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When writing tests it's only a matter of time before you need to create a "fake" version of an internal — or external — service. This is commonly referred to as mocking. WebdriverIO provides utility functions to help you out. You can import { fn, spyOn, mock, unmock } from '@wdio/browser-runner' to access it. See more information about the available mocking utilities in the API docs.


In order to validate whether certain function handler are called as part of your component tests, the @wdio/browser-runner module exports mocking primitives you can use to test, if these functions have been called. You can import these methods via:

import { fn, spy } from '@wdio/browser-runner'

By importing fn you can create a spy function (mock) to track its execution and with spyOn track a method on an already created object.

The full example can be found in the Component Testing Example repository.

import React from 'react'
import { $, expect } from '@wdio/globals'
import { fn } from '@wdio/browser-runner'
import { Key } from 'webdriverio'
import { render } from '@testing-library/react'

import LoginForm from '../components/LoginForm'

describe('LoginForm', () => {
it('should call onLogin handler if username and password was provided', async () => {
const onLogin = fn()
render(<LoginForm onLogin={onLogin} />)
await $('input[name="username"]').setValue('testuser123')
await $('input[name="password"]').setValue('s3cret')
await browser.keys(Key.Enter)

* verify the handler was called
username: 'testuser123',
password: 's3cret'

WebdriverIO just re-exports @vitest/spy here which is a lightweight Jest compatible spy implementation that can be used with WebdriverIOs expect matchers. You can find more documentation on these mock functions on the Vitest project page.

Of course, you can also install and import any other spy framework, e.g. SinonJS, as long as it supports the browser environment.


Mock local modules or observe third-party-libraries, that are invoked in some other code, allowing you to test arguments, output or even redeclare its implementation.

There are two ways to mock functions: Either by creating a mock function to use in test code, or writing a manual mock to override a module dependency.

Mocking File Imports​

Let's imagine our component is importing a utility method from a file to handle a click.

export function handleClick () {
// handler implementation

In our component the click handler is used as following:

import { handleClick } from './utils.js'

export class SimpleButton extends LitElement {
render() {
return html`<button @click="${handleClick}">Click me!</button>`

To mock the handleClick from utils.js we can use the mock method in our test as following:

import { expect, $ } from '@wdio/globals'
import { mock, fn } from '@wdio/browser-runner'
import { html, render } from 'lit'

import { SimpleButton } from './LitComponent.ts'
import { handleClick } from './utils.js'

* mock named export "handleClick" of `utils.ts` file
mock('./utils.ts', () => ({
handleClick: fn()

describe('Simple Button Component Test', () => {
it('call click handler', async () => {
render(html`<simple-button />`, document.body)
await $('simple-button').$('>>> button').click()

Mocking Dependencies​

Suppose we have a class that fetches users from our API. The class uses axios to call the API then returns the data attribute which contains all the users:

import axios from 'axios';

class Users {
static all() {
return axios.get('/users.json').then(resp =>

export default Users

Now, in order to test this method without actually hitting the API (and thus creating slow and fragile tests), we can use the mock(...) function to automatically mock the axios module.

Once we mock the module we can provide a mockResolvedValue for .get that returns the data we want our test to assert against. In effect, we are saying that we want axios.get('/users.json') to return a fake response.

import axios from 'axios'; // imports defined mock
import { mock, fn } from '@wdio/browser-runner'

import Users from './users.js'

* mock default export of `axios` dependency
mock('axios', () => ({
default: {
get: fn()

describe('User API', () => {
it('should fetch users', async () => {
const users = [{name: 'Bob'}]
const resp = {data: users}

// or you could use the following depending on your use case:
// axios.get.mockImplementation(() => Promise.resolve(resp))

const data = await Users.all()


Subsets of a module can be mocked and the rest of the module can keep their actual implementation:

export const foo = 'foo';
export const bar = () => 'bar';
export default () => 'baz';

The original module will be passed into the mock factory which you can use to e.g. partially mock a dependency:

import { mock, fn } from '@wdio/browser-runner'
import defaultExport, { bar, foo } from './foo-bar-baz.js';

mock('./foo-bar-baz.js', async (originalModule) => {
// Mock the default export and named export 'foo'
// and propagate named export from the original module
return {
__esModule: true,
default: fn(() => 'mocked baz'),
foo: 'mocked foo',

describe('partial mock', () => {
it('should do a partial mock', () => {
const defaultExportResult = defaultExport();
expect(defaultExportResult).toBe('mocked baz');

expect(foo).toBe('mocked foo');

Manual Mocks​

Manual mocks are defined by writing a module in a __mocks__/ (see also automockDir option) subdirectory. If the module you are mocking is a Node module (e.g.: lodash), the mock should be placed in the __mocks__ directory and will be automatically mocked. There's no need to explicitly call mock('module_name').

Scoped modules (also known as scoped packages) can be mocked by creating a file in a directory structure that matches the name of the scoped module. For example, to mock a scoped module called @scope/project-name, create a file at __mocks__/@scope/project-name.js, creating the @scope/ directory accordingly.

├── config
├── __mocks__
│ ├── axios.js
│ ├── lodash.js
│ └── @scope
│ └── project-name.js
├── node_modules
└── views

When a manual mock exists for a given module, WebdriverIO will use that module when explicitly calling mock('moduleName'). However, when automock is set to true, the manual mock implementation will be used instead of the automatically created mock, even if mock('moduleName') is not called. To opt out of this behavior you will need to explicitly call unmock('moduleName') in tests that should use the actual module implementation, e.g.:

import { unmock } from '@wdio/browser-runner'



In order to get mocking to work in the browser, WebdriverIO rewrites the test files and hoists the mock calls above everything else (see also this blog post on the hoisting problem in Jest). This limits the way you can pass in variables into the mock resolver, e.g.:

import dep from 'dependency'
const variable = 'foobar'

* ❌ this fails as `dep` and `variable` are not defined inside the mock resolver
mock('./some/module.ts', () => ({
exportA: dep,
exportB: variable

To fix this you have to define all used variables inside the resolver, e.g.:

* ✔️ this works as all variables are defined within the resolver
mock('./some/module.ts', async () => {
const dep = await import('dependency')
const variable = 'foobar'

return {
exportA: dep,
exportB: variable


If you are looking for mocking browser requests, e.g. API calls, head over to the Request Mock and Spies section.

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