expanded on No Router Instance

errors/no-router-instance.md

@@ -2,8 +2,13 @@
 
 #### Why This Error Occurred
 
+Next.js is universal, which means it executes code first server-side with NodeJS, then on the client-side, `window` object is not defined in NodeJS, so some methods are not supported or availale at build time.
 During SSR you might have tried to access a router method `push`, `replace`, `back`, which is not supported.
 
 #### Possible Ways to Fix It
 
-Move any calls to router methods to `componentDidMount` or add a check such as `typeof window !== 'undefined'` before calling the methods
+In a class Component, move any calls to router methods to `componentDidMount` lifecycle method or add a check such as `typeof window !== 'undefined'` before calling the methods
+
+In a functional Component you can move the code into the `useEffect` hook. checking `typeof window !== 'undefined'` also works in a functional component.
+
+This way the calls to the router methods are only executed on the client (in the browser), thus ensuring access to the `window` object.