why does this return undefined. Solving a coding challenge

Question

// Multiplicative Persistence Challenge
function persistence(num) {
var x, y, product;
x = num.toString().length; //length of inputed number
y = num; //The inputed Number

//Check the Number
if (x > 0){
console.log('Continue');
y.toString();

for(var i = 0; i < y.length; i++ ){
product *= y[i];
console.log(product);
}

return product;

}
else('Not Enough Digits To determine')
}
var output = persistence(23);
console.log(output);

Answer 1

Two issues: product isn't a number so you can't multiply with it and y isn't a string so you can't index through it. Add a value:
var x, y, product=1; // Line 3
Save the string:
y=y.toString(); // Line 10

Answer 2

So I need to set a value before I use them?

Answer 3

Yes, you need to set a value first, it's called initialization. Also, may I suggest using 'let' instead of 'var', as 'var'?

Answer 4

y.toString() //line 10 not define y, as new value
y = y.toString() // need to