https://www.codewars.com/kata/5a405ba4e1ce0e1d7800012e/javascript

index.js

function trunkSize (n) {
  return Math.floor(n / 3)
}

function drawTopOfTree (chars, n) {
    const width = (n - 1) * 2 + 1
    let currentPos = 0
    return new Array(n).fill(0).map((_, index) => {
      const placeholders = new Array(index + 1).fill('x').join(' ')
      const spaceLeft = width - placeholders.length
      const borders = ''.padStart(spaceLeft / 2, ' ')
      return borders + placeholders.replace(/x/g, () => chars[currentPos++ % chars.length]) + borders
    })
  }


module.exports = {
  trunkSize,
  drawTopOfTree
}

if (require.main === module) {
  const [,, chars, rawN] = process.argv
  const n = parseInt(rawN, 10)
  console.log(drawTopOfTree(chars, n).join('\n'))
  for (let i = 0; i < trunkSize(n); ++i) {
    console.log('|'.padStart(n, ' '))
  }
}

index.spec.js

const assert = require('assert/strict')
const { trunkSize, drawTopOfTree } = require('..')

describe('Christmas Tree', () => {
  describe('Computing the trunk size', () => {
    const expectations = [{
      size: 3,
      trunk: 1
    }, {
      size: 4,
      trunk: 1
    }, {
      size: 5,
      trunk: 1
    }, {
      size: 6,
      trunk: 2
    }, {
      size: 8,
      trunk: 2
    }, {
      size: 13,
      trunk: 4
    }]

    expectations.forEach(({ size, trunk }) => {
      it(`returns ${trunk} for n = ${size}`, () => {
        assert.equal(trunkSize(size), trunk)
      })
    })
  })

  describe('Top of the tree', () => {
    it('drawing the tree with \'x\' char and n = 3', () => {
      assert.deepEqual(drawTopOfTree('x', 3), [
        '  x  ',
        ' x x ',
        'x x x'
      ])
    })

    it('drawing the tree with \'*\' char and n = 3', () => {
      assert.deepEqual(drawTopOfTree('*', 3), [
        '  *  ',
        ' * * ',
        '* * *'
      ])
    })

    it('drawing the tree with \'*@\' chars and n = 3', () => {
      assert.deepEqual(drawTopOfTree('*@', 3), [
        '  *  ',
        ' @ * ',
        '@ * @'
      ])
    })

    it('drawing the tree with \'x\' char and n = 4', () => {
      assert.deepEqual(drawTopOfTree('x', 4), [
        '   x   ',
        '  x x  ',
        ' x x x ',
        'x x x x'
      ])
    })

    it('drawing the tree with \'1234\' chars and n = 4', () => {
      assert.deepEqual(drawTopOfTree('1234', 4), [
        '   1   ',
        '  2 3  ',
        ' 4 1 2 ',
        '3 4 1 2'
      ])
    })
  })
})