封装绑定事件的处理函数（事件代理）

    <ul id="list">
      <li id="li1">项目一</li>
      <li>项目二</li>
      <li>项目三</li>
      <li>项目四</li>
    </ul>

      const list = document.getElementById(‘list‘)
      const li1 = document.getElementById(‘li1‘)
      function addEvent(ele, type, selector, cb) {
        if (!cb) {
          cb = selector
          selector = null
        }
        ele.addEventListener(type, function (e) {
          // e.stopPropagation()
          const target = e.target
          if (selector) {
            if (target.matches(selector)) cb.call(target, e)
          } else {
            cb.call(target, e)
          }
        })
      }
      addEvent(li1, ‘click‘, function (e) {
        console.log(1, e.target.innerHTML)
        console.log(2, this.innerHTML)
        console.log(this === e.target)
      })
      addEvent(list, ‘click‘, ‘li‘, function (e) {
        console.log(1, e.target.innerHTML)
        console.log(2, this.innerHTML)
        console.log(this === e.target)
      })