要领定义

export function fireKeyEvent(el, evtType, keyCode) {
  var evtObj;
  if (document.createEvent) {
      if (window.KeyEvent) {//firefox 浏览器下模仿事宜
          evtObj = document.createEvent('KeyEvents');
          evtObj.initKeyEvent(evtType, true, true, window, true, false, false, false, keyCode, 0);
      } else {//chrome 浏览器下模仿事宜
          evtObj = document.createEvent('UIEvents');
          evtObj.initUIEvent(evtType, true, true, window, 1);

          delete evtObj.keyCode;
          if (typeof evtObj.keyCode === "undefined") {//为了模仿keycode
              Object.defineProperty(evtObj, "keyCode", { value: keyCode });                       
          } else {
              evtObj.key = String.fromCharCode(keyCode);
          }

          if (typeof evtObj.ctrlKey === 'undefined') {//为了模仿ctrl键
              Object.defineProperty(evtObj, "ctrlKey", { value: true });
          } else {
              evtObj.ctrlKey = true;
          }
      }
      el.dispatchEvent(evtObj);

  } else if (document.createEventObject) {//IE 浏览器下模仿事宜
      evtObj = document.createEventObject();
      evtObj.keyCode = keyCode
      el.fireEvent('on' + evtType, evtObj);
  }
}

要领挪用

fireKeyEvent(document.getElementById('aa'),'keydown',40);