報錯Cannot read property 'length' of undefined ?

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(shù)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
    }
  }