why the function method inside of the object doesnt work?

Question

so im doing a object exercise about 2 guys and i have to calculte their BMI. my code is like this:

  const mark = {
  firstName: "Mark",
  lastName: "Miller",
  Mass: "78 kg",
  Height: "1.69 m",

  calcBMI: function () {
    return (
      this.Mass / this.Height ** 2 - this.Mass / (this.Height * this.Height)
    );
  },
};

const john = {
  firstName: "John",
  lastName: "Smith",
  Mass: "92 kg",
  Height: "1.95 m",

  calcBMI: function () {
    return (
      this.Mass / this.Height ** 2 - this.Mass / (this.Height * this.Height)
    );
  },
};

if (john.calcBMI > mark.calcBMI) {
  console.log(
    `${john.firstName} has a BMI of ${john.calcBMI} which is higher than ${
      mark.firstName
    } who has a BMI of ${mark.calcBMI()}`
  );
} else {
  console.log(
    `${mark.firstName} has a BMI of ${mark.calcBMI} which is higher than ${
      john.firstName
    } who has a BMI of ${john.calcBMI()}`
  );
}

the problem is the "this"? im having a problem with calcBMI function which its not doing the calculations of BMI