The term ["render prop"](https://cdb.reacttraining.com/use-a-render-prop-50de598f11ce) refers to a simple technique for sharing code between React components using a prop whose value is a function.
## Use Render Props for Cross-Cutting Concerns
Components are the primary unit of code reuse in React, but it's not always obvious how to share the state or behavior that one component encapsulates to other components that need that same state.
For example, the following component tracks the mouse position in a web app:
<p>The current mouse position is ({this.state.x}, {this.state.y})</p>
</div>
);
}
}
```
As the cursor moves around the screen, the component displays its (x, y) coordinates in a `<p>`.
Now the question is: How can we reuse this behavior in another component? In other words, if another component needs to know about the cursor position, can we encapsulate that behavior so that we can easily share it with that component?
Since components are the basic unit of code reuse in React, let's try refactoring the code a bit to use a `<Mouse>` component that encapsulates the behavior we need to reuse elsewhere.
```js
// The <Mouse> component encapsulates the behavior we need...
{/* ...but how do we render something other than a <p>? */}
<p>The current mouse position is ({this.state.x}, {this.state.y})</p>
</div>
);
}
}
class MouseTracker extends React.Component {
render() {
return (
<div>
<h1>Move the mouse around!</h1>
<Mouse/>
</div>
);
}
}
```
Now the `<Mouse>` component encapsulates all behavior associated with listening for `mousemove` events and storing the (x, y) position of the cursor, but it's not yet truly reusable.
For example, let's say we have a `<Cat>` component that renders the image of a cat chasing the mouse around the screen. We might use a `<Cat mouse={{ x, y }}>` prop to tell the component the coordinates of the mouse so it knows where to position the image on the screen.
As a first pass, you might try rendering the `<Cat>`*inside `<Mouse>`'s `render` method*, like this:
We could just swap out the <p> for a <Cat> here ... but then
we would need to create a separate <MouseWithSomethingElse>
component every time we need to use it, so <MouseWithCat>
isn't really reusable yet.
*/}
<Catmouse={this.state}/>
</div>
);
}
}
class MouseTracker extends React.Component {
render() {
return (
<div>
<h1>Move the mouse around!</h1>
<MouseWithCat/>
</div>
);
}
}
```
This approach will work for our specific use case, but we haven't achieved the objective of truly encapsulating the behavior in a reusable way. Now, every time we want the mouse position for a different use case, we have to create a new component (i.e. essentially another `<MouseWithCat>`) that renders something specifically for that use case.
Here's where the render prop comes in: Instead of hard-coding a `<Cat>` inside a `<Mouse>` component, and effectively changing its rendered output, we can provide `<Mouse>` with a function prop that it uses to dynamically determine what to render–a render prop.
Instead of providing a static representation of what <Mouse> renders,
use the `render` prop to dynamically determine what to render.
*/}
{this.props.render(this.state)}
</div>
);
}
}
class MouseTracker extends React.Component {
render() {
return (
<div>
<h1>Move the mouse around!</h1>
<Mouserender={mouse=> (
<Catmouse={mouse}/>
)}/>
</div>
);
}
}
```
Now, instead of effectively cloning the `<Mouse>` component and hard-coding something else in its `render` method to solve for a specific use case, we instead provide a `render` prop that `<Mouse>` can use to dynamically determine what it renders.
More concretely, **a render prop is a function prop that a component uses to know what to render.**
This technique makes the behavior that we need to share extremely portable. To get that behavior, render a `<Mouse>` with a `render` prop that tells it what to render with the current (x, y) of the cursor.
One interesting thing to note about render props is that you can implement most [higher-order components](/react/docs/higher-order-components.html) (HOC) using a regular component with a render prop. For example, if you would prefer to have a `withMouse` HOC instead of a `<Mouse>` component, you could easily create one using a regular `<Mouse>` with a render prop:
```js
// If you really want a HOC for some reason, you can easily
// create one using a regular component with a render prop!
function withMouse(Component) {
return class extends React.Component {
render() {
return (
<Mouserender={mouse=> (
<Component{...this.props}mouse={mouse}/>
)}/>
);
}
}
}
```
So using a render prop makes it possible to use either pattern.
### Be careful when using Render Props with React.PureComponent
Using a render prop can negate the advantage that comes from using [`React.PureComponent`](https://reactjs.org/docs/react-api.html#reactpurecomponent) if you create the function inside a `render` method. This is because the shallow prop comparison will always return `false` for new props, and each `render` in this case will generate a new value for the render prop.
For example, continuing with our `<Mouse>` component from above, if `Mouse` were to extend `React.PureComponent` instead of `React.Component`, our example would look like this:
```js
class Mouse extends React.PureComponent {
// Same implementation as above...
}
class MouseTracker extends React.Component {
render() {
return (
<div>
<h1>Move the mouse around!</h1>
{/*
This is bad! The value of the `render` prop will
be different on each render.
*/}
<Mouserender={mouse=> (
<Catmouse={mouse}/>
)}/>
</div>
);
}
}
```
In this example, each time `<MouseTracker>` renders, it generates a new function as the value of the `<Mouse render>` prop, thus negating the effect of `<Mouse>` extending `React.PureComponent` in the first place!
To get around this problem, you can sometimes define the prop as an instance method, like so:
```js
class MouseTracker extends React.Component {
constructor(props) {
super(props);
// This binding ensures that `this.renderTheCat` always refers
// to the *same* function when we use it in render.
this.renderTheCat = this.renderTheCat.bind(this);
}
renderTheCat(mouse) {
return <Catmouse={mouse}/>
}
render() {
return (
<div>
<h1>Move the mouse around!</h1>
<Mouserender={this.renderTheCat}/>
</div>
);
}
}
```
In cases where you cannot bind the instance method ahead of time in the constructor (e.g. because you need to close over the component's props and/or state) you should extend `React.Component` instead.
