easyLogic ErrorsJavaScript

The silent NaN

A developer asked an AI assistant to calculate a cart line total from a quantity field. The suggested code passes every manual test where a number is typed, but when a customer clears the field before checkout, the page silently shows a broken price instead of stopping them.

cart.js

function calculateLineTotal(price, quantityInput) {
  const quantity = parseInt(quantityInput);
  return price * quantity;
}

const total = calculateLineTotal(499, ''); // customer cleared the field
renderTotal(total);

What does renderTotal(total) end up displaying, and why does nobody notice?

'₹NaN' — parseInt('') returns NaN, and NaN silently propagates through the multiplication with no error thrown.

'₹0' — parseInt('') defaults to 0, so the total is simply zero.

'₹499' — an empty quantity is treated as a quantity of 1.

The function throws a TypeError before renderTotal is ever called.