How to detect if screen size has changed to mobile in React?

What I did is adding an event listener after component mount:

componentDidMount() {
    window.addEventListener("resize", this.resize.bind(this));
    this.resize();
}

resize() {
    this.setState({hideNav: window.innerWidth <= 760});
}

componentWillUnmount() {
    window.removeEventListener("resize", this.resize.bind(this));
}

EDIT:
To save state updates, I changed the “resize” a bit, just to be updated only when there is a change in the window width.

resize() {
    let currentHideNav = (window.innerWidth <= 760);
    if (currentHideNav !== this.state.hideNav) {
        this.setState({hideNav: currentHideNav});
    }
}

UPDATE: Time to use hooks!
If you’re component is functional, and you use hooks – then you can use the useMediaQuery hook, from react-responsive package.

import { useMediaQuery } from 'react-responsive';

...

const isMobile = useMediaQuery({ query: `(max-width: 760px)` });

After using this hook, “isMobile” will be update upon screen resize, and will re-render the component. Much nicer!