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:
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 usingElement.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' });
Specifications
Specification | Status | Comment |
---|---|---|
DOM The definition of 'attachShadow()' in that specification. |
Living Standard |
Browser compatibility
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