re: Solving the Digital Root Algorithm using JavaScript VIEW POST


Can I cheat?

function d_root(num) {
  return (num % 9) || 9;



I am really curious how do you know this?


I learnt it first as a math trick for children in Vietnam: to know if something is divisible by 9, you calculate its digitial root and see if the digital root is divisible by 9 (same goes for 3).

When I grew up, I was curious and tried to prove that mathematically, which is how I ended up with digitalRoot(num) = num % 9 (or 9 if divisible by 9) :D

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