Random Password Generator JavaScript

I am using window prompts and window.confirm to ask the user if they want to use certain characters in their password. (I know I can do checkboxes with some HTML, but this must be JS). How do I pass the confirm answers to the function and then use Math.floor(Math.random() * length)?

const key_strings = {
    lowercase: 'abcdefghijklmnopqrstuvwxyz',
    uppercase: 'ABCDEFGHIJKLMNOPQRSTUVWXYZ',
    number: '0123456789',
    symbol: " !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~",
};

function generatePassword() {
  passwordCharSet = "";

  var length = window.prompt("Enter a number from 8 to 128 for password length.");

  var lowercase = window.confirm("Would you like to use lowercase letters?");
    if (lowercase == true) {
        passwordCharSet += lowercase;
    };

  var uppercase = window.confirm("Would you like to use uppercase letters?");
    if (uppercase == true) {
        passwordCharSet += uppercase;
    };

  var symbols = window.confirm("Would you like to use symbols?");
    if (symbols == true) {
        passwordCharSet += symbols;
    };

  var numbers = window.confirm("Would you like to use numbers?");
    if (numbers == true) {
        passwordCharSet += numbers;
    };

  for (let i = 0; i < length; i++) {
    _______________ Math.floor(Math.random() * length)
  }
  }

I'm not sure what else to put with the math.floor either.