js动态添加脚本

function addScript(url) {
  var head = document.getElementsByTagName('head')[0]
  var script = document.createElement('script')
  script.src = url
  script.type = 'text/javascript'
  script.defer = true
  head.appendChild(script)
}
addScript('https://cdn.bootcss.com/lodash.js/4.17.10/lodash.js')