CustomElementRegistry.whenDefined()

The whenDefined() method of the CustomElementRegistry interface returns a Promise that resolves when the named element is defined.

Syntax

customElements.whenDefined(name): Promise<CustomElementConstructor>;

Parameters

name
Custom element name.

Return value

A Promise that will be fulfilled with the custom element's constructor when a custom element becomes defined with the given name. (If such a custom element is already defined, the returned promise will be immediately fulfilled.)

Exceptions

Exception Description
SyntaxError If the provided name is not a valid custom element name, the promise rejects with a SyntaxError.

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.

<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>

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(){
  const promises = [...undefinedElements].map(
    button => customElements.whenDefined(button.localName)
  );

  // Wait for all the children to be upgraded
  await Promise.all(promises);
  // then remove the placeholder
  container.removeChild(placeholder);
}

removePlaceholder();

Specifications

Specification
HTML Standard (HTML)
# dom-customelementregistry-whendefined-dev

Browser compatibility

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