SharePoint migration can sometimes cause issues with custom JavaScript files due to various reasons:

1. Compatibility: SharePoint 2019 might have some differences or updates compared to earlier versions, which could affect how JavaScript files interact with the platform.
2. Migration Process: During the migration process, customizations and scripts may not migrate correctly or might need adjustments to work seamlessly with the new environment.
3. Security Changes: SharePoint updates and migrations often come with security improvements, and some older JavaScript code might not meet the new security requirements.

To troubleshoot JavaScript issues after migration, consider the following steps:

1. Review JavaScript Code: Check the JavaScript code for any errors or outdated syntax that might cause it not to work in SharePoint 2019.
2. Check Browser Compatibility: Ensure that the custom JavaScript is compatible with the browsers used in SharePoint 2019.
3. Debugging: Use browser developer tools to debug and identify any errors or issues with the JavaScript code.
4. Verify URL Paths: If the JavaScript references any specific URLs, ensure that the paths are correctly updated for the SharePoint 2019 environment.
5. Review Console Logs: Check the browser console logs for any error messages related to the JavaScript code.
6. Consult Migration Documentation: If you used a migration tool or service, refer to their documentation or support resources for guidance on handling custom JavaScript during the migration.