You have to use both.
componentDidMount()
componentDidMount() is invoked immediately after a component is mounted. Initialization that requires DOM nodes should go here. If you need to load data from a remote endpoint, this is a good place to instantiate the network request. Setting state in this method will trigger a re-rendering.
componentDidUpdate()
componentDidUpdate() is invoked immediately after updating occurs. This method is not called for the initial render.
You also could place it into the render() method which seems like it’s appropriate for your case since you always want to check the focus. Then you don’t need to put it into componentDidMount() and componentDidUpdate()