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jest mock instance

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Restu Wahyu Saputra answered on November 20, 2020 Popularity 10/10 Helpfulness 2/10

answer jest mock instance

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jest mock instance

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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 javascript

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Contributed on Nov 20 2020

Restu Wahyu Saputra

0 Answers Avg Quality 2/10

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mock function in jest

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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 functio

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Contributed on Dec 27 2023

Determined Dove

0 Answers Avg Quality 2/10

mock function jest

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/*
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

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Contributed on Jun 19 2022

yusuf_fazeri

0 Answers Avg Quality 2/10

jest mock call

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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

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Contributed on Nov 20 2020

Restu Wahyu Saputra

0 Answers Avg Quality 2/10

jest mock instance

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