Simple DOM example

웹 컴포넌트의 중요한 측면은 캡슐화입니다. 마크업 구조, 스타일 그리고 동작을 페이지 내의 다른 코드와
- 분리하고 숨긴채로 유지하여 서로 충돌하지 않으며, 코드가 좋고 깨끗하게 되도록 하는 중요한 측면입니다.
- Shadow DOM API 는 이러한 캡슐화의 핵심이며, 숨겨지고 분리된 DOM 을 엘리먼트에 달 수 있는
- 방법입니다. 현재 문서는 Shadow DOM 의 기본적인 사용을 다루고 있습니다.

Note: Shadow DOM 은 Firefox (63 and onwards), Chrome, Opera, and Safari 에서 기본으로 지원되고 있습니다. 새로운 Chromium 기반의 Edge (75 and onwards) 또한 지원하고 있습니다. 그러나 구버전의 Edge 는 지원하지 않습니다.

High-level view

This article assumes you are already familiar with the concept of the DOM (Document Object Model) — a tree-like structure of connected nodes that represents the different elements and strings of text appearing in a markup document (usually an HTML document in the case of web documents). As an example, consider the following HTML fragment:

Shadow DOM allows hidden DOM trees to be attached to elements in the regular DOM tree — this shadow DOM tree starts with a shadow root, underneath which can be attached to any elements you want, in the same way as the normal DOM.

You can affect the nodes in the shadow DOM in exactly the same way as non-shadow nodes — for example appending children or setting attributes, styling individual nodes using element.style.foo, or adding style to the entire shadow DOM tree inside a {{htmlelement("style")}} element. The difference is that none of the code inside a shadow DOM can affect anything outside it, allowing for handy encapsulation.

Note that the shadow DOM is not a new thing by any means — browsers have used it for a long time to encapsulate the inner structure of an element. Think for example of a {{htmlelement("video")}} element, with the default browser controls exposed. All you see in the DOM is the <video> element, but it contains a series of buttons and other controls inside its shadow DOM. The shadow DOM spec has made it so that you are allowed to actually manipulate the shadow DOM of your own custom elements.

Basic usage

You can attach a shadow root to any element using the {{domxref("Element.attachShadow()")}} method. This takes as its parameter an options object that contains one option — mode — with a value of open or closed:

open means that you can access the shadow DOM using JavaScript written in the main page context, for example using the {{domxref("Element.shadowRoot")}} property:

If you attach a shadow root to a custom element with mode: closed set, you won't be able to access the shadow DOM from the outside — myCustomElem.shadowRoot returns null. This is the case with built in elements that contain shadow DOMs, such as <video>.

If you are attaching a shadow DOM to a custom element as part of its constructor (by far the most useful application of the shadow DOM), you would use something like this:

When you've attached a shadow DOM to an element, manipulating it is a matter of just using the same DOM APIs as you use for the regular DOM manipulation:

Working through a simple example

Now let's walk through a simple example to demonstrate the shadow DOM in action inside a custom element — <popup-info-box> (see a live example also). This takes an image icon and a text string, and embeds the icon into the page. When the icon is focused, it displays the text in a pop up information box to provide further in-context information. To begin with, in our JavaScript file we define a class called PopUpInfo, which extends HTMLElement:

Inside the class definition we define the element's constructor, which defines all the functionality the element will have when an instance of it is instantiated.

Creating the shadow root

Creating the shadow DOM structure

Next, we use some DOM manipulation to create the element's internal shadow DOM structure:

Styling the shadow DOM

After that we create a {{htmlelement("style")}} element and populate it with some CSS to style it:

Attaching the shadow DOM to the shadow root

Using our custom element

Once the class is defined, using the element is as simple as defining it, and putting it on the page, as explained in Using custom elements:

Internal versus external styles

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In the above example we apply style to the Shadow DOM using a {{htmlelement("style")}} element, but it is perfectly possible to do it by referencing an external stylesheet from a {{htmlelement("link")}} element instead.

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For example, take a look at this code from our popup-info-box-external-stylesheet example (see the source code):

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// Apply external styles to the shadow dom
-const linkElem = document.createElement('link');
-linkElem.setAttribute('rel', 'stylesheet');
-linkElem.setAttribute('href', 'style.css');
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-// Attach the created element to the shadow dom
-shadow.appendChild(linkElem);

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Note that {{htmlelement("link")}} elements do not block paint of the shadow root, so there may be a flash of unstyled content (FOUC) while the stylesheet loads.

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Many modern browsers implement an optimization for {{htmlelement("style")}} tags either cloned from a common node or that have identical text, to allow them to share a single backing stylesheet. With this optimization the performance of external and internal styles should be similar.

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