Element.attachShadow()
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>
).
The following is a list of elements you can attach a shadow root to:
Syntax
attachShadow(init)
Parameters
init
-
A object that 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
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, the element cannot have a shadow attached to it, or the static property
disabledFeatures
has been given a value of"shadow"
in the element definition.
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 |
---|
DOM Standard # ref-for-dom-element-attachshadow① |
Browser compatibility
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