If you are trying to access "window" object from HTML DOM in your Next.js app and have this error message:

Unhandled Rejection (ReferenceError): window is not defined

Just keep you code as the example:

componentDidMount() {
   console.log('window.innerHeight', window.innerHeight);
}

Another solution to just execute your code during rendering on the client side only, is:

if (typeof window !== "undefined") {
   var width = window.innerWidth;     
}

I hope you enjoy it!

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