实现一个异步加载远端js的方法loadScript

该方法接收一个标识远端js地址的url参数，加载成功后方法返回一个fulfilled promise，且远端js能够执行；加载失败或者超时5s则返回rejected promise

/**
     * 加载外部的js文件
     * @param url 要加载的js的url地址
     * 
     */
function loadScript(url) {
    return new Promise( (resolve, reject) => {
        let _script = document.createElement('script')
        _script.setAttribute('charset', 'utf-8')
        _script.setAttribute('type', 'text/javascript')
        _script.setAttribute('src', url)
        document.getElementsByTagName('head')[0].appendChild(_script)

        timeout = setTimeout(() => {
            reject('timeout') //rejected
        }, 5000)

        _script.onload = () => { //加载完成
            clearTimeout(timeout)
            resolve() //fulfilled
        }
        _script.onerror = () => { //加载失败
            reject('fail') //rejected
        }
    })
}

参考：

• https://www.cnblogs.com/xdp-gacl/p/3927417.html