How can I attach to a stateless component's ref in React?

You can useuseRef hook which is available since v16.7.0-alpha.

EDIT: You're encouraged to use Hooks in production as of 16.8.0 release!

Hooks enable you to maintain state and handle side effects in functional components.

function TextInputWithFocusButton() {
  const inputEl = useRef(null);
  const onButtonClick = () => {
    // `current` points to the mounted text input element
    inputEl.current.focus();
  };
  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

Read more in Hooks API documentation

EDIT: You now can with React Hooks. See the answer by Ante Gulin.

You can't access React like methods (like componentDidMount, componentWillReceiveProps, etc) on stateless components, including refs. Checkout this discussion on GH for the full convo.

The idea of stateless is that there isn't an instance created for it (state). As such, you can't attach a ref, since there's no state to attach the ref to.

Your best bet would be to pass in a callback for when the component changes and then assign that text to the parent's state.

Or, you can forego the stateless component altogether and use an normal class component.

From the docs...

You may not use the ref attribute on functional components because they don't have instances. You can, however, use the ref attribute inside the render function of a functional component.

function CustomTextInput(props) {
  // textInput must be declared here so the ref callback can refer to it
  let textInput = null;

  function handleClick() {
    textInput.focus();
  }

  return (
    <div>
      <input
        type="text"
        ref={(input) => { textInput = input; }} />
      <input
        type="button"
        value="Focus the text input"
        onClick={handleClick}
      />
    </div>
  );  
}

This is late but I found this solution much better. Pay attention to how it uses useRef & how properties are available under current property.

function CustomTextInput(props) {
  // textInput must be declared here so the ref can refer to it
  const textInput = useRef(null);

  function handleClick() {
    textInput.current.focus();
  }

  return (
    <div>
      <input
        type="text"
        ref={textInput} />
      <input
        type="button"
        value="Focus the text input"
        onClick={handleClick}
      />
    </div>
  );
}

For more reference check react docs