代码如下

var ByteBuffer = function (org_buf,offset) {
	this.byte = function(val,index){
        if(val == undefined || val == null){
           _list.push(_org_buf.readUInt8(_offset));
           _offset+=1;
        }else{
            _list.splice(index != undefined ? index : _list.length,0,{t:Type_Byte,d:val,l:1});      <---疑问点
            _offset += 1;
        }
        return this;
    };
}

<—_list插入的 index != undefined ? 是代表什么含义啊