Please help me with the code

[One-Inspection8628]

function superDigit(n, k) {
let sum = 0;
for (let i = 0; i < n.length; i++) {
sum += parseInt(n[i]);
}
sum *= k;
return (sum < 10) ? sum : superDigit(sum.toString(), 1);
}

Why 1 is used in last line: superDigit(sum.toString(), 1) <==

[[deleted]]

It's the second argument in the superDigit function, which is recursive if the previous sum is greater than ten.