Cant solve grade-school exercise

Sorry in advance for my bad English. I have been struggling with the last task for the excercise grade-school, in wich i have to make the roaster not accessable form outside the module but dont know how to do it.
(Edit: this is the code, though I dont know how to use codeblocks here)

//
// This is only a SKELETON file for the ‘Grade School’ exercise. It’s been provided as a
// convenience to get you started writing code faster.
//

export class GradeSchool {

constructor(){
this.list={}

}
roster() {
return this.list
}

add(student,grade) {
for(let key in this.list){
if(this.list[key]==student)delete this.list[key]
}
if(this.list?.[grade])this.list[grade].push(student)
else this.list[grade]=[student]
for(let key in this.list){
this.aux=this.list[key]
this.aux.sort()
}

}

grade(gradeNum) {
if(this.list?.[gradeNum])return this.list[gradeNum]
else return []
}
}

Can you share your code and the error message? Please use a codeblock and not a screenshot.