Breaking News: Grepper is joining You.com. Read the official announcement!

mock function in jest

Add Answer

Determined Dove answered on November 20, 2020 Popularity 10/10 Helpfulness 4/10

answer mock function in jest

related jest mock method by name

related mock callback function jest

related jest mock implementation once

related mock function jest

related jest mock instance

related jest mock call

mock function in jest

Comment

There are a few different ways to approach it.
You can mock only foo using jest.spyOn and something like mockImplementation:

import { MyClass } from './MyClass';
import * as FooFactory from './somewhere/FooFactory';

describe('test MyClass', () => {
  test('construct', () => {
    const mock = jest.spyOn(FooFactory, 'foo');  // spy on foo
    mock.mockImplementation((arg: string) => 'TEST');  // replace implementation

    const c = new MyClass('test');
    expect(c).toBeDefined();
    expect(c.getState()).toEqual('TEST');  // SUCCESS

    mock.mockRestore();  // restore original implementation
  });
});

Similarly, you can auto-mock FooFactory with jest.mock, then provide an implementation for foo:

import { MyClass } from './MyClass';
import * as FooFactory from './somewhere/FooFactory';

jest.mock('./somewhere/FooFactory');  // auto-mock FooFactory

describe('test MyClass', () => {
  test('construct', () => {
    const mockFooFactory = FooFactory as jest.Mocked<typeof FooFactory>;  // get correct type for mocked FooFactory
    mockFooFactory.foo.mockImplementation(() => 'TEST');  // provide implementation for foo

    const c = new MyClass('test');
    expect(c).toBeDefined();
    expect(c.getState()).toEqual('TEST');  // SUCCESS
  });
});

You can also mock FooFactory using a module factory passed to jest.mock:

import { MyClass } from './MyClass';

jest.mock('./somewhere/FooFactory', () => ({
  foo: () => 'TEST'
}));

describe('test MyClass', () => {
  test('construct', () => {
    const c = new MyClass('test');
    expect(c).toBeDefined();
    expect(c.getState()).toEqual('TEST');  // SUCCESS
  });
});

And finally, if you plan to use the same mock across multiple test files you can mock the user module by creating a mock at ./somewhere/__mocks__/FooFactory.ts:

export function foo(arg: string) {
  return 'TEST';
}

...then call jest.mock('./somewhere/FooFactory'); to use the mock in the test:

import { MyClass } from './MyClass';

jest.mock('./somewhere/FooFactory');  // use the mock

describe('test MyClass', () => {
  test('construct', () => {
    const c = new MyClass('test');
    expect(c).toBeDefined();
    expect(c.getState()).toEqual('TEST');  // SUCCESS
  });
});

Popularity 10/10 Helpfulness 4/10 Language javascript

Source: Grepper

Tags: function javascript

Link to this answer
Share Copy Link

Contributed on Dec 27 2023

Determined Dove

0 Answers Avg Quality 2/10

Closely Related Answers

jest mock method by name

Comment

import Foo from './Foo';
import Bar from './Bar';

jest.mock('./Bar');

describe('Foo', () => {
  it('should return correct foo', () => {
    // As Bar is already mocked,
    // we just need to cast it to jest.Mock (for TypeScript) and mock whatever you want
    (Bar.prototype.runBar as jest.Mock).mockReturnValue('Mocked bar');
    const foo = new Foo();
    expect(foo.runFoo()).toBe('real foo : Mocked bar');
  });
});

Popularity 10/10 Helpfulness 3/10 Language javascript

Source: stackoverflow.com

Tags: javascript j

Link to this answer
Share Copy Link

Contributed on Jul 19 2021

Tame Tapir

0 Answers Avg Quality 2/10

mock callback function jest

Comment

const bodyToAssertAgainst = {};
globals.request.post = jest.fn().mockImplementation((obj, cb) => {
    cb(null, bodyToAssertAgainst);
});

Popularity 10/10 Helpfulness 3/10 Language javascript

Source: stackoverflow.com

Tags: callback callbac

Link to this answer
Share Copy Link

Contributed on May 24 2022

Jumping Boy

0 Answers Avg Quality 2/10

jest mock implementation once

Comment

const myMockFn = jest
  .fn()
  .mockImplementationOnce(cb => cb(null, true))
  .mockImplementationOnce(cb => cb(null, false));

myMockFn((err, val) => console.log(val));
// > true

myMockFn((err, val) => console.log(val));
// > false

Popularity 10/10 Helpfulness 2/10 Language javascript

Source: jestjs.io

Tags: implementation impl

Link to this answer
Share Copy Link

Contributed on Mar 07 2022

Proud Pony

0 Answers Avg Quality 2/10

mock function jest

Comment

/*
The Mock Function
The goal for mocking is to replace something we don’t control with something 
we do, so it’s important that what we replace it with has all the features 
we need.

The Mock Function provides features to:

1. Capture calls
2. Set return values
3. Change the implementation

The simplest way to create a Mock Function instance is with jest.fn()
*/

test("returns undefined by default", () => {
  const mock = jest.fn();

  let result = mock("foo");

  expect(result).toBeUndefined();
  expect(mock).toHaveBeenCalled();
  expect(mock).toHaveBeenCalledTimes(1);
  expect(mock).toHaveBeenCalledWith("foo");
});

Popularity 10/10 Helpfulness 2/10 Language javascript

Source: medium.com

Tags: function functio

Link to this answer
Share Copy Link

Contributed on Jun 19 2022

yusuf_fazeri

0 Answers Avg Quality 2/10

jest mock instance

Comment

test("mock.instances", () => {
  const mockFn = jest.fn();

  const a = new mockFn();
  const b = new mockFn();

  mockFn.mock.instances[0] === a;
  mockFn.mock.instances[1] === b;
});

Popularity 10/10 Helpfulness 2/10 Language javascript

Source: Grepper

Tags: instance instanc

Link to this answer
Share Copy Link

Contributed on Nov 20 2020

Restu Wahyu Saputra

0 Answers Avg Quality 2/10

jest mock call

Comment

test("mock.calls", () => {
  const mockFn = jest.fn();
  mockFn(1, 2);

  expect(mockFn.mock.calls).toEqual([[1, 2]]);
});

Popularity 10/10 Helpfulness 2/10 Language javascript

Source: Grepper

Tags: call

Link to this answer
Share Copy Link

Contributed on Nov 20 2020

Restu Wahyu Saputra

0 Answers Avg Quality 2/10

mock function in jest

Contents

More Related Answers

mock function in jest

Closely Related Answers

jest mock method by name

mock callback function jest

jest mock implementation once

mock function jest

jest mock instance

jest mock call

Grepper

Documentation

Social

Legal

Contact

Oops, You will need to install Grepper and log-in to perform this action.