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What is the Render props pattern? The Render props pattern is a way for us to create a component that provides some kind of data to a child component.

Why would we want that? Well imagine that we wanted to do something of the following:

fetching data, wouldn’t it be nice to have a component that abstracts away all of the mess of HTTP and just serves you the data when it’s done?
A/B testing, as you launch an app into production you will eventually want to improve but you might not know the best way forward or you might want to release often and push the code to production but some feature is not yet ready to see the light of day, so you want to be able to conditionally decide whether something is visible or not.

If you have any of the scenarios above you have reusable functionality. With reusable functionality, you most likely want to abstract that away into a function or a component, we’re gonna opt for the latter.

Wouldn’t it be nice if we can create components for this functionality and just serve it up to some component? That child component would be unaware that it is being served up data.

In a sense, this resembles what we do with Providers but also how container components wrap presentation components. This all sounds a bit vague so let’s show some markup how this could look:

const ProductDetail = ({ product }) => ( 
  <React.Fragment> 
    <h2>{product.title}</h2> 
    <div>{product.description}</div> 
  </React.Fragment> ) 

<Fetch url="some url where my data is" 
  render={(data) => <ProductDetail product={data.product} /> }
/>

As we can see above we have two different components ProductDetail and Fetch. ProductDetail just looks like a presentation component. Fetch on the other hand looks a bit different. It has a property url on it and it seems like it has a render property that ends up rendering our ProductDetail.

Render props explained

We can reverse engineer this and figure out how this works.

Let’s have a look at the code again:

<Fetch url="some url where my data is" 
  render={(data) => <ProductDetail product={data.product} /> }
/>

Our Fetch component has an attribute render that seems to take a function that ends up producing JSX. Here is the thing, the whole render-props pattern is about us invoking a function in our return method. Let me explain that by showing some code:

class Fetch extends React.Component { 
  render() { 
    return this.props.render(); 
  } 
}

This is what the pattern is, at its simplest. The way we use the Fetch component means we at least need to send something into the this.props.render() call. Let's just extract the function invocation bit above and look at it:

(data) => <ProductDetail product={data.product} />

We can see above that that we need a parameter data and data seems to be an object. Ok, so where does data come from? Well thats the thing with our Fetch component, it does some heavy lifting for us namely carrying out HTTP calls.

Creating a component for HTTP

Let’s add some life cycle methods to Fetch so it looks like this:

class Fetch extends React.Component { 
  state = { 
    data: void 0, 
    error: void 0 
  } 
  componentDidMount() { 
    this.fetchData(); 
  } 
  async fetchData() { 
    try { 
      const response = await fetch(this.props.url); 
      const json = await response.json(); 
      this.setState({ data: json }); 
    catch (err) { 
      this.setState({ error: err }) 
    } 
  } 
  render() { 
    if (!this.state.data) return null; 
    else return this.props.render(this.state.data); 
  } 
}

Quick note, if you are unfamiliar with void 0, that’s just setting something to undefined

Ok, now we have fleshed out our component a little. We’ve added the method fetchData() that makes HTTP call,s given this.props.url and we can see that our render() method renders null if this.state.data isn't set, but if the HTTP call finished we invoke this.props.render(data) with our JSON response.

However, it lacks three things:

handling error, we should add logic to handle error
handling loading, right now we render nothing if thefetch() call hasn't finished, that isn't very nice
handling this.props.url, this prop might not be set initially and it might be changed over time, so we should handle that

Handling errors

We can easily handle this one by changing our render() method a little, to cater to if this.state.error is set, after all we have already written logic that sets this.state.error in our catch clause in the fetchData() method.

Here goes:

class Fetch extends React.Component { 
  state = { 
    data: void 0, 
    error: void 0 
  } 
  componentDidMount() { 
    this.fetchData(); 
  } 
  async fetchData() { 
    try { 
      const response = await fetch(this.props.url); 
      const json = await response.json(); 
      this.setState({ data: json }); 
    catch (err) { 
      this.setState({ error: err }) 
    } 
  } 
  render() { 
    const { error, data, loading } = this.state; 
    if(error) return this.props.error(error); 
    if (data) return this.props.render(data); 
    else return null; 
  } 
}

Above we added handling of this.state.error by invoking this.props.error(), so that is a thing we need to reflect once we try to use the Fetch component.

Handling loading

for this one we just need to add a new state loading and updated the render() method to look at the said property, like so:

class Fetch extends React.Component { 
  state = { 
    data: void 0, 
    error: void 0,
    loading: false 
  } 
  componentDidMount() { 
    this.fetchData(); 
  } 
  async fetchData() { 
    try { 
      this.setState({ loading: true }); 
      const response = await fetch(this.props.url); 
      const json = await response.json(); 
      this.setState({ data: json }); 
      this.setState({ loading: false }); 
    catch (err) { 
      this.setState({ error: err }) 
    } 
  }

  render() { 
    const { error, data, loading } = this.state; 
    if(loading) return <div>Loading...</div> 
    if(error) return this.props.error(error); 
    if (data) return this.props.render(data);
    else return null; 
  } 
}

Now, above we are a bit sloppy handling the loading, yes we add an if for it but what we render can most likely be improved using a nice component that looks like a spinner or a ghost image, so that's worth thinking about.

Handling changes to this.props.url

It’s entirely possible that this URL can change and we need to cater to it unless we plan on using the component like so

in which case you should skip this section and look at the next section instead ;)

The React API recently changed, before the change we would’ve needed to add the life cycle method componentWillReceiveProps() to look at if a prop changed, it's considered unsafe, however, so we must instead use

componentDidUpdate(prevProps) { 
  if (this.props.url && this.props.url !== prevProps.url){
    this.fetchData(this.props.url); 
  } 
}

That’s it, that’s what we need, let’s show the full code for this component:

class Fetch extends React.Component { 
  state = { 
    data: void 0, 
    error: void 0,
    loading: false 
  } 
  componentDidMount() { 
    this.fetchData(); 
  } 
  componentDidUpdate(prevProps) { 
    if (this.props.url && this.props.url !== prevProps.url) {     
      this.fetchData(this.props.url); 
    } 
  } 
  async fetchData() { 
    try { 
      this.setState({ loading: true }); 
      const response = await fetch(this.props.url); 
      const json = await response.json(); 
      this.setState({ data: json }); 
      this.setState({ loading: false }); 
    catch (err) { 
      this.setState({ error: err }) 
    } 
  } 
  render() {
    const { error, data, loading } = this.state; 
    if(loading) return <div>Loading...</div>
    if(error) return this.props.error(error);
    if(data) return this.props.render(data); 
    else return null; 
  } 
}

To use our component we can now type:

<Fetch 
  url={url-to-product} 
  render={(data) => <ProductDetail product={data.product} />} 
  error={(error) => <div>{error.message}</div>} 
/>

A/B Testing

Let’s move on to our next case. We will sooner or later have probably two major reasons for wanting to show code conditionally using this component:

it’s not ready yet, we want to deploy often and we may want to show a new feature only to our Product Owner so we can gather feedback, so if we would be able to control the showing of these components content with a flag, that would be great
A/B test, let’s say we don’t know which new Checkout page we want to go to, within our e-commerce app, then it would be great if we can send half the users to version1 and the other half to version 2. In such a scenario you might have two different pages but if the difference is minor, like the toggling of a few sections, then this could be a good candidate.

Ok, let’s look at how we would be using this component:

<FeatureFlag 
  flag={showAlternateSection} 
  render={()=> <div>Alternate design</div>} 
  else={()=> <div>Normal design</div>} 
/>

Above we have a component FeatureFlag and the following attributes, let's break down how we mean to use them:

flag, this would be the name of the feature flag, most likely a string
render, this would be a method we invoke given that the feature flag is enabled
else, this would be a method we invoke if feature flag is disabled or non-existent

Building our component

Ok, we know how we intend to use our component, let’s try to build it:

class FeatureFlag extends React.Component { 
  state = { 
    enabled: void 0 
  } 

  componentDidMount() { 
    const enabled = localStorage.getItem(this.props.flag) === 'true'; 
    this.setState({ enabled }); 
  } 
  render() { 
    if(enabled) return this.props.render(); 
    else if(enabled === false) return this.props.else(); 
    else return null; 
  } 
}

Ok, so introduce three states here:

true, when we know the flag is true
false, when we know the flag is false
void 0/undefined, when the flags value hasn’t been resolved yet

Why do we need three states? Well, we want to make sure it renders exactly what it should be rendering and that it doesn’t show something it shouldn’t, if only for a millisecond.

Ok, that sounds a bit nuts, localStorage.getItem() is usually fast to respond.

Yea sure, ok, I might be a wee bit crazy, but what if the flag value isn’t in localStorage but it resides on a service that we need to call, then it might take some time to get the value back...

So imagine our componentDidMount() looks like this instead:

async componentDidMount() { 
  const enabled = await flagService.get(this.props.flag);
  this.setState({ enabled }); 
}

That’s a minor change if you want to place your flags in a service instead of localStorage:

Summary

Render props pattern is all about rendering a function that itself renders JSX and as you can see you can create quite powerful and useful components based on this design pattern.

Hope you found this article useful, if you did, please give me some claps.

Oh, I would love it if you left me some links in the comments to Components you built using the Render props pattern.

Stay safe, remember it’s never your fault, it’s the compiler ;)

The amazing Render props pattern for React.js — lifecycle begone!