一、作用

二、判定事件是否被支持

备注

• canUseDOM() 由 ExecutionEnvironment#canUseDOM 实现

/**
 * Checks if an event is supported in the current execution environment.
 * 检查当前执行环境是否支持某个事件
 *
 * NOTE: This will not work correctly for non-generic events such as `change`,
 * `reset`, `load`, `error`, and `select`.
 * 注意：这对于非通用事件（如 `change`、`reset`、`load`、`error` 和 `select`）将无法正常工作。
 *
 * Borrows from Modernizr.
 * 借鉴自 Modernizr
 *
 * @param {string} eventNameSuffix Event name, e.g. "click".
 * @return {boolean} True if the event is supported.
 * @internal
 * @license Modernizr 3.0.0pre (Custom Build) | MIT
 */
function isEventSupported(eventNameSuffix: string): boolean {
  if (!canUseDOM) {
    return false;
  }

  const eventName = 'on' + eventNameSuffix;
  let isSupported = eventName in document;

  if (!isSupported) {
    const element = document.createElement('div');
    element.setAttribute(eventName, 'return;');
    isSupported = typeof (element as any)[eventName] === 'function';
  }

  return isSupported;
}