React: Not Everything is State

When working with React we get introduced to state as a way if storing component's data. There is another way that generally gets overlooked: class properties.

Example

class AnnoyingButton extends React.Component {
  intervalId = null; // Here!
  id = generateUniqueId(); // Here!
  state = {
    isDisabled: false,
  };
  componentDidMount() {
    this.intervalId = setInterval(() => {
      this.setState(({ isDisabled }) => ({
        isDisabled: !isDisabled,
      }));
    }, 100);
  }
  componentWillUnmount() {
    console.log(`Unmounting ID: ${this.id}`);
    clearInterval(this.intervalId);
  }
  render() {
    return <button disabled={this.state.isDisabled} />;
  }
}

Nothing special about the component; the interesting bits are the id and intervalId variables. We need some way of preventing memory leaks when the component unmounts. In componentDidMount we save the interval's ID to a class property. Then, use the property to clear the interval and console.log the component's unique ID.

You might be thinking, "Why chouldn't we just store this information in the component's state?" React re-renders the component, and any child components, when the state changes. If the render method never uses a piece of the state it could be causing unwanted renders--making your application slower!

When to use class properties: If state needs to be stored, but is never used as a part of the view/render of a component.