1-window-hook.ts

const useWindowSize = () => {
    // This doesn't handle resizes or SSR, only used for demos
    const [height, setHeight] = useState(window.innerHeight);
    const [width, setWidth] = useState(window.innerWidth);
  
    return {height, width};
}

2-window-hook-invalid-name.ts

// ❌ Not allowed, the function name must start with `use`
const getWindowSize = () => {
    const [height, setHeight] = useState(window.innerHeight);
    const [width, setWidth] = useState(window.innerWidth);
  
    return {height, width};
}

3-window-hook-invalid-root.ts

const useWindowSize = () => {
		const [height, setHeight] = useState(window.innerHeight);
  	const [width, setWidth] = useState(window.innerWidth);
  
    return {height, width};
}

// ❌ Not allowed, you must use a hook _inside_
//                                a component or another hook
const {height, width} = useWindowSize();

const Component = () => {
  return <p>Height is: {height}</p>
}

4-window-hook-invalid-conditional.ts

const useWindowSize = () => {
	// ❌ Not allowed, you cannot `return` before using a hook
  	if (bool) return {height: 0, width: 0}
    const [height, setHeight] = useState(window.innerHeight);

    let widthProps;
    // ❌ Also not allowed, you can't use hooks inside of conditionals
    if (bool) {
      widthProps = useState(window.innerWidth);      
    }
  
    return {height, widthProps};
}