Скачать или смотреть Resolving the Uncaught TypeError when Using classList.add in JavaScript

Learn how to fix the `Uncaught TypeError` when trying to use `classList.add` on an element selected with `getElementsByClassName` in JavaScript.
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This video is based on the question https://stackoverflow.com/q/62365552/ asked by the user 'ned' ( https://stackoverflow.com/u/12355433/ ) and on the answer https://stackoverflow.com/a/62365572/ provided by the user 'Majed Badawi' ( https://stackoverflow.com/u/7486313/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the Uncaught TypeError when Using classList.add in JavaScript

Have you ever encountered the annoying Uncaught TypeError: Cannot read property 'add' of undefined error while trying to manipulate classes in JavaScript? If so, you’re not alone! This is a common issue many developers face when using the getElementsByClassName method. In this guide, we'll explain the problem and provide a clear solution to get you back on track in no time.

Understanding the Problem

When attempting to add a class to an element in the DOM using the classList.add method, you might expect it to work seamlessly. However, if you're using getElementsByClassName, you may run into a snag. Here's a quick overview of the situation based on the provided code:

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

The key issue here is that getElementsByClassName returns a live HTMLCollection, which is similar to an array. This means you're getting a collection of elements, not a single element, and hence cannot directly invoke classList.add on it.

Error Explanation

The specific error, Cannot read property 'add' of undefined, occurs because when you try to call classList.add on box, the variable box is not a single DOM element – it's a collection (even if it contains only one element). Therefore, JavaScript is unable to find the classList property, resulting in an "undefined" error.

A Simple Solution

To resolve this issue, you simply need to specify which element from the collection you want to manipulate. Here's how to do it:

Step 1: Access the First Element

You can do this by adding an index to the getElementsByClassName call. In most cases, you'll want the first element (which is at index 0) like this:

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

Step 2: Use classList.add

Now that you have a single DOM element, you can safely use classList.add to add a new class as follows:

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

Complete Code Example

To illustrate the full solution, here’s a snippet that includes HTML, CSS, and the fixed JavaScript code:

HTML

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

CSS

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

JavaScript

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

Final Thoughts

Now you should be able to confidently use classList.add without encountering the Uncaught TypeError. By simply accessing the first element of the collection returned by getElementsByClassName, you ensure that you have a valid reference to a single DOM element.

If you ever face similar issues, remember: always check whether you're working with an individual element or a collection. Happy coding!