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Learn why you might encounter an `Uncaught ReferenceError` in JavaScript and how to effectively solve it, especially when using Webpacker with Rails.
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Understanding Uncaught ReferenceError in JavaScript

If you have ever stumbled upon an Uncaught ReferenceError message in JavaScript, then you know how daunting it can be. It indicates that you're trying to use a piece of code (like a function or variable) that hasn't been defined. One common scenario where this problem might occur is when you're working with Ruby on Rails applications that have recently been migrated to Rails 6 and are now using Webpacker for JavaScript management. In this guide, we will explore the steps you can take to pinpoint the issue and efficiently solve it.

The Problem at Hand

Let's look at the error you might encounter:

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

This indicates that the JavaScript function editMap cannot be found when the document is being loaded. Here’s a closer look at the relevant JavaScript code that you might have in your Rails application:

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

In your application's script files (e.g., application.js), you may have defined the editMap function as follows:

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

This setup likely worked perfectly before the migration, but with Webpacker, you might be facing a scope-related issue.

The Solution

Understanding Webpack's Module System

Webpack treats each file as a module. Consequently, any variables or functions declared within these files are not automatically available in the global scope. This modularity helps avoid global variable collisions but can cause issues like the one you're experiencing.

Making Your Function Globally Accessible

To fix the Uncaught ReferenceError, you need to expose your function to the global scope. You can achieve this by assigning your function to the window object. Here’s how you can do it:

Modify your function declaration in your application.js file (or wherever your function is defined):

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

Check your imports: If you've split your JavaScript codes into multiple files using Webpacker, ensure that the file where editMap is declared is being imported correctly.

Final Touches and Considerations

Testing: After you make these changes, refresh your web application and check the console for any remaining errors.

Documentation: Be sure to document any significant changes you make to ensure your future self (or other developers) understands the rationale.

Conclusion

Migrating your Rails application to a new version and using Webpacker can initially seem overwhelming, especially with unexpected errors like Uncaught ReferenceError. Understanding how Webpack manages JavaScript scopes and explicitly assigning functions to the window object can prevent these issues. By applying the strategies outlined in this guide, you can smoothly transition your code and avoid headaches down the line.

Feel free to share your experiences or ask any further questions in the comments below!