Count consecutive chunks of a number array

index.js

const assert = require('assert')

function f(n) {
  const [result, counter, num] = (n + '').split('').map(Number)
    .reduce(([result, count, num], n) => {
      if (num == null) return [result, 1, n]

      if (n !== num) return [
        [...result, [count, num]],
        1,
        n
      ]

      return [result, count + 1, num]
    }, [
      [],   // acc
      0,    // counter
      null, // current num
    ])

  return [...result, [counter, num]]
}

assert.deepStrictEqual(f(11), [[2, 1]]);
assert.deepStrictEqual(f(21), [[1, 2], [1, 1]]);
assert.deepStrictEqual(f(9000), [[1, 9], [3, 0]]);
assert.deepStrictEqual(f(222222222222), [[12, 2]]);
assert.deepStrictEqual(f(2331), [[1, 2], [2, 3], [1, 1]]);