Product of Digits of Sum

Create a function that takes one, two or more numbers as arguments and adds them together to get a new number. The function then repeatedly multiplies the digits of the new number by each other, yielding a new number, until the product is only 1 digit long. Return the final product.

Examples

sumDigProd(16, 28) ➞ 6
// 16 + 28 = 44
// 4 * 4 =  16
// 1 * 6 = 6

sumDigProd(0) ➞ 0

sumDigProd(1, 2, 3, 4, 5, 6) ➞ 2

Notes

The input of the function is at least one number.