Software Engineering Coding Challenge 4 (Javascript)

/**
 * Requirements:
 *
 * Write a function that accepts an integer and returns a “validation” number. This validation number is calculated by
 * adding all the digits in the input.
 *
 * If the result of this sum has more than a single digit, another iteration is required,
 * repeat the process until a single digit number is calculated.
 */

function calculateValidationNumber(input) {}

// calculateValidationNumber(444444); // Output: 6