How to fix the "window is not defined" error in SvelteKit

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Justin Ahinon

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While working with SvelteKit, you may encounter the "window is not defined" error. This issue arises when attempting to access the window object in a script tag or importing a function that utilizes the window object.

This post explores the reasons behind this error and how to address it.

tl; dr : Utilize SvelteKit's browser browser module to check if you are in the browser or the server. Alternatively, use Svelte's onMount onMount lifecycle function to run code involving the window object.

Why do we encounter the "window is not defined" error?

In SvelteKit, components are initially rendered server-side by default. Once this server-side rendering is done, the components are hydrated in the browser. During the component's lifecycle, there will be instances when certain browser-specific objects, such as window , are not accessible.

For example, when attempting to access the window object within a script tag:

code loading...

Executing this code results in an internal server error being displayed in the browser, while the Node.js console shows:

code loading...

Resolving the "window is not defined" error

Depending on your specific use case, there are several ways to address this error.

Utilize the SvelteKit browser module

SvelteKit offers a built-in module called browser that can be used to determine whether your code is executing in a browser or server environment. You can use it to resolve the "window is not defined" error as follows:

code loading...

In this example, we only execute the code that utilizes the window object when we are in a browser environment.

Employ the onMount lifecycle function

Another approach to resolving the "window is not defined" error is to use Svelte's onMount lifecycle function. This function is called when a component is added to the DOM, making it ideal for running code that depends on the window object.

code loading...

By using these methods, you can successfully overcome the "window is not defined" error in SvelteKit.

Wrapping Up

In summary, the "window is not defined" error in SvelteKit can be effectively resolved by using either the SvelteKit browser module or Svelte's onMount lifecycle function. These techniques ensure that your code only accesses the window object when running in a browser environment, avoiding errors and providing a smooth development experience.

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