very concise 😀👍. It never crossed my mind.

wow!

Even though I also consider this as clever, wouldn't this be best practice (or maybe better practice)?

Same!

I am so disappointed in myself right now.

Excellent solution

came to the comments for this. Thanks!

This is a clever solution

Insane

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

Your reply is 6 months old, so I assume you figured it out, but for those reading and wondering the same.
||[] this part of the code, if no match was found, it returns an empty array and if you call .length on an empty array, it will return 0

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