newSquare.js

export const Square = () => {
  const   createLength = (length) => {
    return 'x'.repeat(length)
  }

  const createHeight = (length) => {
    return 'x' + ' '.repeat(length - 2) + 'x'
  }

  const createArray = (length) => {
    return Array.from('x'.repeat(length))
  }

  const isFirstOrLastElement = (index, last) => {
    return (index === 0 || index === last)
  }

  const createLengthAndHeight = (element, index, array) => {
    if (isFirstOrLastElement(index, array.length - 1)) {
      return createLength(array.length)
    } else {
      return createHeight(array.length)
    }
  }

  const createSquare = (length) => {
    return createArray(length)
      .map(createLengthAndHeight.bind(this))
  }

  const drawSquare = (length) => {
    return createSquare(length).join('\n')
  }

  return { drawSquare }
}

square.js

class Square {
  constructor () {
    this.size = 0
  }

  createLength (length) {
    return 'x'.repeat(length)
  }

  createHeight (length) {
    return 'x' + ' '.repeat(length - 2) + 'x'
  }

  createArray (length) {
    return Array.from('x'.repeat(length))
  }

  isFirstOrLastElement (index, last) {
    return (index === 0 || index === last)
  }

  createLengthAndHeight (element, index, array) {
    if (this.isFirstOrLastElement(index, array.length - 1)) {
      return this.createLength(array.length)
    } else {
      return this.createHeight(array.length)
    }
  }

  createSquare (length) {
    return this.createArray(length)
      .map(this.createLengthAndHeight.bind(this))
  }

  drawSquare (length) {
    return this.createSquare(length).join('\n')
  }
}

squareSpec.js

const square = require('../square')

describe('Square should', () => {
  it('draw a square of length 2', () => {
    expect(square.drawSquare(2)).toEqual('xx\nxx')
  })

  it('draw a square of length 3', () => {
    expect(square.drawSquare(3)).toEqual('xxx\nx x\nxxx')
  })

  it('draw a square of length 4', () => {
    expect(square.drawSquare(4)).toEqual('xxxx\nx  x\nx  x\nxxxx')
  })
})