Higher order components in React

A higher-order component (HOC) is an advanced technique in React for reusing component logic. HOC is not part of the React API. It is a pattern that emerges from React’s compositional nature and that can be reused in different programming languages as well.

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Concretely, a higher-order component is a function that takes a component and returns a new component.

const CoolerComponent = higherOrderComponent(NotAsCoolComponent);

Let’s say you have a component that fetches and shows a list of dogs from some API source:

Photo by Anoir Chafik on Unsplash

class DogsList extends React.Component {
  constructor(props) {
    super(props);
    this.handleChange = this.handleChange.bind(this);
    this.state = {
      // "APIDataSource" is some global api data source
      dogs: APIDataSource.getDogs()
    };
  }

  componentDidMount() {
    APIDataSource.addChangeListener(this.handleChange);
  }

  componentWillUnmount() {
    APIDataSource.removeChangeListener(this.handleChange);
  }

  handleChange() {
    // Update component state whenever the data source changes
    this.setState({
      dogs: APIDataSource.getDogs()
    });
  }

  render() {
    return (
      <div>
        {this.state.dogs.map((dog) => (
          <Dog dog={dog} key={dog.id} />
        ))}
      </div>
    );
  }
}

Later you need to create a component that fetches a cute cat from API and shows it in a text block:

Photo by Joe Cleary on Unsplash

class CuteCat extends React.Component {
  constructor(props) {
    super(props);
    this.handleChange = this.handleChange.bind(this);
    this.state = {
      cuteCat: APIDataSource.getCuteCat(props.id)
    };
  }

  componentDidMount() {
    APIDataSource.addChangeListener(this.handleChange);
  }

  componentWillUnmount() {
    APIDataSource.removeChangeListener(this.handleChange);
  }

  handleChange() {
    this.setState({
      cuteCat: APIDataSource.getCuteCat(this.props.id)
    });
  }

  render() {
    return <TextBlock text={this.state.cuteCat} />;
  }
}

DogsList and CuteCat aren’t identical — they call different methods on APIDataSource, and they render different JSX. However, some of their functionalities overlap:

• On mount, add a change listener to APIDataSource.
• Inside the listener, call setState whenever the data source changes.
• On unmount, remove the change listener.

We can write a function that creates components, like DogsList and CuteCat, that subscribe to APIDataSource. The function will accept as one of its arguments a child component that receives the subscribed data as a prop. Let’s call the function withDataSubscription:

const DogsListWithSubscription = withDataSubscription(
  DogsList,
  (APIDataSource) => APIDataSource.getDogs()
);

const CuteCatWithSubscription = withDataSubscription(
  CuteCat,
  (APIDataSource, props) => APIDataSource.getCuteCat(props.id)
);
function withDataSubscription(WrappedComponent, selectData) {
  // returns another component
  return class extends React.Component {
    constructor(props) {
      super(props);
      this.handleChange = this.handleChange.bind(this);
      this.state = {
        data: selectData(APIDataSource, props)
      };
    }

    componentDidMount() {
      // ... that takes care of the subscription...
      APIDataSource.addChangeListener(this.handleChange);
    }

    componentWillUnmount() {
      APIDataSource.removeChangeListener(this.handleChange);
    }

    handleChange() {
      this.setState({
        data: selectData(APIDataSource, this.props)
      });
    }

    render() {
      // Renders the wrapped component with data
      // We pass through any additional props
      return <WrappedComponent data={this.state.data} {...this.props} />;
    }
  };
}

And that’s it! Go on to my other article to find out HOCs I use the most.