The Element.attachShadow()
method attaches a shadow DOM tree to the specified element and returns a reference to its ShadowRoot
.
The Element.attachShadow()
method attaches a shadow DOM tree to the specified element and returns a reference to its ShadowRoot
.
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:
js
attachShadow(options)
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 is false
.
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 a slot
attribute which matches the name
attribute of a <slot>
within this shadow root will be assigned to that slot. Any top-level children of the host with no slot
attribute will be assigned to a <slot>
with no name
attribute (the "default slot") if one is present.
manual
Elements are not automatically assigned to <slot>
elements. Instead, they must be manually assigned with HTMLSlotElement.assign()
. Its default value is named
.
Returns a ShadowRoot
object.
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.
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" });
Specification |
---|
DOM Standard # ref-for-dom-element-attachshadow① |
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
attachShadow |
53 | 79 | 63 | No | 40 | 10 | 53 | 53 | 63 | 41 | 10 | 6.0 |
init_delegatesFocus_parameter |
53 | 79 | 94 | No | 40 | 13.1 | 53 | 53 | 94 | 41 | 13.4 | 6.0 |
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https://developer.mozilla.org/en-US/docs/Web/API/Element/attachShadow