Element: attachShadow() method
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
js
attachShadow(options)
Parameters
options
-
An object which contains 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
:js
element.attachShadow({ mode: "open" }); element.shadowRoot; // Returns a ShadowRoot obj
closed
-
Denies access to the node(s) of a closed shadow root from JavaScript outside it:
js
element.attachShadow({ mode: "closed" }); element.shadowRoot; // Returns null
delegatesFocus
Optional-
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. Its default value isfalse
. slotAssignment
Optional-
A string specifying the slot assignment mode for the shadow DOM tree. This can be one of:
named
-
Elements are automatically assigned to
<slot>
elements within this shadow root. Any descendants of the host with aslot
attribute which matches thename
attribute of a<slot>
within this shadow root will be assigned to that slot. Any top-level children of the host with noslot
attribute will be assigned to a<slot>
with noname
attribute (the "default slot") if one is present. manual
-
Elements are not automatically assigned to
<slot>
elements. Instead, they must be manually assigned withHTMLSlotElement.assign()
. Its default value isnamed
.
Return value
Returns a ShadowRoot
object.
Exceptions
InvalidStateError
DOMException
-
The element you are trying to attach to is already a shadow host.
NotSupportedError
DOMException
-
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.
js
// 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
const wcParent = this.parentNode;
function countWords(node) {
const text = node.innerText || node.textContent;
return text
.trim()
.split(/\s+/g)
.filter((a) => a.trim().length > 0).length;
}
const count = `Words: ${countWords(wcParent)}`;
// Create a shadow root
const shadow = this.attachShadow({ mode: "open" });
// Create text node and add word count to it
const text = document.createElement("span");
text.textContent = count;
// Append it to the shadow root
shadow.appendChild(text);
// Update count when element content changes
setInterval(() => {
const 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
BCD tables only load in the browser