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, how it works, and how you can use it in your projects.

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?

The Shadow DOM is basically a feature that helps you to keep the HTML and CSS of a component separate from the rest of the webpage. It is a part of a larger concept idea known as Web Components, which also consists of templates and custom elements. By using this feature, you can write styles that are only applicable to your component, without affecting anything else on the page. Essentially, it lets you attach a hidden, isolated section to an HTML element. That hidden section is referred to as the “shadow tree”, and the element to which it is attached is called the “shadow host”. The invisible boundary that separates this section from the rest of the page is known as the “shadow boundary”, and everything inside begins with the “shadow root”.

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

• Shadow host: This is the regular HTML element that holds the shadow content.

• Shadow tree: This is the internal structure (like HTML tags) that is present inside the isolated part.

• Shadow boundary: This is basically the invisible line where the 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 are building components for the web, sometimes you want to keep the styles and structure of one part of your webpage separate from everything else in the page. The Shadow DOM API in JavaScript makes this possible.

Below are the steps to create one from scratch:

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 DOM will be attached.

Example:

Explanation:

In the above code, the <div> element with the ID shadow-container is just a normal HTML element.

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. This root is where all isolated content and styles will live. All HTML and CSS elements 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 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 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 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 DOM using JavaScript is given below for your reference:

Code:

Output:

Benefits of Using Shadow DOM

1. Encapsulation: Encapsulation 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 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 <aDOM, 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

Let’s take a quick overview of the difference between Shadow DOM and Virtual DOM in the table below.

Potential Security Concerns with Shadow DOM

1. Hidden Malicious Code:  Attackers might hide harmful scripts inside isolated sections, making detection harder.

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 of the 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 this 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 this 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, using clear names helps in maintaining readability and avoiding confusion when the project grows

3. Always keep your scripts and structure inside the Shadow DOM so it stays private and separate from the main page.

4. You should also use slots wisely to pass content from the outside while maintaining 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. It 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.

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What is Shadow DOM – FAQs