NaN is allowing a combination of numbers and letters in this code

Why is NaN allowing a combination of numbers and letters in my code?

let gameIsStarted = false;
let guesses = [];
const startButton = document.getElementById('startButton');
startButton.addEventListener('click', function() {
gameIsStarted = !gameIsStarted;
if (gameIsStarted) {
let n = 0;
while (n <= 0) {
n = Math.round(parseFloat(prompt('You have begun to play. What is the highest number you wish to be able to guess?')));
if (isNaN(n)) {
alert('That is not a number! Please enter a number.');
n = 0;
} else if (n < 1) {
alert('Please enter a number 1 and above.');
n = 0;
}
}
let secretNum = secretNumber(n);
guess(secretNum);
function secretNumber(n) {
return Math.floor(Math.random() * n) + 1;
}
function userGuess() {
let guessValue = parseInt(prompt('What do you think the number is?'));
guessValue = Math.round(guessValue);
return guessValue;
}
function guess(secretNum) {
let userGuessValue = userGuess();
while (userGuessValue !== secretNum) {
switch (true) {
case isNaN(userGuessValue):
alert('That is not a number!');
userGuessValue = userGuess();
break;
case userGuessValue <= 0:
alert('That number is too low. Pick a number that is at least 1.');
userGuessValue = userGuess();
break;
case userGuessValue > n:
alert('That number is not in range, try again.');
userGuessValue = userGuess();
break;
case userGuessValue > secretNum:
alert('No, try a lower number.');
userGuessValue = userGuess();
break;
case userGuessValue < secretNum:
alert('No, try a higher number.');
userGuessValue = userGuess();
break;
}
}
alert('You got it!');
}
}
gameIsStarted = false;
});

I have tried to use Metacharacters, I have asked everyone I know, but this will allow things like 13a or 17w. When I looked up the documentation on NaN, it worked there, so why is it not implementing in my code?

> this will allow things like 13a or 17w

The culprits are parseInt and parseFloat. They allow digits to be followed by other characters.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

console.log(parseInt('777$#@!*')); // 777
console.log(parseFloat('123.56abcd')); // 123.56

<!-- end snippet -->

To validate the input strictly, use unary number coersion.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

console.log(+'777$#@!*'); // NaN
console.log(+'777'); // 777
console.log(+'123.56abcd'); // NaN
console.log(+'123.56'); // 123.56

<!-- end snippet -->

Then you can perform other checks depending on what sort of number you need.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function testNumber (value) {
console.log('Value:', value);
console.log('> integer OR fractional:', Number.isFinite(value));
console.log('> integer:', Number.isInteger(value));
console.log('> fractional:', Number.isFinite(value) && value != Math.floor(value));
}
testNumber(+'777$#@!*');
testNumber(+'777.888');
testNumber(+'1234');

<!-- end snippet -->