randomMocks.test.js

const crypto = require('crypto')

describe('Predictable Randomness', () => {
  describe('Math.random', () => {
    const firstResult = 0.37454011430963874
    const secondResult = 0.7965429842006415
    beforeEach(() => Math.__clearChances__())
    it('should produce these two numbers first by default', () => {
      expect(Math.random()).toEqual(firstResult)
      expect(Math.random()).toEqual(secondResult)
    })
    it('should produce these two numbers again (due to the beforeEach)', () => {
      expect(Math.random()).toEqual(firstResult)
      expect(Math.random()).toEqual(secondResult)
    })
    it('should be affected by intermittent calls', () => {
      expect(Math.random()).toEqual(firstResult)
      Math.random()
      expect(Math.random()).not.toEqual(secondResult)
    })
    it('should not be affected by new calls with a different seed', () => {
      expect(Math.random()).toEqual(firstResult)
      Math.random('never mind me')
      expect(Math.random()).toEqual(secondResult)
    })
    it('should produce a different but repeatable sequence for a custom seed', () => {
      expect(Math.random("I'm different!")).toEqual(0.16998715861700475)
      expect(Math.random("I'm different!")).toEqual(0.5475940068718046)
    })
    it('should produce a different but repeatable sequence for a custom seed', () => {
      expect(Math.random("I'm different!")).toEqual(0.16998715861700475)
      expect(Math.random("I'm different!")).toEqual(0.5475940068718046)
    })
    it('should produce yet a different sequence for a different custom seed', () => {
      expect(Math.random('hi')).toEqual(0.6855331424158067)
      expect(Math.random('hi')).toEqual(0.565650706877932)
    })
  })
  describe('crypto.randomBytes', () => {
    const b64it = buffer => buffer.toString('base64') // for brevity's sake
    const firstResult = 'X8vzLrvHmZgncicZDnXdVQ=='
    const secondResult = 'mSS1pgUO+LjV8DYALv4ung=='
    beforeEach(() => crypto.__clearChances__())
    it('should produce these two strings first by default', () => {
      expect(b64it(crypto.randomBytes(16))).toEqual(firstResult)
      expect(b64it(crypto.randomBytes(16))).toEqual(secondResult)
    })
    it('should produce a buffer of the correct length', () => {
      expect(crypto.randomBytes(16).length).toEqual(16)
      expect(crypto.randomBytes(Math.floor(Math.random() * 100) + 50).length).toEqual(87)
    })
    it('should pass the buffer back through the callback without a custom seed', () => {
      const callback = jest.fn()
      crypto.randomBytes(16, callback)
      expect(callback.mock.calls.length).toEqual(1)
      expect(callback.mock.calls[0][0]).toBeNull()
      expect(callback.mock.calls[0][1]).toBeInstanceOf(Buffer)
    })
    it('should pass the buffer back through the callback with a custom seed', () => {
      const callback = jest.fn()
      crypto.randomBytes(16, 'custom seed', callback)
      expect(callback.mock.calls.length).toEqual(1)
      expect(callback.mock.calls[0][0]).toBeNull()
      expect(callback.mock.calls[0][1]).toBeInstanceOf(Buffer)
    })
    it('should produce these two strings again (due to the beforeEach)', () => {
      expect(b64it(crypto.randomBytes(16))).toEqual(firstResult)
      expect(b64it(crypto.randomBytes(16))).toEqual(secondResult)
    })
    it('should be affected by intermittent calls', () => {
      expect(b64it(crypto.randomBytes(16))).toEqual(firstResult)
      crypto.randomBytes(16)
      expect(b64it(crypto.randomBytes(16))).not.toEqual(secondResult)
    })
    it('should not be affected by new calls with a different seed', () => {
      expect(b64it(crypto.randomBytes(16))).toEqual(firstResult)
      crypto.randomBytes(16, 'Never mind me')
      expect(b64it(crypto.randomBytes(16))).toEqual(secondResult)
    })
    it('should produce a different but repeatable sequence for a custom seed', () => {
      expect(b64it(crypto.randomBytes(16, "I'm different!"))).toEqual('K4yK2B8VGxxISK0d73h46Q==')
      expect(b64it(crypto.randomBytes(16, "I'm different!"))).toEqual('BqU35hvdIw0EZiIm77Dltg==')
    })
    it('should produce a different but repeatable sequence for a custom seed', () => {
      expect(b64it(crypto.randomBytes(16, "I'm different!"))).toEqual('K4yK2B8VGxxISK0d73h46Q==')
      expect(b64it(crypto.randomBytes(16, "I'm different!"))).toEqual('BqU35hvdIw0EZiIm77Dltg==')
    })
    it('should produce yet a different sequence for a different custom seed', () => {
      expect(b64it(crypto.randomBytes(16, 'hi'))).toEqual('r5DNZ0l1eCaYnMrzazXzNQ==')
      expect(b64it(crypto.randomBytes(16, 'hi'))).toEqual('zUzV3TuaKZlnp7vRDRxo4w==')
    })
  })
})