lib/index.esm.js
function addEvent(el, type, handler) {
  var once = arguments.length > 3 && arguments[3] !== undefined ? arguments[3] : false;
  var capture = arguments.length > 4 && arguments[4] !== undefined ? arguments[4] : false;

  if (capture !== undefined && !isBoolean(capture) && supportsPassive) {
    capture = { passive: true };
  }
  if (once) {
    var oldHandler = handler;
    handler = function () {
      return function () {
        for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key++) {
          args[_key] = arguments[_key];
        }

        oldHandler.apply(this, args);
        removeEvent(el, type, handler, once, capture);
      };
    }();
    /* 将包装后的方法记录到缓存中 */
    addEventCache(el, type + '_once', oldHandler, handler);
  }

  el.addEventListener(type, handler, capture);
}
function addEvent(el, type, handler) {
  var once = arguments.length > 3 && arguments[3] !== undefined ? arguments[3] : false;
  var capture = arguments.length > 4 && arguments[4] !== undefined ? arguments[4] : false;

  if (capture !== undefined && !isBoolean(capture) && supportsPassive) {
    capture = { passive: true };
  }
  if (once) {
    var oldHandler = handler;
    handler = function () {
      return function () {
        for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key++) {
          args[_key] = arguments[_key];
        }

        oldHandler.apply(this, args);
        removeEvent(el, type, handler, once, capture);
      };
    }();
    /* 将包装后的方法记录到缓存中 */
    addEventCache(el, type + '_once', oldHandler, handler);
  }

  el.addEventListener(type, handler, capture);
}
为HTML元素添加事件监听
@param {HTMLElement} el 目标元素
@param {String} type 事件名称
@param {Function} handler 处理函数
@param {Boolean} once 是否只监听一次
@param {Boolean|Object} capture 是否在捕获阶段监听，这里也可以传入 { passive: true } 表示被动模式