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I am writing tests with jest in which I want to spy on some lodash functions, that I import separately in the module (opposed to importing the whole lodash module as _), e.g.

/** matrix.js **/
import shuffle from 'lodash/shuffle'
import pick from 'lodash/pick'

// ...

/**
 * Shuffles the order of the rows in the matrix. If a column/variable name
 * is specified, only the rows in this column are shuffled.
 *
 * @export
 * @param {array} matrix The matrix to be shuffled
 * @param {array} columns  Array containing the variable/column to be shuffled
 * @returns {array}
 */
export function shuffleVert (matrix, columns) {
  if (typeof (columns) === 'undefined' || (isArray(columns) && columns.length === 0)) {
    return shuffle(matrix)
  } else if (!isArray(columns)) {
    throw new TypeError('Invalid argument for columns specified to shuffleVert. Expects an array containing column names')
  } else {
    let grouped = unstack(matrix)
    let cols = pick(grouped, columns)
    cols = Object.entries(cols).reduce((prev, [key, values]) => {
      prev[key] = shuffle(values)
      return prev
    }, {})
    return stack({ ...grouped, ...cols })
 }

The shuffleVert function shuffles all rows of a matrix, or only those of the specified columns. Because it is difficult to test functions with a random output, to my knowledge, I just want to test if the shuffle and pick function of lodash have been called inside the tested function.

I currently have implemented a working spy procedure in my test module, but I don't think it's conventional or efficient, and I just think there must be a better way to do this...

/* matrix.test.js */
import {
  shuffleVert,
} from 'matrix'

/** Generate a mock functions to spy on lodash */
const mockShuffle = jest.fn()
jest.mock('lodash/shuffle', () => a => {
  const shuffle = jest.requireActual('lodash/shuffle')
  mockShuffle()
  return shuffle(a)
})

const mockPick = jest.fn()
jest.mock('lodash/pick', () => (a, b) => {
  const pick = jest.requireActual('lodash/pick')
  mockPick()
  return pick(a, b)
})

describe('reverseRows', () => {
  let srcMatrix
  beforeEach(() => {
    srcMatrix = [
      { number: 1, word: 'one' },
      { number: 2, word: 'two' },
      { number: 3, word: 'three' }
    ]
    mockShuffle.mockClear()
    mockPick.mockClear()
  })

  it('should shuffle the rows of the entire matrix with no argument for columns', () => {
    shuffleVert(srcMatrix)
    // 2 is weird, but seems correct.
    // It appears the shuffle function calls itself recursively
    expect(mockShuffle).toHaveBeenCalledTimes(2)
  })

  it('should only shuffle the rows of columns that were specified', () => {
    shuffleVert(srcMatrix, ['word'])
    expect(mockShuffle).toHaveBeenCalledTimes(2)
    expect(mockPick).toHaveBeenCalledTimes(2)
  })
})

I know jest has a spyOn functionality, but that only appears to work on object methods, thus

import * as lodash from 'lodash'
const shuffleSpy = jest.spyOn(lodash, 'shuffle')

results in the error Cannot spyOn on a primitive value; undefined given

What is generally the best way in jest to spy on methods of a module? Is there a better implementation for doing what I'm trying to achieve? Or is this already the way to go?

I am writing tests with jest in which I want to spy on some lodash functions, that I import separately in the module (opposed to importing the whole lodash module as _), e.g.

/** matrix.js **/
import shuffle from 'lodash/shuffle'
import pick from 'lodash/pick'

// ...

/**
 * Shuffles the order of the rows in the matrix. If a column/variable name
 * is specified, only the rows in this column are shuffled.
 *
 * @export
 * @param {array} matrix The matrix to be shuffled
 * @param {array} columns  Array containing the variable/column to be shuffled
 * @returns {array}
 */
export function shuffleVert (matrix, columns) {
  if (typeof (columns) === 'undefined' || (isArray(columns) && columns.length === 0)) {
    return shuffle(matrix)
  } else if (!isArray(columns)) {
    throw new TypeError('Invalid argument for columns specified to shuffleVert. Expects an array containing column names')
  } else {
    let grouped = unstack(matrix)
    let cols = pick(grouped, columns)
    cols = Object.entries(cols).reduce((prev, [key, values]) => {
      prev[key] = shuffle(values)
      return prev
    }, {})
    return stack({ ...grouped, ...cols })
 }

The shuffleVert function shuffles all rows of a matrix, or only those of the specified columns. Because it is difficult to test functions with a random output, to my knowledge, I just want to test if the shuffle and pick function of lodash have been called inside the tested function.

I currently have implemented a working spy procedure in my test module, but I don't think it's conventional or efficient, and I just think there must be a better way to do this...

/* matrix.test.js */
import {
  shuffleVert,
} from 'matrix'

/** Generate a mock functions to spy on lodash */
const mockShuffle = jest.fn()
jest.mock('lodash/shuffle', () => a => {
  const shuffle = jest.requireActual('lodash/shuffle')
  mockShuffle()
  return shuffle(a)
})

const mockPick = jest.fn()
jest.mock('lodash/pick', () => (a, b) => {
  const pick = jest.requireActual('lodash/pick')
  mockPick()
  return pick(a, b)
})

describe('reverseRows', () => {
  let srcMatrix
  beforeEach(() => {
    srcMatrix = [
      { number: 1, word: 'one' },
      { number: 2, word: 'two' },
      { number: 3, word: 'three' }
    ]
    mockShuffle.mockClear()
    mockPick.mockClear()
  })

  it('should shuffle the rows of the entire matrix with no argument for columns', () => {
    shuffleVert(srcMatrix)
    // 2 is weird, but seems correct.
    // It appears the shuffle function calls itself recursively
    expect(mockShuffle).toHaveBeenCalledTimes(2)
  })

  it('should only shuffle the rows of columns that were specified', () => {
    shuffleVert(srcMatrix, ['word'])
    expect(mockShuffle).toHaveBeenCalledTimes(2)
    expect(mockPick).toHaveBeenCalledTimes(2)
  })
})

I know jest has a spyOn functionality, but that only appears to work on object methods, thus

import * as lodash from 'lodash'
const shuffleSpy = jest.spyOn(lodash, 'shuffle')

results in the error Cannot spyOn on a primitive value; undefined given

What is generally the best way in jest to spy on methods of a module? Is there a better implementation for doing what I'm trying to achieve? Or is this already the way to go?

• javascript
• unit-testing
• lodash
• jestjs
• es6-modules

Share Improve this question Follow edited Dec 6, 2018 at 12:59 Daniel Schreij asked Dec 6, 2018 at 10:50 Daniel SchreijDaniel Schreij 76311 gold badge1111 silver badges2828 bronze badges 5

• Is lodash specified in package.json? It looks like lodash is undefined in jest.spyOn(lodash, 'shuffle'). – Dinesh Pandiyan Commented Dec 6, 2018 at 11:02
• Yes, lodash is present and is used throughout the app. – Daniel Schreij Commented Dec 6, 2018 at 12:58
• Did you ever get this issue resolved? I'm currently struggling with the same problem. – Khaladin Commented Jul 28, 2020 at 19:53
• This is too lang ago so I don't really remember how I tackled this. I think I took a pletely different approach from using jest spies though... – Daniel Schreij Commented Aug 4, 2020 at 9:15
• 1 Does this answer your question? How to spy on a default exported function with Jest? – Xeltor Commented Oct 27, 2021 at 18:56

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1 Answer 1

Sorted by: Reset to default 3

Please try this and let me know if it works for you

import { shuffleVert } from 'matrix';
const shuffleVertRef = {
    shuffleVert
};
let spyShuffleVert;

beforeEach(() => {
    spyShuffleVert = jest.spyOn(shuffleVertRef, "shuffleVert");
}

afterEach(() => {
    jest.clearAllMocks();
}

it('should shuffle the rows of the entire matrix with no argument for columns', () = {
    let srcMatrix = [
        { number: 1, word: 'one' },
        { number: 2, word: 'two' },
        { number: 3, word: 'three' }
    ];

    shuffleVert(srcMatrix);
    expect(spyShuffleVert).toHaveBeenCalledTimes(2);
}

Then if you want to check any other function execution times, just add them to a/that constant and spyOn them below.

I too don't have a full understanding of this, but from the little I read, the latest versions of JavaScript have this principle called "single source of truth" which is required for some things to work properly.

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