Fixing the Uncaught ReferenceError in Svelte: How to Reference Global Variables Inside onMount

Learn how to resolve the `Uncaught ReferenceError` when trying to access a variable defined inside the `onMount` lifecycle function in Svelte. This guide will help you correctly scope your variables.
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Fixing the Uncaught ReferenceError in Svelte: How to Reference Global Variables Inside onMount

When working with Svelte, you may encounter common scope-related errors, particularly when trying to access variables inside lifecycle methods like onMount. In this article, we will tackle a specific issue—getting an Uncaught ReferenceError when referencing a global variable defined inside onMount. We’ll break down the problem, explain why it happens, and present a simple solution.

Understanding the Problem

You might have initialized an object within the onMount function, like this:

[[See Video to Reveal this Text or Code Snippet]]

However, if you then try to access mapy outside of onMount:

[[See Video to Reveal this Text or Code Snippet]]

You will likely encounter an error in the console stating:

[[See Video to Reveal this Text or Code Snippet]]

Why Does This Error Happen?

The issue arises because of scope. When you declare a variable inside a function (like onMount), it is only available within that function. This means that the mapy variable is local to the onMount function and cannot be accessed outside of it.

The Solution

To fix this error, you need to declare your variable at the top level of your script. This makes it accessible throughout your module, including inside the onMount lifecycle function. Here’s how to properly structure your code:

Step 1: Declare the Variable at the Top Level

You should declare mapy before the onMount function:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Accessing the Variable

Now you can safely reference mapy in your AddPoint function:

[[See Video to Reveal this Text or Code Snippet]]

Explained Changes

By declaring let mapy; at the top level, mapy now retains its value throughout the lifecycle of your Svelte component.

The optional chaining (mapy ?) ensures that if mapy is not yet initialized for any reason, your code won’t throw an error, but will instead set Center to undefined.

Conclusion

Svelte's reactivity model can sometimes lead to confusion regarding variable scope, especially when using lifecycle methods. By declaring your variables at the top level, you can avoid common pitfalls like the Uncaught ReferenceError. Now you should be able to access your mapy object freely within your component's functions, making your Svelte application more robust and functional.

Feel free to experiment with this solution and adapt it to your specific use case, ensuring your Svelte applications run smoothly without scope-related issues!