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Next.js: run code only on the server side or client side in Next.js

The Valley of Code

Your Web Development Manual

How to write code that's only executed on one side of your stack: frontend or backend

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.

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