CustomElementRegistry.whenDefined()

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

Syntax

customElements.whenDefined(name).then(function() {
  // Do something related to the element
})

Parameters

name

Custom element name.

Return value

A Promise that resolves when the custom element is defined. If the custom element is already defined, the promise is resolved immediately.

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>

  let container = document.getElementById('menu-container');
  let placeholder = container.querySelector('.menu-placeholder');
  // Fetch all the children of menu that are not defined yet.
  let undefinedElements = container.querySelectorAll(':not(:defined)');
  let promises = [...undefinedElements].map(button => {
    return customElements.whenDefined(button.localName);
  });
  // Wait for all the children to be upgraded, 
  // then remove the placeholder.
  Promise.all(promises).then(() => {
    container.removeChild(placeholder);
  });

Browser compatibility