Why Is My jQuery Code Failing with the '$ is not defined' Error?

JQuery Mystery: $ is Not Defined

Encountering the infamous "$ is not defined" error in jQuery can be a perplexing issue. This article explores the root causes and provides practical solutions to resolve it effectively.

The Problem:

A simple jQuery click event fails to execute, returning the dreaded "$. is not defined" error despite the alleged inclusion of a jQuery reference in the site master. The script is confirmed to be resolving correctly, but the issue persists.

The Causes:

The error typically stems from one of three scenarios:

1. The JavaScript file is not loading into the page.
2. A corrupted version of jQuery exists, overwriting the $ variable.
3. JavaScript is running before the page and jQuery fully load.

The Solution:

1. Ensure Script Loading:

Verify the script is called correctly. It should resemble the following:

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js" type="text/javascript"></script>

Ensure it does not have the async or defer attributes.

2. Check Script Loading in Firebug:

In the Firebug net panel, examine if the script is loaded successfully. If highlighted red with a "404" error, the file is not loading.

3. Encapsulate Code:

Wrap all jQuery code within a block such as:

$(document).ready(function () {
  //your code here
});

This ensures the code executes only after jQuery initialization.

4. Avoid Premature Plugin Loading:

Ensure no plugins are loaded before jQuery core. Plugins extend "$", and prematurely loading them overwrites jQuery internals.

5. Separate Non-jQuery Code:

Non-jQuery code can be separated from jQuery-dependent code using document.readyState. This optimizes performance and prevents early jQuery execution errors.

Remember, thorough investigation and adherence to these solutions can effectively resolve the "$ is not defined" issue, allowing you to harness the power of jQuery seamlessly.

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