What is Shadow DOM?

While building modern web applications, you often want to create components that are contained within themselves and don’t interfere with the rest of the page. This is where Shadow DOM is used. It provides you with a powerful way to encapsulate HTML, CSS, and JavaScript inside a web component, so that your code becomes cleaner and more modular. 

In this blog, we will discuss what Shadow DOM is, why it is used, its working, and how you can use it in your projects. So let’s get started!

Table of Contents:

• What is a DOM?
• What Is Shadow DOM?
• Creating a Shadow DOM using JavaScript
  • Step 1: Pick an HTML Element (Shadow Host)
  • Step 2: Attach a Shadow Root to the Element
  • Step 3: Add HTML and CSS Inside the Shadow Root
• Benefits of Using Shadow DOM
• Shadow DOM vs. Virtual DOM
• Potential Security Concerns with Shadow DOM
• Best Practices for Using Shadow DOM in JavaScript
• Conclusion

What is a DOM?

Before you dive into what a Shadow DOM is, you must first understand what a DOM is. A DOM (Document Object Model) is basically a programming interface that helps you to interact and manipulate a webpage with the help of JavaScript. With the help of DOM, you can change how the web page looks, the way it is structured, and the content it shows. Given below is an image that demonstrates the structure of a DOM:

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What Is Shadow DOM?

Shadow DOM is basically a feature that helps you to keep the HTML and CSS of a component separate from the remaining web page. It is a part of a bigger idea known as Web Components, which also consists of templates and custom elements. With the help of Shadow DOM, you can write styles that are only applicable to your component, without affecting anything else on the page.

Shadow DOM helps you add a hidden section to an HTML element. That hidden part is called the “Shadow DOM”, and the element that is connected to the Shadow DOM is called the “Shadow Host”. You can add more hidden sections inside the Shadow Host, which, as a result, creates a “Shadow Tree”.

From the above diagram, you can see that there are 4 important terminologies in Shadow DOM that you should know about:

• Shadow host: Basically, this is your normal HTML element to which you insert the Shadow DOM.

• Shadow tree: This is the hidden structure (like HTML tags) that is present inside the Shadow DOM.

• Shadow boundary: This is basically the invisible line where the Shadow DOM ends and the content of the normal page begins.

• Shadow root: This is the starting point or the main container of everything that is present inside the Shadow DOM.

Creating a Shadow DOM using JavaScript

When you’re building components for the web, sometimes you want to keep the styles and structure of one part of your webpage separate from everything else. The Shadow DOM API in JavaScript makes this possible. With JavaScript, you can easily create a Shadow DOM that is hidden from the rest of the page and doesn’t affect any other styles or scripts.

Now, let us talk about the steps on how you can create a Shadow DOM using JavaScript.

Step 1: Pick an HTML Element (Shadow Host)

The first thing you need is a regular HTML element. This element will act as the Shadow Host; in simple words, it’s the placeholder where your Shadow DOM will be attached.

Example:

Explanation:

In the above code, the <div> element with the ID shadow-container is just a normal HTML element. But when you attach a Shadow DOM to it, it becomes a Shadow Host.

Step 2: Attach a Shadow Root to the Element

After you have picked your HTML element, in the next step, you have to attach a Shadow Root to the HTML element using JavaScript. The Shadow Root is the starting point of your Shadow DOM. All the HTML and CSS will go to the Shadow Root.

Code:

Explanation:

The above JS code is used to select the #shadow-container element, and then it creates an open Shadow DOM inside it. This allows you to add isolated content and styles.

Step 3: Add HTML and CSS Inside the Shadow Root

After you have created the Shadow Root, HTML and CSS can be inserted inside it by using innerHTML. This content will only exist inside the Shadow DOM, and it won’t affect the rest of your page.

Example:

Output:

The above JS code is used to add custom styles and a paragraph inside the Shadow DOM. This is done by keeping the styles and the paragraph separate from the rest of the page.

Now we will combine the above 3 steps. The combined code for creating a Shadow DOM using JavaScript is given below for your reference:

Code:

Output:

Benefits of Using Shadow DOM

1. Encapsulation: Encapsulation in Shadow DOM means that the CSS and JavaScript present inside the Shadow DOM will stay separate from the rest of the page. The styles and scripts that are used to write in the Shadow DOM will only affect what is inside it; they won’t affect the components that are outside the Shadow DOM. It helps you to avoid problems where styles are accidentally affected by other components in the web page. This is called “CSS bleeding”.

2. Independence: Since the Shadow DOM is connected to a specific element, it works like its own little world. This means you can build, test, and use that part separately from the rest of your website or app. It’s really helpful when you’re working on big projects because each part can be handled on its own without getting in the way of other parts. It also helps the browser work faster by only updating the parts that actually need changes.

3. Reusability: One of the best things about using Shadow DOM is that it makes your components reusable. Since everything a component needs, its styles and behavior, is kept inside the Shadow DOM, you can use it in different places without rewriting anything. This saves you time and helps keep your app consistent because the component will always look and work the same way, no matter where you use it.

Shadow DOM vs. Virtual DOM

The difference between Shadow DOM and Virtual DOM is given below in a tabular format:

Potential Security Concerns with Shadow DOM

1. Hidden Malicious Code: Since content inside Shadow DOM is hidden from the main DOM, attackers could hide harmful scripts that are harder to detect.

2. Phishing or UI Spoofing: A malicious person could use the Shadow DOM in order to create fake buttons or forms that look real. This tricks the users into saving the sensitive info.

3. Limited Visibility in Dev Tools: By default, some Shadow DOM content may not be easy to inspect, making it harder for developers to spot security issues.

4. CSS Isolation can be misused: This is because the files present inside the Shadow DOM don’t get leaked. Hence, the attackers can cover the UI elements that are harmful in order to track users visually.

5. Script Execution Risks: If untrusted content is placed inside a Shadow DOM, it can still run scripts and pose the same risks as normal DOM elements.

Best Practices for Using Shadow DOM in JavaScript

1. To prevent CSS style conflicts with the rest of the page, write CSS in the Shadow DOM.

2. Even though the styles are not scoped properly, by using clear names, you can keep your Shadow DOM well organized.

3. Always keep your scripts and structure private so that your Shadow DOM stays private and self-contained.

4. Slots allow carrying outside content while still holding onto the Shadow DOM structure.

5. Since Shadow DOM is isolated, test it as a lone unit to make sure it works perfectly on its own.

Conclusion

In modern web development, Shadow DOM gives you a clean and powerful way to build self-contained components. Shadow DOM helps you avoid style and script conflicts, makes your code easier to manage, and keeps everything organized. Whether you’re working on a small project or a large application, using Shadow DOM can help you create more reliable and reusable parts of your website. Once you understand it clearly, you’ll see how much easier it makes your development workflow. If you are an aspiring Web Developer, explore our blog and enroll in our Web Development Course.

What is Shadow DOM – FAQs