Javascript – Looping through an object to display the grades of a student object and the function displays undefined, what could be the reason?

I am doing an online course and practising what I am learning.
I just wanted to create a function, that takes an object and loops through an array element of the object using forEach.

So I don’t want to change anything. The reason why I am doing it just to practice forEach method combined with an object.

When I use this following function, it displays this:

15
18
14
17
undefined

Normally it shouldn’t display undefined at the end. I wonder, what I did wrong with the coding.
Can you help me please?

    let student = {
    firstName: "Sam", //string
    lastName: "Doe", // string
    hasGraduated: false, // boolean
    age: 21, // number
    grades: [15, 18, 14, 17], // array of numbers
    interests: ["computers", "books"] // array of strings
};

function showContent(obj){
  obj.grades.forEach(function(grade){
    console.log(grade);
  })
}
console.log(showContent(student));