React sub-components

Every React project I’ve worked on, whether it was personal or work related, got big enough at some point that their codebase became hard to understand. Every little change required more thinking but lead to a lot of inconsistencies and hacks. Among the many issues I had with such codebases, the lack of reusability of some views was the main one: it lead to a lot of copying/pasting code of complex components/views to ensure they look the same, and the resulting duplicated code didn’t make it easier to maintain nor to test. Using a sub-component pattern can help to fix all these issues.

What exactly is a sub-component

For this article, we’ll consider the following view as our main example: a simple Article view to render a title, subtitle, content, metadata and comments of an article object. We’ve all dealt with such views, and they can be really problematic for the reasons stated in the intro.

class MyArticleView extends React.Component {
  ...
  render() {
    return (
      <div className={css.mainContainer}>
        <div className={css.wrapper}>
          <div className={css.titleContainer}>
            <div className={css.title}>
              <span>{this.renderTitle()}</span>
            </div>
            <div className={css.subtitle}>
              <div className={css.subtitleBox}> {this.renderSubtitle()}</div>
            </div>
          </div>
          <ul className={css.articlemetadata}>
            <li className={css.item}>{this.renderAuthor()}</li>
            <li className={css.item}>{this.renderDate()}</li>
          </ul>
        </div>
        <div className={css.contentArticle}>
          <div className={css.contentTextStyle}>{this.renderMainContent()}</div>
          <span className={css.inlineComments}>{this.renderComments()}</span>
        </div>
      </div>
    );
  }
}

By using sub-components we can render the same exact view, but with a much more readable code and a reusable component. This is what the result can look like:

class MyArticleView extends React.Component {
  ...
  render() {
    return (
      <Article>
        <Article.Title>{this.renderTitle()}</Article.Title>
        <Article.Subtitle>{this.renderSubtitle()}</Article.Subtitle>
        <Article.Metadata>
          {this.renderAuthor()}
          {this.renderDate()}
        </Article.Metadata>
        <Article.Content>{this.renderContent()}</Article.Content>
        <Article.Comments>{this.renderComments}</Article.Comments>
      </Article>
    );
  }
}

In this context, sub-components are defined as components which have their own definition declared within another parent component, and can only be used in the context of that parent component. In the example above, the Title component for instance only exists within the scope of the Article component. It can’t be rendered on its own. I’m personally not sure about the name, but this is the best term I’ve found to refer to this pattern that I’ve learned to appreciate in my projects. Sub-components can be seen in multiple libraries such as Recharts or Semantic-UI. The latter refers to sub-components as Modules, Collections and Views in its library, and gives you the ability to render views the same way as stated above. This kind of pattern is really beneficial:

• to keep views consistent: you can actually show any kind of data using the Article component above. What matters here is that regardless of its purpose, it will look the same across the whole app.
• to keep your code tight and clean: Title, Comments, Subtitle, Metadata only make sense within Article and will only be able to be used within it (i.e. where they make sense, since these components are only used in the context of an “Article”).
• to have easily testable views: for testing such components, Jest and snapshot testing are our allies. It gives us the ability to quickly test any combination of sub-components when using Article. We’ll see how to use Jest to test such a pattern later.

How to build sub-components

In this section we’re going to build the Article component step by step, first by trying to implement the Title sub-component. The first thing we need in order to build sub-components within a component is a util to find children by “type” or “name” so React will know how to render our Title sub-component. We’ll pass two parameters to this util:

• children: the list of children of Article
• component: the component we want to find within the list of children, in our example it will be Title. Here’s how the util findByType looks like:

import React from 'react';
const findByType = (children, component) => {
  const result = [];
  /* This is the array of result since Article can have multiple times the same sub-component */
  const type = [component.displayName] || [component.name];
  /* We can store the actual name of the component through the displayName or name property of our sub-component */
  React.Children.forEach(children, child => {
    const childType =
      child && child.type && (child.type.displayName || child.type.name);
    if (type.includes(childType)) {
      result.push(child);
    }
  });
  /* Then we go through each React children, if one of matches the name of the sub-component we’re looking for we put it in the result array */
  return result[0];
};
export default findByType;

Now that we have our findByType util, we can start writing our Article component and the Title sub-component:

import React, { Component } from 'react';
import findByType from './findByType';
import css from './somestyle.css';
// We instantiate the Title sub-component
const Title = () => null;
class Article extends Component {
  // This is the function that will take care of rendering our Title sub-component
  renderTitle() {
    const { children } = this.props;
    // First we try to find the Title sub-component among the children of Article
    const title = findByType(children, Title);
    // If we don’t find any we return null
    if (!title) {
      return null;
    }
    // Else we return the children of the Title sub-component as wanted
    return <div className={css.title}>{title.props.children}</div>;
  }
  render() {
    return (
      <div className={css.mainContainer}>
        <div className={css.wrapper}>
          <div className={css.titleContainer}>
           {this.renderTitle()}
          </div>
        </div>
      </div>
    );
  }
}
// Lastly we expose the Title sub-component through Article
Article.Title = Title;
export default Article;

We now have the ability to use the Article component and its Title sub-component as such:

<Article>
  <Article.Title>
    My Article Title
  </Article.Title>
</Article>

In order to extend our set of sub-components, we simply need to instantiate each one of them, write their corresponding render function, and call it in the main render function. Below you will find the fully implemented component with all its sub-components:

// @flow
import React, { Component } from 'react';
import type { Node } from 'react';
import findByType from './findByType';
import css from './styles.css';

const Title = () => null;
const Subtitle = () => null;
const Metadata = () => null;
const Content = () => null;
const Comments = () => null;

type Props = {
  children?: Node,
  className?: string,
};

class Article extends Component<Props> {
  static Title: Function;
  static Subtitle: Function;
  static Metadata: Function;
  static Content: Function;
  static Comments: Function;
  
  renderTitle() {
    const { children } = this.props;
    const title = findByType(children, Title);
    if (!title) {
      return null;
    }
    return <div className={css.title}>{title.props.children}</div>;
  }

  renderSubtitle() {
    const { children } = this.props;
    const subtitle = findByType(children, Subtitle);
    if (!subtitle) {
      return null;
    }
    return (
      <div className={css.subtitle}>
        <div className={css.subtitleBox}>{subtitle}</div>
      </div>
    );
  }

  renderMetadata() {
    const { children } = this.props;
    const metadata = findByType(children, Metadata);

    if (!metadata) {
      return null;
    }

    return (
      <ul className={css.articlemetadata}>
        {metadata.props.children.map(child => {
          return <li className={css.item}>{child}</li>;
        })}
      </ul>
    );
  }

  renderContentAndComment() {
    const { children } = this.props;
    const content = findByType(children, Content);
    const comments = findByType(children, Comment);

    if (!content) {
      return null;
    }

    return (
      <div className={css.contentArticle}>
        <div className={css.contentTextStyle}>{content.props.children}</div>
        <span className={css.inlineComments}>
          {comments && comments.props.children}
        </span>
      </div>
    );
  }

  render() {
    const { children, className, ...rest } = this.props;

    return (
      <div className={css.mainContainer}>
        <div className={css.wrapper}>
          <div className={css.titleContainer}>
            {this.renderTitle()}
            {this.renderSubtitle()}
          </div>
          {this.renderMetadata()}
          {this.renderContentAndComment()}
        </div>
      </div>
    );
  }
}

Article.Title = Title;
Article.Subtitle = Subtitle;
Article.Metadata = Metadata;
Article.Content = Content;
Article.Comments = Comments;

export default Article;

Note: the renderMetadata function is really interesting in this example, it shows how it is possible to use a single render function for two different sub-components.

Using Jest and snapshot testing to test sub-components

Snapshot testing our sub-components is probably the quickest and safest way to make sure that any combination of sub-components within the Article component will render properly. To do this we’re going to use both Jest and Enzyme. Here’s how you can write tests for our example:

import React from 'react';
import { mount } from 'enzyme';
import Article from '../';

// First we declare some mocks
const Content = () => <div>[Mock] Content</div>;
const Subtitle = () => <div>[Mock] Subtitle</div>;
const Comments = () => <div>[Mock] Comments</div>;
const Metadata = () => <div>[Mock] Metadata</div>;
const Title = () => <div>[Mock] Title</div>;
const Subtitles = () => <div>[Mock] Subtitles</div>;

it('Renders with all the sub-components', () => {
  // Then we try to render the Article component with the desired sub-components
  const component = mount(
    <Article>
      <Article.Title>
        <Title />
      </Article.Title>
      <Article.Subtitle>
        <Subtitle />
      </Article.Subtitle>
      <Article.Metadata>
        <Metadata />
      </Article.Metadata>
      <Article.Content>
        <Content />
      </Article.Content>
      <Article.Comments>
        <Comments />
      </Article.Comments>
    </Article>
  );
  // Finally we check it matches its snapshot stored in the project
  expect(component).toMatchSnapshot();
});

it('Renders with only the Content and Comments', () => {
  // We can iterate the same process again with a different combination of sub-components
  const component = mount(
    <Article>
      <Article.Content>
        <Content />
      </Article.Content>
      <Article.Comments>
        <Comments />
      </Article.Comments>
    </Article>
  );
  expect(component).toMatchSnapshot();
});

it('Renders with a Title and without a subtitle', () => {
  const component = mount(
    <Article>
      <Article.Title>
        <Title />
      </Article.Title>
      <Article.Metadata>
        <Metadata />
      </Article.Metadata>
      <Article.Content>
        <Content />
      </Article.Content>
      <Article.Comments>
        <Comments />
      </Article.Comments>
    </Article>
  );
  expect(component).toMatchSnapshot();
});
```## One last note
While writing this article I noticed that sub-components wouldn’t render on IE 11 and Edge once bundled with Babel 6.26.0 and Webpack 3.10. Maybe it affects other versions, I haven’t checked yet, but all I know is that it only affected the bundled app, it worked fine when the project was running with Webpack Dev Server.

What happened? The culprit here was found when debugging the `findByType` util. `child.type.displayName || child.type.name` was returning `undefined` on IE and Edge for the following reason: “`type` *here is a reference to the component constructor. So if you do `child.type.name`, it references the name property on the constructor -- no supported in IE.”*

Reference: [https://github.com/facebook/react/issues/9803](https://github.com/facebook/react/issues/9803)

As a workaround I added a static variable called displayName for each one of my sub-components to ensure that they have a name. Here’s how it should look like on our example:

...

const Title = () => null; Title.displayName = 'Title';

const Subtitle = () => null; Subtitle.displayName = 'Subtitle';

const Metadata = () => null; Metadata.displayName = 'Metadata';

const Content = () => null; Content.displayName = 'Content';

const Comments = () => null; Comments.displayName = 'Comments';

...

***
If you liked this article don’t forget to hit the “rocket” button and if you have any other questions I’m either reachable on [Twitter](http://twitter.com/MaximeHeckel), or from my [website.](https://maximeheckel.com/)

*Part Two [Here](https://javascript.works-hub.com/learn/react-sub-components-part-2-using-the-new-context-api-aba70)*