References

There will always be scenarios where you need a direct reference to the DOM-Element or Component that was rendered by Preact. Refs allow you to do just that.

A typical use case for it is measuring the actual size of a DOM node. While it's possible to get the reference to the component instance via ref we don't generally recommend it. It will create a hard coupling between a parent and a child which breaks the composability of the component model. In most cases it's more natural to just pass the callback as a prop instead of trying to call the method of a class component directly.

createRef

The createRef function will return a plain object with just one property: current. Whenever the render method is called, Preact will assign the DOM node or component to current.

class Foo extends Component {
  ref = createRef();

  componentDidMount() {
    console.log(this.ref.current);
    // Logs: [HTMLDivElement]
  }

  render() {
    return <div ref={this.ref}>foo</div>
  }
}

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Callback Refs

Another way to get the reference to an element can be done by passing a function callback. It's a little more to type, but it works in a similar fashion as createRef.

class Foo extends Component {
  ref = null;
  setRef = (dom) => this.ref = dom;

  componentDidMount() {
    console.log(this.ref);
    // Logs: [HTMLDivElement]
  }

  render() {
    return <div ref={this.setRef}>foo</div>
  }
}

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If the ref callback is defined as an inline function it will be called twice. Once with null and then with the actual reference. This is a common error and the createRef API makes this a little easier by forcing user to check if ref.current is defined.

Putting it all together

Let's say we have a scenario where we need to get the reference to a DOM node to measure its width and height. We have a simple component where we need to replace the placeholder values with the actual measured ones.

class Foo extends Component {
  // We want to use the real width from the DOM node here
  state = {
    width: 0,
    height: 0,
  };

  render(_, { width, height }) {
    return <div>Width: {width}, Height: {height}</div>;
  }
}

Measurement only makes sense once the render method has been called and the component is mounted into the DOM. Before that the DOM node won't exist and there wouldn't make much sense to try to measure it.

class Foo extends Component {
  state = {
    width: 0,
    height: 0,
  };

  ref = createRef();

  componentDidMount() {
    // For safety: Check if a ref was supplied
    if (this.ref.current) {
      const dimensions = this.ref.current.getBoundingClientRect();
      this.setState({
        width: dimensions.width,
        height: dimensions.height,
      });
    }
  }

  render(_, { width, height }) {
    return (
      <div ref={this.ref}>
        Width: {width}, Height: {height}
      </div>
    );
  }
}

Run in REPL

That's it! Now the component will always display the width and height when it's mounted.