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function digital_root(n) {
var digits = n.toString().split("").map(Number);
var sum = 0;
for (var i = 0; i < digits.length; i++) {
sum += digits[i];
}
var sumString = sum.toString();
if (sumString.length > 1) {
var sumDigits = sumString.split("").map(Number);
var firstSumDigit = sumDigits.slice(0);
var lastSumDigit = sumDigits.slice(-1);
return firstSumDigit[0] + lastSumDigit[0];
} else {
return sum;
}
}
A1 under study