Parts of the calculator module unit test

test-calculator.js

const {describe, it} = require('mocha')
const {expect} = require('chai')
const calculator = require('../../lib/calculator')

describe('calculator', function () {
  const stream = (characters, calculatorState = calculator.initialState) =>
    !characters
      ? calculatorState
      : stream(characters.slice(1),
               calculator.nextState(calculatorState, characters[0]))

  it('should show initial display correctly', () => {
    expect(calculator.initialState.display).to.equal('0')
  })
  it('should replace 0 in initialState', () => {
    expect(stream('4').display).to.equal('4')
  })
//...