vrandom/tests/shuffle.test.js

an easy way to work with random numbers in javascript

const shuffle = require('../lib/shuffle.js')

describe('shuffle', () => {
  it('returns array of same length of arg array', () => {
    const array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    const actual = shuffle(array)
    expect(typeof actual).toBe('object')
    expect(actual.length).toBe(array.length)
  })

  it('copys array, shuffles it and returns new array leaving original array unchanged', () => {
    const array = [1, 2, 3, 4, 5]
    const actual = shuffle(array)
    expect(actual).not.toBe(array)
    expect(array).toStrictEqual([1, 2, 3, 4, 5])
  })

  it.each([
    // Unfortunately, JS doesn't allow for seeding Math.random. That might change with proposals like
    // https://www.proposals.es/proposals/Math.seededRandoms() in the future.
    // For now, can mock Math.random -- unless there's a better, less tedious way.
    { array: [1, 2, 3, 4, 5], mockValues: [0.9, 0.9, 0.9, 0.9, 0.9], expected: [1, 2, 3, 4, 5] },
    { array: [1, true, 3], mockValues: [0.0105, 0.9595, 0.42], expected: [3, true, 1] },
    { array: [1], mockValues: [0.99891], expected: [1] },
    { array: [1], mockValues: [0.0021], expected: [1] },
    { array: ['a', false], mockValues: [0.0021, 0.03683], expected: [false, 'a'] },
    {
      array: ['a', 'b', 'c', 'd', 'e', 'f'],
      mockValues: [0.4, 0.0003, 0.999, 0.9451, 0.9, 0.9],
      expected: ['e', 'b', 'f', 'd', 'a', 'c']
    }
  ])(
    'shuffle($array) should return $expected when Math.random returns $mockValues individually',
    ({ array, mockValues, expected }) => {
      jest.spyOn(Math, 'random')

      mockValues.forEach(mockValue => {
        Math.random.mockReturnValueOnce(mockValue)
      })

      const actual = shuffle(array)

      expect(actual).toStrictEqual(expected)

      Math.random.mockRestore()
    }
  )
})