Jest Mock Props

Extends jest to allow easy mocking of object and module properties.

Also see jest-mock-module for non destructive spying on imported modules by automatically shimming module properties

Introduction

Getting Started

Install the extension using npm or yarn

npm install -D 'jest-mock-props'

API Reference

Mock Properties

Mock properties are "spies" that let you control the behavior of a property that is accessed indirectly by some other code.

These are the methods available on every mocked property spy object.

spy.mockClear()

Equivalent to mockReset.

spy.mockReset()

Removes any mocked values.

This is useful when you want to completely reset a property back to its initial value.

spy.mockRestore()

Restores the original (non-mocked) value.

This is useful when you want to mock properties in certain test cases and restore the original value in others.

spy.mockValue(value)

Accepts a value that should be result of accessing the mocked property.

Note: This is the same function used when setting the mocked property directly; e.g. obj.mockedProp = 'newValue'

spy.mockValueOnce(value)

Accepts a value that will be result of a single access to the mocked property. Can be chained so that multiple accesses produce different results. See example.

Note: When the mocked property runs out of values defined with mockValueOnce, it will have the default value set with obj.mockedProp = 'defaultValue' or spy.mockValue(defaultValue).

The Jest Object

The jest object needs to be extended in every test file. This allows mocked properties to be reset and restored with jest.resetAllMocks and jest.restoreAllMocks respectively.

jest.isMockProp(object, propertyName)

Determines if the given object property has been mocked.

jest.spyOnProp(object, propertyName)

Note: This is aliased as jest.spyOn as of v1.9.0, overriding the existing jest.spyOn to use spyOnProp when spying on a regular object property.

Creates a mock property attached to object[propertyName] and returns a mock property spy object, which controls all access to the object property. Repeating spying on the same object property will return the same mocked property spy.

Note: By default, spyOnProp preserves the object property value. If you want to overwrite the original value, you can use jest.spyOnProp(object, propertyName).mockValue(customValue) or jest.spyOn(object, methodName, accessType?) to spy on a getter or a setter.

Example

video.js

const video = {
    length: 1000,
};

module.exports = video;

video.test.js

import * as mockProps from "jest-mock-props";
mockProps.extend(jest);

const video = require("./video");

it("mocks video length", () => {
    const spy = jest.spyOn(video, "length");
    spy.mockValueOnce(200)
        .mockValueOnce(400)
        .mockValueOnce(600);
    expect(video.length).toEqual(200);
    expect(video.length).toEqual(400);

    video.length = 800;
    expect(video.length).toEqual(800);

    spy.mockReset();
    expect(video.length).toEqual(1000);
    expect(jest.isMockProp(video, "length")).toBe(true);

    spy.mockRestore();
    expect(jest.isMockProp(video, "length")).toBe(false);
});

Related

• Spy on JS modules using jest mock