報錯如下:Uncaught TypeError: Cannot read property 'length' of undefined請問是哪里出問題了, 該怎么解決, 謝謝html  changeIndex (index){    console.log(index)    this.activeIndex = index    this.sportDatabaseOrderBy = this.sportDatabase.slice(0) // 數組深拷貝    for (let i = 0; i < this.sportDatabaseOrderBy.length; i++) {       let firstLetter = py.getFullChars(this.sportDatabaseOrderBy[i].name).slice(0,1)       this.sportDatabaseOrderBy[i].firstLetter = firstLetter    }  }