What’s the purpose of if (typeof window !== ‘undefined’) in Nextjs?

The if (typeof window !== 'undefined') check is a common pattern in JavaScript, especially in the context of web development with libraries like React and Next.js. This condition is used to determine whether the code is running in a browser environment or on the server.

Here’s an explanation of how it works:

1. typeof window: The typeof operator in JavaScript is used to check the type of a variable. In this case, it checks the type of the window object.
2. 'undefined': The comparison checks if the type of window is not equal to the string literal 'undefined'. In JavaScript, the value 'undefined' represents an uninitialized or non-existent variable.

Putting it together, typeof window !== 'undefined' checks if the window object is defined. The window object is specific to browser environments and represents the global window containing the DOM (Document Object Model) and other browser-related features.

The reason for using this check is to differentiate between code execution on the server and the client (browser). During server-side rendering in frameworks like Next.js, certain objects like window may not be defined, as the code is executed on the server. By wrapping code inside this condition, you ensure that it is only executed on the client side where the window object exists.

In the specific context of your code:

javascript

if (typeof window !== 'undefined')
 { // Your client-side code here... } 

It ensures that the fetchData logic, which involves accessing the window object and client-specific functionality, is only executed on the client side and not during server-side rendering. This helps prevent errors related to window being undefined on the server side.