Why is My jQuery '$ is not defined'?

JQuery: The Elusive $ is not defined

The error message "$ is not defined" is frequently encountered by programmers and indicates that jQuery is not properly integrated into the web page. This error can stem from three fundamental causes:

1. Improper Script Loading: Ensure that your jQuery script file is correctly linked and loaded on the page. Verify that the script tag has the proper 'src' attribute pointing to the correct jQuery file and that it does not have any 'async' or 'defer' attributes, which can hinder the loading process.
2. Botched jQuery Version: Inspect the jQuery file to identify any possible tampering or errors. Ensure that the file you are using is an official release and has not been altered or overwritten by plugins or external code.
3. Premature JavaScript Execution: JavaScript code should be executed after the page has fully loaded to prevent errors caused by premature execution. Place your jQuery code within a '$(document).ready' block or its equivalent to ensure that it runs after jQuery is initialized. Avoid running jQuery code before the page has completed loading.

To troubleshoot these issues, perform the following steps:

• Script Verification: Double-check that the script is being properly referenced with the correct URL and attributes.
• Firebug Net Panel: Utilize Firebug's net panel to verify that the jQuery file is successfully loaded. A red highlight with "404" indicates an issue with file loading.
• Functional Code Block: Wrap your jQuery code within an appropriate code block, such as "$(document).ready," to guarantee its execution after jQuery initialization.
• Plugin Loading Order: Ensure that jQuery is loaded before any plugins that may extend its "$" object. Preloading plugins can cause conflicts and errors.
• Selective Code Placement: Not all code requires jQuery functionality. Move code that can operate independently of jQuery outside the '$(document).ready' block to improve performance and avoid unnecessary execution.

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