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How to use shadow dom in Polymer

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by chandramouliprabuoff dot icon Updated: Jan 24, 2024

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The Shadow DOM, or Shadow Document Object Model, is a web standard. It allows the encapsulation of DOM and CSS in a scoped and isolated subtree. It helps make reusable parts for websites. It does this by encapsulating their styling and structure. This prevents styles or scripts from leaking into or affecting the rest of the document.

  1. HTML5 Origins: HTML5 created the Shadow DOM to fix problems in old web development. It focuses on styling and encapsulation.
  2. Styling and Encapsulation Enhancement: Styling and encapsulation got better. It helps developers make components with clear styles and behavior, all enclosed.
  3. Component Creation Focus: The main goal was to help developers create modular components. This promotes code organization and reusability.
  4. Browsers: Over time, major web browsers recognized the utility of the Shadow DOM. They adopted it across platforms. Chrome, Firefox, Safari, and Edge now work with the Shadow DOM.
  5. Web Development: Major browsers adopting it played a big role in modern web development. It made the Shadow DOM a fundamental technology in modern web development practices.
  6. Standardization Platforms: Major browsers adopting the Shadow DOM helped make it a standard. It ensures a consistent and reliable experience for developers across different platforms.
  7. Popularity: More developers liked using the Shadow DOM as they moved to component-based styles. This made it a big part of web development.

Benefits of the Shadow DOM:

  • The shadow boundary encapsulates styles and functionality. This prevents unintended interference with other elements on the page.
  • The Shadow DOM reduces the risk of conflicts by encapsulating styles and scripts. This improves performance. It also enhances performance by avoiding global scope pollution.
  • You can reuse components built using the Shadow DOM. There are no concerns about style clashes. This promotes modular and maintainable code.

Each Shadow DOM instance has a shadow root. It serves as the boundary for styles and structure. The shadow root isolates elements within it from the rest of the document. Shadow DOM allows for the use of slots. Slots are placeholders within a component. They inject content from the main document. This facilitates the composition of components and enhances flexibility

Examples of Websites/Applications using Shadow DOM:

  • YouTube employs the Shadow DOM to encapsulate video player components. This prevents external styles or scripts from interfering with its functionality.
  • Netflix uses the Shadow DOM to make neat, separate parts for its user interface. This enhances maintainability and prevents global styling conflicts.

In conclusion, the Shadow DOM is a crucial aspect of modern web development. It provides encapsulation, reusability, and improved performance. The HTML5 specification roots its history, and major browsers have embraced it. The benefits extend to encapsulation, improved performance, and modular code. The workings involve shadow roots and slots. Popular platforms like YouTube and Netflix leverage the Shadow DOM for component-based development.

Fig 1: Preview of the output that you will get on running this code from your IDE

Code

Can I select Polymer elements that exist in the Shadow DOM?

Lines of Code : 38dotLicense : Strong Copyleft (CC BY-SA 4.0)

Dependent Libraries : 
<my-app>
  #shadow-root
    <h3 part="header">My App Header</h3>
    <my-dialog>
      #shadow-root
        <p part="header">My Dialog Header</p>
        <my-alert>
          #shadow-root
            <span part="header">My Alert Header</span>
        </my-alert>
        <my-alert>
          #shadow-root
            <span part="header">My Alert Header</span>
        </my-alert>
    </my-dialog>
</my-app>

document.querySelector('my-app').shadowRoot.querySelector('my-dialog').shadowRoot.querySelector('my-alert');

<my-app id="app">
  #shadow-root
    <h3 part="header">My App Header</h3>
    <my-dialog id="dialog">
      #shadow-root
        <p part="header">My Dialog Header</p>
        <my-alert id="alert1">
          #shadow-root
            <span part="header">My Alert Header</span>
        </my-alert>
        <my-alert id="alert2">
          #shadow-root
            <span part="header">My Alert Header</span>
        </my-alert>
    </my-dialog>
</my-app>

document.querySelector('my-app').$.dialog.$.alert1

Instruction

  1. Install Visual Studio Code IDE on your computer.
  2. Create a new HTML file.
  3. Copy the code using the 'Copy' button and create an HTML tag then paste the code inside the tag into that HTML file.
  4. Copy the controller.js code and paste in the JS file created.
  5. Create a script tag inside the body tag. Then keep the function lines inside the script tag(refer to preview as well).
  6. ⁠Add the script tag below the div tag.
  7. Save and run the HTML file directly from the file location to generate the output


I found this code snippet by searching for 'How to use shadow dom in Polymer' .you can try any such use case!

Environment Tested

I tested this solution in the following versions. Be mindful of changes when working with other versions.

  1. The solution is created in Visual Studio Code 1.75.1.


Using this solution, we are able to process form using angular js with simple steps. This process also facilities an easy way to use, hassle-free method to create a hands-on working version of code which would help us to use shadow dom in Polymer to follow object.

FAQ

1. What is the event model in relation to shadow DOM and how does it function?

The event model in the context of the Shadow DOM refers to how events propagate through the DOM tree. Events in the Shadow DOM follow a process called "retargeting." The shadow host is the target when an event occurs in a shadow tree. Then, the system retargets it to the appropriate shadow tree node, ensuring encapsulation.


2. How does the shadow DOM tree differ from the main document DOM tree?

The shadow DOM tree is distinct from the main document DOM tree. The main document DOM represents the entire webpage structure. The shadow DOM tree encapsulates specific components. It creates isolated scopes for styles and behavior.


3. What is the purpose of a DOM boundary in relation to shadow doms?

A DOM boundary, in relation to shadow DOMs, acts as a separation between the shadow DOM and the main document DOM. It makes sure that styles and events in the shadow DOM don't mess with the rest of the document. This provides encapsulation and modularity.


4. Can developers access hidden DOM trees, and if they can, how?

You can't reach hidden parts inside the shadow DOM from the main document. The shadow DOM provides encapsulation. The internal structure and styling details hide by it. They are not manipulable from outside the shadow boundary.


5. How does following DOM structure affect the use of shadow doms?

Using shadow DOMs makes it simpler and cleaner to organize website parts into separate sections. Each section can have its own styles and actions. It allows for the creation of modular, reusable elements. They don't interfere with the global document structure. This promotes a cleaner and more maintainable codebase.

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