Observer is used to replace useObserver and can work with function components. Compare these two code snippets:

const BugsHeader = observer(() => {
    const store = React.useContext(StoreContext);
    return (<h1>{store.bugsCount} Bugs!</h1>)
});

const BugsHeader = () => {
    const store = React.useContext(StoreContext);
    return useObserver(()=> <h1>{store.bugsCount} Bugs!</h1>);
}

At first used the second code block and it worked. Then I replaced the “userObserver” with “Observer” and it throwed error. It takes me a while to find out how to fix it by using the first code block.