function Student() {
  this.grades = [];
  this.average = average;
  this.addGrades = addGrades;
}

function addGrades(gradeArgs) {
  this.grades.push(gradeArgs);
};
function average() {
  var total = 0;
  for(var i = 0; i < this.grades.length; i += 1){
    total += this.grades[i];
  }
  return total/this.grades.length;
};

var Sagar = new Student();
Sagar.addGrades(20);Sagar.addGrades(30);Sagar.addGrades(40);
console.log(Sagar.average());