How to swap first and last digits of a number using for loop in Javascript

I need to ask user to input a 3 digit number and then swap the first and last numbers using a for loop. This is what I have so far, but I’m stuck. Using a for loop seems illogical, but that is what I have to do:

num = prompt("Please input a number with 3 digits.");
let firstDigit = num[0];
let secondDigit = num[1];
let lastDigit = num[2];

for (firstDigit < 10; secondDigit < 10; lastDigit < 10); {
    console.log(lastDigit + secondDigit + firstDigit);
}

Please help!

Thanks