The || operator checks for falsy values and 0 is a falsy value, which would accidentally apply the default tax rate. To fix this, instead of checking for a falsy value, you can use the nullish coalescing (??) operator.

function calcPrice(price, tax, discount) {
  tax = tax ?? 0.05;
  discount = discount ?? 0;

  return price - price * discount + price * tax;
}

The nullish coalescing operator works because it returns its right-hand side operand when its left-hand side operand is null or undefined (NOT 0), and otherwise returns its left-hand side operand.