动态加载JavaScript文件

function loadScript(url,cb){
    let script = document.createElement('script')
    script.type = 'text/javascript'
    if(script.readyState){
        script.onreadystatechange = function(){
            if(script.readyState == 'loaded' || script.readyState == 'complete'){
                script.onreadystatechange = null
                cb && cb()
            }
        }
    }else{
        script.onload = function(){
            cb && cb()
        }
    }
    script.src = url
    document.getElementsByTagName('body')[0].appendChild(script)
}