Jest Mock Props


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

Introduction

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Install the extension using npm or yarn

``sh
npm install -D 'jest-mock-props'
`

API Reference

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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).

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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

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`js
const video = {
length: 1000,
};

module.exports = video;
`

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`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);
});
``