报错Cannot read property 'length' of undefined ?

Question

报错如下:
Uncaught TypeError: Cannot read property 'length' of undefined
请问是哪里出问题了, 该怎么解决, 谢谢
html

  changeIndex (index){
    console.log(index)
    this.activeIndex = index
    this.sportDatabaseOrderBy = this.sportDatabase.slice(0) // 数组深拷贝
    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
    }
  }

Answer 1

在for loop时， this.sportDatabaseOrderBy是undefined.

Answer 2

检查this.sportDatabaseOrderBy是否有值

Answer 3

你的this.sportDatabaseOrderBy是underfined，你检查下this.sportDatabase

Answer 4

在这钟报length错误的情况下，一般都是这个值是undefined，或是特殊情况下取不到

Answer 5

当取this.sportDatabaseOrderBy的值为undefined时，length属性就取不到值了；
需要先进行判断，this.sportDatabaseOrderBy != undefined判断之后，再执行this.sportDatabaseOrderBy.length。