CustomElementRegistry: whenDefined() method
The whenDefined()
method of the
CustomElementRegistry
interface returns a Promise
that
resolves when the named element is defined.
Syntax
js
whenDefined(name)
Parameters
name
-
The name of the custom element.
Return value
A Promise
that fulfills with the custom element's constructor when a custom element becomes defined with the given name. If a custom element has already been defined with the name, the promise will immediately fulfill.
The promise is rejected with a SyntaxError
DOMException
if the name is not a valid custom element name.
Examples
This example uses whenDefined()
to detect when the custom elements that
make up a menu are defined. The menu displays placeholder content until the actual menu
content is ready to display.
html
<nav id="menu-container">
<div class="menu-placeholder">Loading…</div>
<nav-menu>
<menu-item>Item 1</menu-item>
<menu-item>Item 2</menu-item>
…
<menu-item>Item N</menu-item>
</nav-menu>
</nav>
js
const container = document.getElementById("menu-container");
const placeholder = container.querySelector(".menu-placeholder");
// Fetch all the children of menu that are not yet defined.
const undefinedElements = container.querySelectorAll(":not(:defined)");
async function removePlaceholder() {
// Filter the elements down to unique localNames
const tags = new Set(
[...undefinedElements].map((button) => button.localName),
);
const promises = [...tags].map((tag) => customElements.whenDefined(tag));
// Wait for all the children to be upgraded
await Promise.all(promises);
// then remove the placeholder
container.removeChild(placeholder);
}
removePlaceholder();
Specifications
Specification |
---|
HTML Standard # dom-customelementregistry-whendefined-dev |
Browser compatibility
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