Why do I report an error when I set _ expanded to true in the data bound to the table component of iview

Why does setting _ expanded to true in the data I bind to the table component of iview report an error
let tempData = response.data.data;

    for (let i = 0; i < tempData.length; iPP) {
      tempData[i].date = tempData[i].list[0].borrow_date.substring(0, 7);
      tempData[i]._expanded = true;
    }
    _this.data1 = tempData;
    

clipboard.png

Mar.05,2021

_ this.data1 = tempData.map (item= > {

)
    item._expanded=true;return item;
})
Menu