Unofficial Preact FAQ

This is a list of common questions we frequently encounter in our slack channel.

How do I upgrade to Preact X?

Check out this guide.

I'm seeing weird $$typeof properties on all objects with Preact X

This happens because the internal shape for vnodes (the data structure we use to compare components) has changed with Preact X. It is not a class anymore. The $$typeof-property is set by preact-compat which sets it on the prototype of a Preact 8 vnode. Uninstall it via npm uninstall preact-compat and switch to preact/compat which is included in the preact npm package.

How do you work with async data in Preact?

Call the async function via useEffect and store the result inside the component's state. That way the component will automatically rerender when the async request is resolved.

function Foo() {
  const [data, setData] = useState(null);

  useEffect(() => {
    fetch("/my-api")
      .then(res => res.json())
      .then(res => setData(res));
  }, []);

  return data === null ? "not loaded" : data;
}