JavaScript for loop that alternates two constants

I’ve been trying to use a for loop to make a code that alternates between two strings and ends with .toUpperCase , but I’m completely stuck. I’m “able” to do it with an array with two strings (and even so it has a mistake, as it ends with the first string of the array…), but not with two separate constants.
Could anyone offer some help?

function repeteFrases(num) {
  const frases = ["frase um", "frase dois"];
  let result = "";
  for (let i = 0; i < num; i++) {
    result += i === num - 1 ? frases[0].toUpperCase() : `${frases[0]}, ${frases[1]}, `;
  }
  return result;
}
console.log(repeteFrases(2));