Pay attention to the CategoryMenu component as it renders the child menu items from the categories array prop:

render() {
  return (
    <ul id="cat-menu">
      {this.props.categories.map(c => (
        <li key={c.name}>
          <CategoryMenuItem
            categoryName={c.name}
            checked={c.name === this.state.selectedCategoryName}
            onSelected={this.onCategorySelected}
          />
        </li>
      ))}
    </ul>
  );
}

In React, to render multiple children commonly through arrays, you use the map operator and return the rendered content for every item. In such cases, React promotes assigning a key prop for every item's rendered root element.

The key prop is a specialized prop that should represent the identity, meaning elements with the same key are considered interchangeable, which can better optimize performance if the collection changes.