Cannot read property 'query' of undefined in Next.js: Causes and How to Fix

Understanding 'Cannot read property 'query' of undefined'

• The error "'Cannot read property 'query' of undefined'" in Next.js typically occurs when developers attempt to access the query object from the Next.js router or the context object before it is properly defined. This often happens when components or functions expect the router to be initialized but, due to asynchronous loading or lifecycle transitions, it's not.

 

• In Next.js, the query object is populated only during specific phases of the app's lifecycle, usually after client-side navigation or during pre-rendering on the server. Attempting to access the query property prematurely can lead to this error if the object is not yet initialized.

 

• Another common cause is attempting to access the query object in a component when it is rendered on the server with server-side rendering (SSR) without passing correct context or props that contain the query data.

 

• The error might also occur when there’s a mismatch in the environment. For instance, during SSR, developers might erroneously attempt to use useRouter or other client-side hooks which aren't available or initialized during server-side execution.

 

• If you use dynamic routing in Next.js, and you fail to handle routes properly, accessing the dynamic portions of a URL via the query object might lead to undefined paths or properties that aren't correctly specified within your component or page logic.

 

import { useRouter } from 'next/router';

function MyComponent() {
  const router = useRouter();

  // Potential cause of error if router is not initialized
  const { id } = router.query;
  
  console.log(id);

  return <div>Item ID: {id}</div>;
}

export default MyComponent;

Possible Scenarios in Code

• If the above code runs client-side but is accessed on the server due to an incorrect rendering strategy, 'router.query' can be undefined. Next.js's router is a client-side only utility, and its initialization happens after the component mounts.

 

• Another case is misunderstanding the asynchronous nature of data fetching in Next.js. When using functions like getServerSideProps or getStaticProps, developers might wrongly assume query is available immediately in client-side code.