js中如何在不影响既有事件监听的前提下新增监听器

function additionalListener(){
    console.log('should do something else');
}

_windowonload = window.onload;
window.onload = function(){
    additionalListener();
    if (someCondition){
        _windowonload();
    }
}

 var onloadProxy = new Proxy(window.onload,{
    apply:function () {
        additionalListener();
        Reflect.apply(...arguments);
    }
  });
  window.onload = onloadProxy;

  Function.prototype.after = function (afterFn) {
      return () => {
         const result = this.apply(this, arguments);
          afterFn.apply(this,arguments);
          return result;
      }
  }
  window.onload = window.onload.after(additionalListener);