layer提示框添加多个按钮选择的实例

function jumpChoose(argu) {
 //询问框
 var index = layer.confirm('下载文件还是在线预览呢？', {
  btn: ['在线预览','下载',"关闭"], //按钮
  shade: false //不显示遮罩
 }, function(){
  //关闭提示框
  layer.close(index);
 }, function(){
  //关闭提示框
  layer.close(index);
 }, function(){
  //关闭提示框
  layer.close(index);
 });
}