All You Need To Know About This Function

All You Need To Know About This Function

Jest Spyon is a well-liked JavaScript testing framework. It is used to check the recordsdata and create mocks or let the consumer management the general Jest’s conduct.All You Need To Know About This Function

In this text, the reader will establish the the explanation why Jest is used as a operate, together with its related examples. Let’s get began!

What Is Jest Spyon?

Jest spyOn is a technique used to spy on a technique name on an object. It is used within the JavaScript testing framework. It can also be very helpful when the Jest mock module or mock operate is just not one of the best software for the job.

Generally, the dependencies are changed with mocks whereas writing unit checks. However, one other technique to supplant dependencies is to make use of Spies; it’s useful throughout unit testing. Spies robotically document the data relying on how they’re referred to as. Therefore, Jest spyOn can mock or spy a operate on an object.


Import {count on, jest, take a look at} from ‘@jest/globals’;

Jest Spyon Mock Functions Examples

There are a number of Jest.Spyon instances of its mock capabilities. Let’s see them together with their examples.

– Function 1: Jest.fn

This operate returns a brand new and unused mock operate in this system. It optionally takes a mock implementation.

Let’s see the instance.

const mockFn = jest.fn();


count on(mockFn).toHaveBeenCalled();

// Mock implementation:

const returnsTrue = jest.fn(() => true);

console.log(returnsTrue()); // true;

– Function 2: Jest.spyon(Object, Methodname)

This operate creates a mock operate that’s much like Jest.fn, but it surely additionally tracks calls to object[methodName]. However, it returns a Jest mock operate. Do do not forget that the Jest.spyOn additionally calls the spied technique by default. This causes a unique behaviour from many of the different take a look at libraries.1674304970 21 All You Need To Know About This Function

Additionally, if the programmer needs to overwrite the unique operate, they’ll use:

  1. spyOn(object, techniqueName).mockImplementation(() => customImplementation)
  2. object[methodName] = jest.fn(() => customImplementation);

Let’s see an instance.

const shortvideo = {

play() {

return true;



module.exports = shortvideo;

const shortvideo = require(‘./shortvideo’);

afterEach(() => {

// restore the spy created with spyOn



take a look at(‘plays shortvideo’, () => {

const spy = jest.spyOn(shortvideo, ‘play’);

const isPlaying =;

count on(spy).toHaveBeenCalled();

count on(isPlaying).toBe(true);


– Function 3: Jest.spyon(Object, Methodname, Accesstype?)

Since the discharge of Jest 22.1.0+, the Jest.spyOn operate takes an non-compulsory third argument of accessType, both ‘get’ or ‘set’. This is proved to be helpful when the programmer needs to spy on a getter or a setter, respectively.

Let’s see the instance:

const shortvideo = {

// it’s a getter!

get play() {

return true;



module.exports = shortvideo;

const audiovoice = {

_volume: false,

// it’s a setter!

set quantity(worth) {

this._volume = worth;


get quantity() {

return this._volume;



module.exports = audiovoice;

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Example take a look at:

const audiovoice = require(‘./audiovoice’);

const shortvideo = require(‘./shortvideo’);

afterEach(() => {

// spy created with spyOn is restored



take a look at(‘plays shortvideo’, () => {

const spy = jest.spyOn(shortvideo, ‘play’, ‘get’); // ‘get’ is handed right here

const isPlaying =;

count on(spy).toHaveBeenCalled();

count on(isPlaying).toBe(true);


take a look at(‘plays audiovoice’, () => {

const spy = jest.spyOn(audiovoice, ‘volume’, ‘set’); // ‘set’ is handed right here

audiovoice.quantity = 100;

count on(spy).toHaveBeenCalled();

count on(audiovoice.quantity).toBe(100);


Jest Spyon Fake Timers and Real Timers

You can change the implementation of a number of timer capabilities like setTimeout() utilizing the jest spyon faux timers operate. Returning the timers to their regular conduct can also be fairly straightforward. You simply have to make use of the jest.useRealTimers() operate to do that.

– Function 1: Jest.usefaketimers

Using the Jest.useFakeTimers() operate, the programmers will use faux timers for all of the checks inside the file till the unique timers are restored by utilizing the Jest.useRealTimers() operate. Moreover, the programmer can name Jest.useFakeTimers() or Jest.useRealTimers() capabilities from wherever, similar to top-level, inside a take a look at block, and many others.

Do be aware that it is a international operation, and it’ll have an effect on different checks inside the identical file. Calling the Jest.useFakeTimers() operate as soon as once more in the identical take a look at file will reset the inner state, such because the timer depend. It may also reinstall faux timers by utilizing the supplied choices.

Let’s see an instance for a greater understanding:

take a look at(‘automatically advance the timers’, () => {

jest.useFakeTimers({advanceTimers: true});


take a look at(‘don’t advance the timers and don’t faux the ‘performance’’, () => {

jest.useFakeTimers({doNotFake: [‘performance’]});


take a look at(‘uninstall all the fake timers for the rest of the tests in the file’, () => {



– Function 2: Jest.useRealTimers()

When utilized in a program, this operate instructs the Jest operate to revive the unique implementations of the efficiency, international date, time, and timer APIs. For instance, the programmer could name Jest.useRealTimers() contained in the “afterEach” hook to revive the timers after every take a look at.1674304972 421 All You Need To Know About This Function

Let’s see the next instance:

afterEach(() {



take a look at(‘do something with the fake timers’, () {



take a look at(‘do something with the real timers’, () {


Jest SpyOn Mock Module

There are numerous Jest spyOn mock modules. However, let’s see one among its capabilities and perceive the idea with the assistance of an instance.

– Function 1: Jest.EnableAutomock()

Using the Jest.enableAutomock() operate in a program permits the automated mocking within the module loader.1674304973 360 All You Need To Know About This Function

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Let’s use an instance to grasp:

export default {

authorize: () {

return ‘gold token’;


isAuthorized: secret => secret === ‘wizard’,




import utils from ‘../utils’;

take a look at(‘original implementation’, () => {

count on(utils.authorize._isMockFunction).toBeTruthy();

count on(utils.isAuthorized._isMockFunction).toBeTruthy();


Testing the Form Render in Javascript After Using Jest Spyon

After all the setup is full, the primary fundamental take a look at is to examine if the display screen initially masses with the textual content and type. The first take a look at the programmer does is to make sure the display screen appears to be like as desired. The code for the testing is:

take a look at(‘Renders the initial heading and form with elements correctly’, () {

render(<App />);

const titleElement = display screen.getByText(/Name Check/o);

count on(titleElement).toBeInTheDocument();

count on(display screen.getByRole(‘form’)).toBeInTheDocument();

count on(display screen.getByRole(‘textbox’)).toBeInTheDocument();

count on(display screen.getByRole(‘button’, {identify: ‘Get Cryptocurrencies’})).toBeInTheDocument();


In this program, the take a look at is appropriately named, and it renders the preliminary heading and type with components appropriately. It follows a couple of steps, similar to:

  1. The App element is rendered upon executing this system.
  2. Then the title ingredient is searched by the textual content supplied within the testing library.
  3. The spyon react testing library might be used as a operate on this program.
  4. After that, the programmer ought to make sure the ingredient visibility within the doc with the toBeInTheDocument technique.

Difference Between Jest Spyon vs Mock

The fundamental distinction between Jest spyon vs mock is that the Jest.mock operate is used to mock a JavaScript module and however, the Jest.spyOn operate is used to mock what the programmer imported from that module. However, it requires an object to spy on.

Let’s take a look at the straightforward instance under:

import { getAddress, getName } from ‘./utils.js’

import * as utilsFunc from ‘./utils.js’

Note that utils.js is a module on this program. The programmer can use Jest.mock to mock this module, and getName and getAddress might be robotically mocked. GetName and getAddress are what the programmer imported from the utils.js module.

The consumer can use Jest.spyOn to return specified values if getName or getAddress known as. However, Jest requires the consumer to first present an object so as to use spyOn. The consumer import considerably impacts how they mock a specific operate in several eventualities. Keep in thoughts among the essential notes:

  1. The programmer ought to use Jest.mock to mock a module.
  2. Use jest.spyOn to mock capabilities from an object.
  3. First, do a mock, then proceed with rendering later.


1. What Does Jest.clearallmocks() Function Do?

The jest.clearAllMocks() operate clears all of the mock.calls, mock.situations, mock.contexts and mock.outcomes properties of all mocks. This operate is equal to calling a .mockClear() operate on each mocked operate. It returns the jest object for chaining.

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2. What Does Jest.restoreallmocks() Do?

The jest.restoreAllMocks() operate restores all of the mocks again to their authentic worth. This operate is equal to calling a .mockRestore() operate on each mocked operate. However, the Jest.restoreAllMocks() solely works when the mock is created with Jest.spyOn.

The different mocks might be required to be performed manually to revive them.

3. What Does the Jest.runonlypendingtimers() Function Do within the Program?

The Jest.runOnlyPendingTimers() operate solely executes the macro-tasks which can be presently pending, similar to duties which can be queued by setTimeout() or setInterval() capabilities, when any of the pending macro-tasks schedule the brand new macro-tasks, these duties is not going to be executed by this name.

This is beneficial in a program when the module being examined schedules a setTimeout(), and its callback schedules one other setTimeout() recursively. This means the scheduling course of by no means stops. In such conditions, utilizing Jest.runOnlyPendingTimers() is beneficial.

4. What Is Jest.advancetimersbytime(Mstorun)?

The jest.advanceTimersByTime(msToRun) operate solely executes the macro activity queue that’s queued by setTimeout() or setInterval() and setImmediate(). Its working mechanism is much like that of Jest.runOnlyPendingTimers() operate. When the programmer calls this API, all of the timers are superior by msToRun milliseconds.

Furthermore, all of the pending macro-tasks which can be queued by way of setTimeout() or setInterval() could be executed inside the given timeframe after utilizing this operate. Moreover, if these macro-tasks schedule new macro-tasks to be executed on the identical timeframe, these duties might be additional executed till there are not any extra macro-tasks left within the queue.


After studying this informative article, the reader will find out about Jest and its recognition, together with examples for higher understanding. Some key factors are:

  • It is a purposeful app with React that guesses the nationality of a given identify by calling an API.
  • Jest spyon has the power to spy on the strategy calls and parameters like Jest Mock/fn.
  • Mock can solely reply with mocks and might’t name the underlying actual code, however Jest can.
  • The Jest spyon imported operate might be utilized on the Jest spyon typescript in addition to on JavaScript too.
  • The jest spyon context needs to be clear when the programmer is utilizing the jest spyon operate with out an object in this system.

Finally, this text will assist the reader use the applying successfully and effectively. Thank you for studying!

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