file文件转换为二进制数据

const beforeUpload = file => {

      var reader = new FileReader();
      reader.readAsDataURL(file);
      //转化二进制流，异步方法
      reader.onload = function(){
        //完成后this.result为二进制流
        console.log(this.result,"0-00-");
        var base64Str = this.result;
        var startNum = base64Str.indexOf("base64,");
        startNum = startNum*1 + 7;
        //去除前部格式信息（如果有需求）
        var baseStr = base64Str.slice(startNum);
        console.log(baseStr,"二进制数据！！！")
      }
 }