Class Components

This class has been showing you the latest APIs for React: hooks. Going forward, these sorts of components will be the default way of writing React. However, the class API still has its uses and isn't going anywhere anytime soon. In this section we're going to go through and learn the basics of it since there's still a lot class code out in the wild and the new API can't do everything the old one can, so it's still useful in some cases.

Let's go make Details.js as a class.

// replace Details.js
import { Component } from "react";
import { withRouter } from "react-router-dom";

class Details extends Component {
  constructor() {
    this.state = { loading: true };

  async componentDidMount() {
    const res = await fetch(
    const json = await res.json();
    this.setState(Object.assign({ loading: false }, json.pets[0]));

  render() {

    if (this.state.loading) {
      return <h2>loading … </h2>;

    const { animal, breed, city, state, description, name } = this.state;

    return (
      <div className="details">
          <h2>{`${animal}${breed}${city}, ${state}`}</h2>
          <button>Adopt {name}</button>

export default withRouter(Details);
  • Every class component extends React.Component. Every class component must have a render method that returns some sort of JSX / markup / call to React.createElement.
  • Not every component needs to have a constructor. Many don't. I'll show you momentarily how you nearly never need to have one. In this case we need it to instantiate the state object (which we'll use instead of useState.) If you have a constructor, you have to do the super(props) to make sure that the props are passed up to React so React can keep track of them.
  • componentDidMount is a function that's called after the first rendering is completed. This pretty similar to a useEffect call that only calls the first time. This is typically where you want to do data fetching. It doesn't have to be async; we just made it async here to make the data fetching easy.
  • Notice instead of getting props via parameters and state via useState we're getting it from the instance variables this.state and this.props. This is how it works with class components. Neither one will you mutate directly.
    • this.state is the mutable state of the component (like useState). You'll use this.setState to mutate it (don't modify it directly.)
    • this.props comes from the parent component, similar to parameter given to the render functions that we pull props out of.
  • withRouter() is called a higher order component and is a bit of an advance concept. Basically we're composing functionality into our component via react-router. Think of useParams: it mixes in functionality from react-router by calling a hook. This is how you get that custom hook behavior of mixing in library functionality with class components. Redux does this too, but otherwise it's not overly common.

Other lifecycle methods

This class doesn't cover all the lifecycle methods but you can imagine having different timings for different capabilities of a component can be useful. For example, if you have a set of props that come in and you need to filter those props before you display them, you can use getDerivedStateFromProps. Or if you need to react to your component being removed from the DOM (like if you're subscribing to an API and you need to dispose of the subscription) you can use componentWillUnmount.

There are lots more you can check out in the React docs here.