Why is NaN value outputted?
<p>I'm trying to solver <a href="https://projecteuler.net/problem=8" rel="nofollow noreferrer">Project Euler #8</a>. It asks to find the largest product of 13 consecutive digits in the following 1000-digit number. Here is my code (in JavaScript):</p>

<pre><code>var bigNumber = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";
for(var i = 0, largest = 0; i &lt; 987; i++) {
  for(var j = 0, product = 1; j &lt; 13; j++)
    product *= parseInt(bigNumber.substring(i*13+j, i*13+j+1));
  largest = Math.max(largest, product);
}
</code></pre>

<p>The answer is stored in <code>largest</code>. For some reason, <code>largest</code> is <code>NaN</code>, and I do not know why.</p>

<p>I've checked all the data types of all the numbers, especially <code>product</code> and <code>largest</code>, and they always turn up as "number." Their values are never infinity, which may <a href="https://stackoverflow.com/a/2801629/2397327">also cause a NaN value</a>.</p>

<p>I was also thinking maybe the products might be too large JavaScript to handle, but I thought it might have thrown a specific error for that.</p>

<p>What is causing this error and how should I solve it?</p>