReactNode

header?: ReactNode

media?: ReactNode

actions?: ReactNode

}

}

and also constraint our render method output within implementation, so we don't render things that aren't needed:

Card component with children as an object map ( named slots ) with render constraints

3. combining default children and children as object ( default and named slot projection )

Now with our last implementation of Card component, if consumers want to render just simple content, they need to write a lot of unnecessary markup:

<Card>

{{

content: <p>The Shiba Inu is the smallest of ...</p>

}}

</Card>

We can tweak our API to leverage both default children and children as an object map, so consumer would be able to use one or another.

Let’s have a look at our final Card implementation:

Card component with both default and named children projection

Quite a lot is happening here, let’s explain our code step by step:

we’re leveraging union types to define our Component props API. Also we had to change type from ReactNode to ReactChild because ReactNode contains within its definition a mixed type 👉 {} which would narrow our children type to {} instead of being NamedChildrenSlots which remove our strict type checking of our children as object. we add runtime check so app will throw when consumer forgot to provide children if props.children is in shape of our NamedChildrenSlots we will tell type checker that and render properly our markup if props.children is not NamedChildrenSlots we just render default markup provided by consumer of our component here we are defining our children guards with TypeScript type guard capabilities

we had to change type from ReactNode to ReactChild because ReactNode contains within it's definition a mixed type 👉 {}

And now we can use both children patterns within our app:

Card component with both default and named children projection usage with proper type-safety

4. props.children as a function ( known also as render props pattern )

While previous patterns kinda mirror element projection that is available in HTML and with ShadowDOM via slot projection (of course not type-safe, in our case type safe 💪), we can go further and introduce more powerful patterns like Render props / Children as a function !

So consumer of our Toggle should be able to write following markup:

<Toggle

{/* (1) */}

onToggle={(value) => console.log('onToggle', value)}

>

{

{/* (2) */}

({ toggle, on }) => (

<button onClick={toggle}>{on ? 'on' : 'off'}</button>

)

}

</Toggle>

Consumer defines what should happen onToggle which will get actual value via function argument. State handling and logic is encapsulated within our Toggle children is a function, which exposes public API of our component, which we can compose and render whatever UI we want 👌. When button will be clicked, internal state of Toggle will change, and our onToggle handler will be executed as well.

With “How the API and usage should look like” covered, let’s implement our Toggle component which via children as a function pattern:

Toggle component with children as a function pattern

Again, quite a lot code over there. Let’s examine various parts of the implementation:

We define our Props , by making children required and defined as a function, which will get public API via argument and onToggle which should be a function that will receive actual on state when internal state changes. Here we define API type from implementation, which is a very powerful TS pattern (source of truth is implementation instead of manualy defined type. When we would refactor getApi() there is no need to refactor the type as it's automatically infered == less boilerplate FTW). Because Classes are resolved as structural types instead of nominal in TypeScript, we can access any class member via property lookup + leveraging conditional types via ReturnType mapped type provided by lib.d.ts. We use similar pattern for as we used in (2.) for defining state. We infer state type definition from implementation of initial state and we also map our state object to have all members readonly to avoid mutation on compile time. here we define state via class property and we mark it as readonly again ( with that if you do this.state = {} within your code you’ll get compile errors as state is immutable ) our private toggle that will change our internal state and calls onToggle callback provided by consumer of our component private method which defines our API exposed via arguments in children function call we double check if children is function, if not we throw ( runtime check ) finally we don’t render any custom markup, instead we’re calling children with our public API

Again both implementation and usage is type safe with top notch DX, without looking up any docs 👀🖖💪.

Let’s see it in action:

Toggle component with children as a function usage within App

5. render props + children as a function

Ok, last thing that we might want to do is to give consumer API flexibility by leveraging render prop instead of children .

Easy. We just need to add render with same annotation as children prop to our type Props definition:

type Props = {

onToggle: (on: boolean) => void

children: (api: API) => ReactNode

render: (api: API) => ReactNode

}

This is not correct though, as consumer of our component will get compile errors because both children and render are required. First “fix” that may comes up my mind is to mark those as optional.

type Props = {

onToggle: (on: boolean) => void

children?: (api: API) => ReactNode

render?: (api: API) => ReactNode

}

Which won’t protect us for use cases, when user completely forgets to define children or render prop.

// No compile errors 😥

<Toggle onToggle={(value) => console.log('onToggle', value)} />

Type Union to the rescue !

type Props = {

onToggle: (on: boolean) => void

} & RenderProps type RenderProps =

| { children: (api: API) => ReactNode }

| { render: (api: API) => ReactNode }

Which widens to:

type Props =

| {

onToggle: (on: boolean) => void

children: (api: API) => ReactNode

}

| {

onToggle: (on: boolean) => void

render: (api: API) => ReactNode

}

With that API, our component should support one or another props pair.

Let's take a look if it does:

Proper type checking for our type union defined children/render props

Yup, it works!

Although intellisense for props defined within union is missing. There is an open bug for it, so hopefully it will be fixed Good thing is that inference still works for function annotation for both children/render props!

missing intellisense for union defined props

Ok let’s look on changes that we did within Toggle implementation:

Final Toggle implementation

We added new type RenderProps which intersects with onToggle prop and with that defines final type Props API new type definition of RenderProps which consist of union of 2 possible objects children/render we’re checking if render is defined by consumer, if yes we render via render prop and exit early from render. hasRender is a type guard which leverages power of conditional types ( we won't explain this as it would be for separate article on it's own) we’re checking if children is defined by consumer and if it's a function. If it is, we render via children as a function. when children nor render are provided, we throw a runtime error 🤖

That’s it! LGTM ! 🤙

5. EXTRA (render props via both children as a function and render prop)

NOTE: I don’t recommend following pattern as it’s highly experimental and breaks guarantees within Toggle component implementation. Just mentioning it here for curious reader 🧐

One more thing. What about preventing consumer to define both render and children at one time? It should be already covered right? By that prop union.

Let’s see:

Oh No Panic! No Errors 🤯… Why Mister Anderson Why ? Well TypeScript doesn’t allow exclusive union types. What needs to be done is to provide and exclusive ( XOR like ) operator to the language, which we can partially implement by conditional types.

type XOR<T, U> = (T | U) extends object

? (Without<T, U> & U) | (Without<U, T> & T)

: T | U type Without<T, U> = { [P in Exclude<keyof T, keyof U>]?: never } // Redefine RenderProps via XOR instead of Union operator

type RenderProps = XOR<

{ children: (api: API) => ReactNode },

{ render: (api: API) => ReactNode }

>

Now we got proper compile errors if both children/render are defined at once, even so we got our intellisense back!

This although broke constraints within Toggle implementation as both render and children are now defined without the need of conditional checks… Which is wrong 😫…

End

Allright we are done here!

We learned how to build highly composable components thanks to props.children React API, which are really not that different from what we get from the browser platform ( Shadow DOM ), even so more powerful thanks to JavaScript.

With that said I hope you’ve learned something new today and that you’ll leverage the power of React children composition with TypeScript sooner than later to provide consumers of your components with fantastic developer experience and type-safety ✌.