conditional rendering of component in Next js - reactjs

Question

I used to render a component depending on the screen width by writing something like this.
function App(props) {
const [mobile, setMobile] = useState(() => window.innerWidth < 576 ? true : false)
return (
<div>
{
mobile ? <ComponentA /> : <ComponentB />
}
</div >
);
}
But now that I'm using Next.js this gives me several errors due to the window.innerWidth reference.
How could I do this?
Thanks in advance.

Answer 1

You are getting a reference error because you cannot access the window object in useState. Instead, you have to set the initial value in useState to undefined or null and use useEffect where window can be referenced to call setMobile(window.innerWidth < 576 ? true : false). finally, in your render method, you can check whether mobile state is set using setMobile (i.e., not undefined or null) and use the defined mobile state value (either true or false) to conditionally render your ComponentA or ComponentB. Also, you need to add window.addEventListener('resize', handleResize) when your App component is mounted and remove it when it is unmounted, which you also do in useEffect since that is where you get reference to window. Otherwise, resizing the browser will not trigger an update to mobile state. Here is a working example:
import React, { useState, useEffect } from 'react'
function App() {
const [mobile, setMobile] = useState(undefined)
useEffect(() => {
const updateMobile = () => {
setMobile(window.innerWidth < 576 ? true : false)
}
updateMobile()
window.addEventListener('resize', updateMobile)
return () => {
window.removeEventListener('resize', updateMobile)
}
}, [])
return typeof mobile !== 'undefined' ? (
mobile ? (
<ComponentA />
) : (
<ComponentB />
)
) : null
}

Answer 2

Assuming you're seeing something along the lines of ReferenceError: window is not defined:
ReferenceError is thrown when a non-existent variable is referenced.
This is occurring because, in NextJS, components are often initially rendered server-side, using NodeJS, before being handed over for clients to consume. Additionally, in NodeJS, there is no such thing as window — hence, window is not defined.
Fortunately, typeof can be used in such cases to safely check variables before attempting to use them (see this SO answer for additional info).
See below for a practical example.
const [mobile, setMobile] = useState(() => {
if (typeof window === 'undefined') return false
return window.innerWidth < 576
})