为什么我通过 typeof 返回“undefined”，我该如何防止这种情况？

以下代码：

"use strict"

 const students = [
  {firstname: "Max", lastname: "Mustermann", age: 21},
  {firstname: "Laura", lastname: "Müller", age: 25},
  {firstname: "Julia", lastname: "Schreiber", age: 30},
  {firstname: "Tobias", lastname: "Lieb", age: 19}
]

console.log( typeof students.age)

退货

undefined

students.push({
  firstname: "Christian",
  lastname:"Schmidt",
  age:null
})

let ageSum = 0

for (const student in students){
 if(typeof student.age){
  ageSum += student.age
 }
}

又来了，我不明白为什么

typeof

undefined

students

age

undefined

age

"use strict"

 const students = [
  {firstname: "Max", lastname: "Mustermann", age: 21},
  {firstname: "Laura", lastname: "Müller", age: 25},
  {firstname: "Julia", lastname: "Schreiber", age: 30},
  {firstname: "Tobias", lastname: "Lieb", age: 19}
]

console.log( typeof students[0].age)
// Here --------------------^