通用事件绑定

function bindEvent(elem, type, selector, fn) {
        if(fn == null) {
            fn = selector
          selector = null
    }
      elem.addEventListener(type, event => {
          const target = event.target
            if(selector) {
                // 代理绑定
          if(target.matches(selector)) {
              fn.call(target. event)
          }
        } else {
                // 普通绑定
            fn.call(target. event)
        }
    })
}

// 普通代理
const btn1 = document.getElementById('btn1')
bindEvent(btn1, 'click', function(event) {
    event.preventDefault()           // 阻止a标签跳转
    const target = event.target
       alert(this.innerHTML)
})

const div1 = document.getElementById('div1')
bindEvent(div3, 'click','a',function(event) {
    event.preventDefault()           // 阻止a标签跳转
    const target = event.target
    if(target.nodeName === 'A') {
        alert(this.innerHTML)
    }
})