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Element.attachShadow() - Web APIs 编辑

The Element.attachShadow() method attaches a shadow DOM tree to the specified element and returns a reference to its ShadowRoot.

Elements you can attach a shadow to

Note that you can't attach a shadow root to every type of element. There are some that can't have a shadow DOM for security reasons (for example <a>), and more besides. The following is a list of elements you can attach a shadow root to:

Any autonomous custom element with a valid name
<article>
<aside>
<blockquote>
<body>
<div>
<footer>
<h1>
<h2>
<h3>
<h4>
<h5>
<h6>
<header>
<main>
<nav>
<p>
<section>
<span>

Syntax

var shadowroot = element.attachShadow(shadowRootInit);

Parameters

shadowRootInit

A ShadowRootInit dictionary, which can contain the following fields:

mode

A string specifying the encapsulation mode for the shadow DOM tree. This can be one of:

open: Elements of the shadow root are accessible from JavaScript outside the root, for example using Element.shadowRoot:
```
element.shadowRoot; // Returns a ShadowRoot obj
```
closed: Denies access to the node(s) of a closed shadow root from JavaScript outside it:
```
element.shadowRoot; // Returns null
```

delegatesFocus

A boolean that, when set to true, specifies behavior that mitigates custom element issues around focusability. When a non-focusable part of the shadow DOM is clicked, the first focusable part is given focus, and the shadow host is given any available :focus styling.

Return value

Returns a ShadowRoot object.

Exceptions

Exception	Explanation
`InvalidStateError`	The element you are trying to attach to is already a shadow host.
`NotSupportedError`	You are trying to attach a shadow root to an element outside the HTML namespace, or the element cannot have a shadow attached to it (see above).

Examples

The following example is taken from our word-count-web-component demo (see it live also). You can see that we use attachShadow() in the middle of the code to create a shadow root, which we then attach our custom element's contents to.

// Create a class for the element
class WordCount extends HTMLParagraphElement {
  constructor() {
    // Always call super first in constructor
    super();

    // count words in element's parent element
    var wcParent = this.parentNode;

    function countWords(node){
      var text = node.innerText || node.textContent
      return text.trim().split(/\s+/g).length;
    }

    var count = 'Words: ' + countWords(wcParent);

    // Create a shadow root
    var shadow = this.attachShadow({mode: 'open'});

    // Create text node and add word count to it
    var text = document.createElement('span');
    text.textContent = count;

    // Append it to the shadow root
    shadow.appendChild(text);

    // Update count when element content changes
    setInterval(function() {
      var count = 'Words: ' + countWords(wcParent);
      text.textContent = count;
    }, 200)
  }
}

// Define the new element
customElements.define('word-count', WordCount, { extends: 'p' });