The read-only composed property of the Event interface returns a boolean value which indicates whether or not the event will propagate across the shadow DOM boundary into the standard DOM.
All UA-dispatched UI events are composed (click/touch/mouseover/copy/paste, etc.). Most other types of events are not composed, and so will return false. For example, this includes synthetic events that are created without their composed option set to true.
Propagation only occurs if the bubbles property is also true. However, capturing only composed events are also handled at host as if they were in AT_TARGET phase. You can determine the path the event will follow through the shadow root to the DOM root by calling composedPath().
A boolean value which is true if the event will cross from the shadow DOM into the standard DOM after reaching the shadow root. (That is, the first node in the shadow DOM in which the event began to propagate.)
If this value is false, the shadow root will be the last node to be offered the event.
In this example, we define two trivial custom elements, <open-shadow> and <closed-shadow>, both of which take the contents of their text attribute and insert them into the element's shadow DOM as the text content of a <p> element. The only difference between the two is that their shadow roots are attached with their modes set to open and closed respectively.
The two definitions look like this:
customElements.define(
"open-shadow",
class extends HTMLElement {
constructor() {
super();
const pElem = document.createElement("p");
pElem.textContent = this.getAttribute("text");
const shadowRoot = this.attachShadow({
mode: "open",
});
shadowRoot.appendChild(pElem);
}
},
);
customElements.define(
"closed-shadow",
class extends HTMLElement {
constructor() {
super();
const pElem = document.createElement("p");
pElem.textContent = this.getAttribute("text");
const shadowRoot = this.attachShadow({
mode: "closed",
});
shadowRoot.appendChild(pElem);
}
},
);
We then insert one of each element into our page:
<open-shadow text="I have an open shadow root"></open-shadow>
<closed-shadow text="I have a closed shadow root"></closed-shadow>
Then include a click event listener on the <html> element:
document.querySelector("html").addEventListener("click", (e) => {
console.log(e.composed);
console.log(e.composedPath());
});
When you click on the <open-shadow> element and then the <closed-shadow> element, you'll notice two things.
- The
composed property returns true because the click event is always able to propagate across shadow boundaries. - A difference in the value of
composedPath for the two elements.
The <open-shadow> element's composed path is this:
Array [ p, ShadowRoot, open-shadow, body, html, HTMLDocument https://mdn.github.io/web-components-examples/composed-composed-path/, Window ]
Whereas the <closed-shadow> element's composed path is a follows:
Array [ closed-shadow, body, html, HTMLDocument https://mdn.github.io/web-components-examples/composed-composed-path/, Window ]
In the second case, the event listeners only propagate as far as the <closed-shadow> element itself, but not to the nodes inside the shadow boundary.