CustomElementRegistry.whenDefined()

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

Syntax

Promise<> customElements.whenDefined(name);

Parameters

name
Custom element name.

Return value

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

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)');

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

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

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'customElements.whenDefined()' in that specification.
Living Standard Initial definition.

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung Internet
Basic support
Experimental
Chrome Full support 66
Notes
Full support 66
Notes
Notes Support for 'Customized built-in elements' as well.
Full support 54
Notes
Notes Support for 'Autonomous custom elements' only.
Edge No support No
Notes
No support No
Notes
Notes Under consideration
Firefox Full support 63
Full support 63
Full support 59
Disabled
Disabled From version 59: this feature is behind the dom.webcomponents.customelements.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.
No support ? — 59
Disabled
Disabled Until version 59 (exclusive): this feature is behind the dom.webcomponents.enabled preference (needs to be set to true) and the dom.webcomponents.customelements.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.
IE No support NoOpera Full support 53
Notes
Full support 53
Notes
Notes Support for 'Customized built-in elements' as well.
Full support 41
Notes
Notes Support for 'Autonomous custom elements' only.
Safari Full support 10.1
Notes
Full support 10.1
Notes
Notes Supports 'Autonomous custom elements' but not 'Customized built-in elements'
WebView Android Full support 66
Notes
Full support 66
Notes
Notes Support for 'Customized built-in elements' as well.
Full support 54
Notes
Notes Support for 'Autonomous custom elements' only.
Chrome Android Full support 66
Notes
Full support 66
Notes
Notes Support for 'Customized built-in elements' as well.
Full support 54
Notes
Notes Support for 'Autonomous custom elements' only.
Edge Mobile No support No
Notes
No support No
Notes
Notes Under consideration
Firefox Android Full support 63
Full support 63
Full support 59
Disabled
Disabled From version 59: this feature is behind the dom.webcomponents.customelements.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.
No support ? — 59
Disabled
Disabled Until version 59 (exclusive): this feature is behind the dom.webcomponents.enabled preference (needs to be set to true) and the dom.webcomponents.customelements.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.
Opera Android Full support 53
Notes
Full support 53
Notes
Notes Support for 'Customized built-in elements' as well.
Full support 41
Notes
Notes Support for 'Autonomous custom elements' only.
Safari iOS Full support 10.1
Notes
Full support 10.1
Notes
Notes Supports 'Autonomous custom elements' but not 'Customized built-in elements'
Samsung Internet Android Full support 6.0
Notes
Full support 6.0
Notes
Notes Support for 'Autonomous custom elements' only.

Legend

Full support  
Full support
No support  
No support
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.
See implementation notes.
See implementation notes.
User must explicitly enable this feature.
User must explicitly enable this feature.

Document Tags and Contributors

Last updated by: sideshowbarker,