Generate random password based on user input in Javascript

The assignment is to prompt for length of the password and character type from the user, then generate a random password. I think the for loop isn't working correctly. The retVal is returned empty because the for loop isn't passing it anything. I tried removing the charAt function and having the Math.floor give me just and index, that just gave me undefinedundefinedundefinedundefinedundefinedundefined. Back with the regular charAt function I'm getting nothing.

var length = prompt("How many characters will your password be? Enter a number between 8 and 128");

//ask for character type
var charType = prompt("Enter a character type: special, numeric, uppercase, lowercase.");

//generate password
function generatePassword() {
  //evaluate character type
  var charSet = "";
  if( charType.toLowerCase === "lowercase" ) {
    charSet = "abcdefghijklmnopqrstuvwxyz";
  } else if( charType.toLowerCase === "uppercase" ) {
    charSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  } else if( charType.toLowerCase === "numeric" ) {
    charSet = "0123456789";
  } else if( charType.toLowerCase === "special" ) {
    charSet = " !\"#$%&'()*+,-./:;<=>?@[\]^_`{|}~";
  } 
  //return value
  var retVal = "";
  //for (var i = 0, n = charSet.length; i < length; i++) {
    for (var i = 0, n = length; i < length; i++) {
    //picks a character within charSet at index of random number
    retVal += charSet.charAt(Math.floor(Math.random() * n));
  }
  console.log(retVal);
  return retVal;
}```