DOMTokenList contains Method

The DOMTokenList represents a set of space-separated tokens, which can be used to manage classes (and other string lists) within the Document Object Model (DOM). It’s often seen when working with CSS classes through properties like classList on DOM elements. One of the most important methods available on DOMTokenList is the contains method, which allows developers to check if a specific token exists.

I. Introduction

A. Overview of DOMTokenList

The DOMTokenList interface is a useful utility for managing lists of space-separated tokens such as CSS class names. This interface provides several methods to manipulate these tokens efficiently without dealing with string parsing manually. It is particularly beneficial in modern web development for managing the classes of HTML elements dynamically.

B. Importance of the contains method in DOM manipulation

The contains method is significant for checking the presence of a specific token within a DOMTokenList. This method provides a straightforward way to determine if an element has a particular class, allowing developers to make decisions based on that information.

II. Definition

A. Explanation of the contains method

The contains method checks whether a certain token (e.g., a class name) is present within the DOMTokenList. It takes a single parameter and returns a boolean value based on its presence.

B. Purpose and functionality

The primary purpose of the contains method is to verify if a specific class is applied to an element, which is essential for conditional styling and DOM manipulation.

III. Syntax

A. Description of the method syntax

element.classList.contains(token);

B. Parameters required

Parameter	Description
token	A string representing the class name to be checked.

IV. Return Value

A. Explanation of the return value

The contains method returns a boolean value: true if the token is found within the DOMTokenList, otherwise it returns false.

B. Conditions for true or false returns

The return value will be true under the following conditions:

The token provided matches an entry within the DOMTokenList.

Conversely, it will return false if:

The token does not exist in the list.

V. Examples

A. Basic example of using the contains method

Here’s a simple example illustrating the use of the contains method:


<div id="myDiv" class="box active">
    This is a box.
</div>

<script>
    const myDiv = document.getElementById('myDiv');
    if (myDiv.classList.contains('active')) {
        console.log('The div has the active class.');
    } else {
        console.log('The div does not have the active class.');
    }
</script>

B. Use cases in real-world scenarios

The contains method is often employed in responsive design. For example, a developer can check if an element has a specific class that indicates its visibility state and take actions accordingly.


<button id="toggleBtn">Toggle Visibility</button>
<div id="content" class="hidden">This is content that can be toggled.</div>

<script>
    const button = document.getElementById('toggleBtn');
    button.addEventListener('click', function() {
        const content = document.getElementById('content');
        if (content.classList.contains('hidden')) {
            content.classList.remove('hidden');
            console.log('Content is now visible.');
        } else {
            content.classList.add('hidden');
            console.log('Content is now hidden.');
        }
    });
</script>

VI. Browser Compatibility

A. Overview of browser support

The contains method is widely supported across all modern browsers including:

Chrome
Firefox
Safari
Edge

As such, developers can confidently use this functionality in production environments without worrying about compatibility issues.

B. Importance for developers

Understanding browser compatibility is crucial as it allows developers to leverage the full potential of web APIs like DOMTokenList and ensure that their applications function correctly across multiple platforms.

VII. Conclusion

A. Summary of key points

The contains method of the DOMTokenList interface is an essential tool for developers to manage class-related checks efficiently. Understanding its syntax, usage, and return values can significantly enhance a developer’s ability to manipulate classes within the DOM.

B. Final thoughts on using the contains method in web development

Incorporating the contains method into your web development toolkit can lead to cleaner and more efficient code, making it easier to implement dynamic styling and conditional logic in your applications.

Frequently Asked Questions (FAQ)

1. What is the difference between contains and classList’s other methods?

The contains method is specifically for checking the existence of a token, while methods like add or remove are intended to modify the token list.

2. Can I use contains for other types of tokens besides class names?

Yes, the contains method works for any DOMTokenList, which could contain various types of tokens, not just class names.

3. What happens if I pass an empty string to the contains method?

Passing an empty string to the contains method will always return false, as it checks for specific tokens.

4. Is there a performance impact of using contains in loops?

While the contains method is efficient, particularly when used in the context of DOMTokenList, using it within tight loops can affect performance. It’s best to minimize its use in performance-critical code.

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