Recognize anything which is not a number and return null or None

I was writing a factorial function and I wanted to add a logic that returns null or None if the user puts anything into the argument instead of numbers like symbols (@#!) or letters (s,z).
how can I recognize them and immediately stop my program and return null or None?
I know some ways but they will make my code really ugly and hard to read and long.

function factorial(n){

let num = Number(n);

// make num - 1 each time and push it into the list
let miniaturizedNum = [];
for (num; num > 0; num--){
    miniaturizedNum.push(num);
}

// Multiply the finalresult with the numbers in the list
// and finalresault is 1 because zero multiplied by anything becomes zero
let finalResault = 1;
for (let i = 0; i < miniaturizedNum.length; i++){
    finalResault *= miniaturizedNum[i];
}

return finalResault;

}

>Solution :

Assuming

• you want to accept strings representing a number and convert them to number

• you want do avoid non integers and negative numbers since you’re calcolating the factorial

  if( typeof( n ) !== 'string' &&
      typeof( n ) !== 'number' &&
      ( '' + n ).match(/^[0-9]+$/) === null ) {
      return null;
  }