why this show return wrong value.

class Student {
    constructor(gpa, credits){
        this.gpa = gpa;
        this.credits = credits;
    }

    stringGPA() {
        return this.gpa.toString();
    }

    get level() {
        if (this.credits > 90 ) {
            return 'Senior';
        } else if (this.credits > 60) {
            return 'Junior';
        } else if (this.credits > 30) {
            return 'Sophomore';
        } else {
            return 'Freshman';
        }
    }

  get major()
  {
    return this._major;
  }

  set major(value)
  {
  this._major = value;
    if(value == 'Junior' || value == 'Senior')
    {
      this._major = value;
    }

    else if(value == 'Freshman' || value == 'Sophomore')
    {
      this._major = 'None';
    }
  }

}

var student = new Student(3.9, 60);
student.major = 'Senior';