在此页

• 句法
• 范例
• 规范
• 浏览器兼容性
• 相关话题

whenDefined() 方法在 CustomElementRegistry interface returns a Promise that resolves when the named element is defined.

句法

Promise<> customElements.whenDefined(name);
					

参数

名称

Custom element name.

返回值

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

异常

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

范例

此范例使用 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);
					

规范

规范	状态	注释
HTML 实时标准 The definition of 'customElements.whenDefined()' in that specification.	实时标准	初始定义。

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	桌面						移动
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
whenDefined	Chrome 66 66 注意事项 注意事项 Support for 'Customized built-in elements' as well. 54 注意事项 注意事项 Support for 'Autonomous custom elements' only.	Edge 79 79 注意事项 注意事项 Support for 'Customized built-in elements' as well. 79 注意事项 注意事项 Support for 'Autonomous custom elements' only.	Firefox 63 63 不支持 59 — 63 Disabled Disabled From version 59 until version 63 (exclusive): this feature is behind the dom.webcomponents.customelements.enabled preference (needs to be set to true ). To change preferences in Firefox, visit about:config. 不支持 ? — 59 Disabled Disabled Until version 59 (exclusive): this feature is behind the dom.webcomponents.enabled preference (needs to be set to true ) 和 preference (needs to be set to ). To change preferences in Firefox, visit about:config.	IE 不支持 No	Opera 53 53 注意事项 注意事项 Support for 'Customized built-in elements' as well. 41 注意事项 注意事项 Support for 'Autonomous custom elements' only.	Safari 10.1 10.1 注意事项 注意事项 Supports 'Autonomous custom elements' but not 'Customized built-in elements'	WebView Android 66 66 注意事项 注意事项 Support for 'Customized built-in elements' as well. 54 注意事项 注意事项 Support for 'Autonomous custom elements' only.	Chrome Android 66 66 注意事项 注意事项 Support for 'Customized built-in elements' as well. 54 注意事项 注意事项 Support for 'Autonomous custom elements' only.	Firefox Android 63 63 不支持 59 — 63 Disabled Disabled From version 59 until version 63 (exclusive): this feature is behind the dom.webcomponents.customelements.enabled preference (needs to be set to true ). To change preferences in Firefox, visit about:config. 不支持 ? — 59 Disabled Disabled Until version 59 (exclusive): this feature is behind the dom.webcomponents.enabled preference (needs to be set to true ) 和 preference (needs to be set to ). To change preferences in Firefox, visit about:config.	Opera Android 47 47 注意事项 注意事项 Support for 'Customized built-in elements' as well. 41 注意事项 注意事项 Support for 'Autonomous custom elements' only.	Safari iOS 10.3 10.3 注意事项 注意事项 Supports 'Autonomous custom elements' but not 'Customized built-in elements'	Samsung Internet Android 9.0 9.0 注意事项 注意事项 Support for 'Customized built-in elements' as well. 6.0 注意事项 注意事项 Support for 'Autonomous custom elements' only.

图例

完整支持

完整支持

不支持

不支持

实验。期望将来行为有所改变。

实验。期望将来行为有所改变。

见实现注意事项。

见实现注意事项。

用户必须明确启用此特征。

用户必须明确启用此特征。

元数据

• 最后修改： 2019 年 3 月 23 日

相关话题

1. CustomElementRegistry
2. 方法

   1. define()
   2. get()
   3. upgrade()
   4. whenDefined()