React component with typed Refs

index.flow.jsx

// @flow

import React from "react";

type Refs = {
  // the ref `foo` is a button (or null, because it might not be mounted).
  foo: HTMLButtonElement | null
};

class Foo extends React.Component {
  // Annotating refs as an object of Refs
  refs: Refs;

  onClickHandler() {
    // `foo` is a HTMLButtonElement.
    if (this.refs.foo) {
      console.log(this.refs.foo.innerHTML)
    }
  }

  render() {
    return <button ref="foo" onClick={this.onClickHandler}>Click</button>
  }
}