js通过HEAD请求方式提前获取下载文件大小,XMLHttpRequest和ajax两种实例

var url = ‘http://‘; //文件下载链接
var fileSize = 0; //下载文件大小    
var xhr = new XMLHttpRequest();
    xhr.open(‘HEAD‘, url, true);    // 也可用POST方式
    xhr.onreadystatechange = () => {
     if (xhr.readyState == 4) {
       if (xhr.status == 200) {
         fileSize = xhr.getResponseHeader(‘Content-Length‘);
         console.log(fileSize)
       } else {
         alert(‘ERROR‘);
       }
     }
    };
    xhr.send()

             $.ajax({
                url: item.m3u8Url,
                async: false,
                timeout: 2000,
                // type: ‘HEAD‘,
                complete: function( xhr,data ){
                  console.log(xhr.getResponseHeader("hls-next"));
                  hlsNext = Math.floor(xhr.getResponseHeader("hls-next")/1000)
                  console.log(‘hlsNext‘,hlsNext)
                }
              });