promise顺序动态加载JavaScript

<script>
  // Promise IE不兼容，需要找shim一下
  let loadScript = function () {
    return function _loadScript(url, callBack) {
      return new Promise(function (resolve) {
        let script = document.createElement('script')
        script.type = 'text/javascript'
        if (script.readyState) {
          // 兼容IE的script加载事件
          script.onreadystatechange = function () {
            // loaded ： 下载完毕 complete： 数据准备完毕。这两个状态ie可能同时出现或者只出现一个
            if (script.readyState === 'loaded' || script.readyState === 'complete') {
              // 防止加载两次
              script.onreadystatechange = null
              callBack()
              // 把函数传递下去，保证能顺序加载js
              resolve(_loadScript)
            }
          }
        } else {
          script.onload = function () {
            callBack()
            resolve(_loadScript)
          }
        }
        script.src = url
        document.head.appendChild(script)
      })
    }
  }()
  loadScript('./p1.js', () => {
    console.log('p1-loading')
  }).then(res => {
    return res('./p2.js', () => { console.log('p2-loading') })
  }).then(res => {
    res('./p3.js', () => { console.log('p3-loading') })
  })
</script>