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jQuery Not Defined: Load Order, Defer, and Compatibility

Jonathan Dough

Web developers frequently encounter a frustrating error that can bring a halt to JavaScript-enhanced user interfaces: “jQuery is not defined.” This error, while simple in appearance, can stem from a range of issues that typically revolve around incorrect load order, improper use of the defer attribute, or compatibility issues. Understanding these root causes and how to resolve them is essential for ensuring seamless user experiences and maintaining functional web applications.

TL;DR: The “jQuery is not defined” error usually occurs when scripts are loaded in the wrong order — a script relying on jQuery is run before jQuery is actually loaded. The use of the defer attribute can prevent this problem when used correctly. Additionally, compatibility issues such as loading jQuery from improperly configured CDNs or mixing different libraries can cause this error. Judicious debugging and script management strategies can help eliminate this common JavaScript error.

Understanding the Error: “jQuery is not defined”

When the browser attempts to execute a script that uses jQuery before the jQuery library itself has fully loaded, it throws the "jQuery is not defined" error. This error halts script execution and leads to non-functional interactive components of the website, such as sliders, modals, or AJAX requests.

At a technical level, the error means the browser does not recognize what jQuery is at the time a script tries to use it — it simply hasn’t seen a definition for it yet.

Common Causes of the Error

Let’s explore the most frequent reasons this error might appear:

Resolving the Load Order Issue

Load order is the most common culprit behind this error. You must ensure that the jQuery library is loaded before any scripts that use it. Here’s how a correct script order should look:

<script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
<script src="/js/my-script.js"></script>

In this example, my-script.js depends on jQuery and will only be evaluated by the browser after jQuery has been loaded.

Using defer and async Correctly

To prevent render-blocking, developers often use async or defer attributes on <script> tags. However, these attributes control how the scripts load and can inadvertently cause dependencies to break.

To ensure jQuery loads first, use defer consistently and preserve the order:

<script src="https://code.jquery.com/jquery-3.6.0.min.js" defer></script>
<script src="/js/my-script.js" defer></script>

Do not mix async and defer unless you understand their implications. Mixing may result in unpredictable execution order.

Checking for CDN and File Path Issues

A simple mistake in the URL, a missing file on your server, or a CDN outage can cause the script to fail entirely. If jQuery is hosted externally, failure in loading the file means jQuery will not be defined.

To check this:

  1. Open your browser’s Developer Tools.
  2. Go to the Network tab and reload the page.
  3. Look for the jQuery script request and verify its status is 200 (OK).

If the file returns 404 or any other error, you’ll need to fix the path or choose a different CDN.

Handling Library Conflicts and Compatibility

If you’re using another JavaScript library that also uses the $ sign (such as Prototype or MooTools), jQuery’s shortcut alias might be overwritten. You can mitigate this with jQuery.noConflict().

var $j = jQuery.noConflict();
// Now use $j instead of $ throughout your code
$j(document).ready(function() {
    $j("#example").text("jQuery works without conflicts!");
});

Additionally, ensure all libraries are compatible with each other and that jQuery is not included multiple times with different versions, which can also result in undefined behavior.

Best Practices to Avoid “jQuery is Not Defined”

To summarize, here are some key best practices:

Advanced Debugging Tips

If the error persists, try the following:

document.addEventListener("DOMContentLoaded", function() {
    // jQuery usage here
});

This ensures your scripts execute only after HTML and scripts are fully parsed.

Conclusion

The “jQuery is not defined” error, while common, is straightforward to debug once you understand JavaScript loading behavior. Correctly ordering your scripts, judiciously using loading attributes, and checking for compatibility issues can save hours of frustration. With best practices in mind, your sites can leverage jQuery’s power without runtime hiccups.

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