Javascript – How to Create Functions to replace text, factorial, check if it’s even

// isEven f(x)
function isEven(arg) {
 return arg%2 === 0
}
// factorial f(x)
function factorial(num) {
 var result = 1;
 for(var i = 2; i <= num; i++) {
  result = result * i;
 }
 return result;
}
// kebabToSnake() Replace a string "-" with '_'s
function kebabToSnake(str) {
 var newStr = str.replace(/-/g , "_");
 return newStr;
}