No, it returns 0, try it yourself. There is even a test for 0 that expects 0.

great solution, but if we use digital_root(0) result 9

i am stupid

same

great solution

I have spent to solve this kata around 14 minutes (
After seing this solution, it is just SUPER! No words 👍

wow how can you come with this solution, it's amazing

It's amazing!

I am so disappointed in myself right now.

Excellent solution

I see... I have to go back to studying.

I just tested it again in codewars with 10,000,000,000 and it still works fine. I also tried with other several higher numbers and it always works fine and returns the expected value:

• 92,000,000,123 and returned 8 (true)
• 999,999,999,999 and returned 9 (true)
• 9,111,999,999,999 and returned 3 (true)
• 88,888,888,888,888 and returned 4 (true)
• 123,123,123,123,123 and returned 3 (true)
• 123,456,789,012,345 and returned 6 (true)

Here you can try the tests:

const chai = require("chai");
const assert = chai.assert;
chai.config.truncateThreshold=0;

describe("Tests", () => {
  it("test", () => {
    assert.strictEqual( digitalRoot(10000000001), 2)
    assert.strictEqual( digitalRoot(92000000123), 8)
    assert.strictEqual( digitalRoot(999999999999), 9)
    assert.strictEqual( digitalRoot(9111999999999), 3)
    assert.strictEqual( digitalRoot(88888888888888), 4)
    assert.strictEqual( digitalRoot(123123123123123), 3)
    assert.strictEqual( digitalRoot(123456789012345), 6)
  });
});

This dude must be a scientist in NASA