In your page components, you can execute code only in the server-side or on the client-side, but checking the window property.

This property is only existing inside the browser, so you can check

if (typeof window === 'undefined') {
}

and add the server-side code in that block.

Similarly, you can execute client-side code only by checking

if (typeof window !== 'undefined') {
}

JS Tip: We use the typeof operator here because we can’t detect a value to be undefined in other ways. We can’t do if (window === undefined) because we’d get a “window is not defined” runtime error

Next.js, as a build-time optimization, also removes the code that uses those checks from bundles. A client-side bundle will not include the content wrapped into a if (typeof window === 'undefined') {} block.

Lessons in this unit:

0:	Introduction
1:	How to analyze the Next.js app bundles
2:	The Next.js App Bundles
3:	Lazy loading modules in Next.js
4:	How to run a script build time in Next.js
5:	▶︎ Next.js: run code only on the server side or client side in Next.js
6:	How to cache data in Next.js globally across all pages at build time