Unit Testing

Jest Setup

// Project setup
$ npm run test -- --watch

// Global setup (optional)
$ npm install -g jest
$ jest --watch

Directory Tree

root
 ├── __mocks__     // mocks
 └── src
      ├── components
      │      └── __test__    // tests components
      └── scene
             └── __test__    // tests scenes

Test folder - This is where all tests should be, which will automatically detected by Jest

Mocks folder - Mock functions or libraries (to return what you want) for your tests to reduce the test scope

Unit Test Example

import React from 'react';
import renderer from 'react-test-renderer';
import { TouchableOpacity } from 'react-native';
import CollapsibleContainer from '../../CollapsibleContainer/index';

describe('<CollapsibleContainer>', () => {
  it('toggles isCollapsed when pressed (starts with collapsed)', () => {
    
    let initialState = true
    
    const inst = renderer.create(<CollapsibleContainer />);
    
    const touchable = inst.root.findByType(TouchableOpacity)
    expect(inst.root.instance.state.isCollapsed).toBe(initialState)
    touchable.props.onPress();
    expect(inst.root.instance.state.isCollapsed).toBe(!initialState)
  });
});

Writing Tests

Create a new file in the following directory

/src/scene/__test__/demo/demo.test.js

Note: Directory in test folders should always be organized based on the directory of original file for easier access.

Example:

/src/scene/AuthCheck/AuthCheck.test.js
/src/scene/__test__/AuthCheck/AuthCheck.test.js

You can skip to the end and see the completed demo.test.js by clicking here

1. Creating a Test Suite

Then, import react and describe your first test suite.

// demo.test.js
import React from 'react';

describe('My Test Suite', () => {
    it('should run test flawlessly', () => {
        // Your tests should be here
    })

    it('should fail the test', () => {
        // Your tests should be here
    })
})

describe() is for creating a test suite. A test suite can contain multiple tests depending on the test scope, it is advised to name it as the component you're testing for better referral.

https://jestjs.io/docs/en/api#describename-fn

it() is for describing the test, you can treat it as a sentence that starts with 'it'. Tests should be contained in this function.

https://jestjs.io/docs/en/api#testname-fn-timeout

Note: it() is an alias of test(), hence using either one will work the same.

2. Writing your first test

let total = 1 + 1

it('should run test flawlessly', () => {
    expect(total).toBe(2)
})

it('should fail the test', () => {
    expect(total).toBe(3)
})

expect() as what the word means, checking if the retrieved data matches what is expected.

.toBe is just one of many ways to compare, the rest can be found in the API docs in the following link https://jestjs.io/docs/en/expect

in short,

expect(received).toBe(expected)

3. Running the test

Run the following command in the root folder

$ npm run test -- --watch

You will see the following menu

Press 'p' to filter by filename, and type in demo.

And it will run the test & will show the following

Jest clearly shows the test suite, test name and what the expected result should be clearly from the console.

4. Importing a Container/Component

First, import these two things:

React Test Renderer
Container that contains the function to test.

Note: The following is only for containers that uses Redux, you can skip this part and import normally if you're testing components/containers that does not use Redux

// demo.test.js
import React from 'react';

import renderer from 'react-test-renderer';
import { MultiCityTravelEditContainer } from '../Travel/MultiCityTravelEditContainer';

This will return an error because the container is exported with Redux. To avoid that, we will need to export the class directly (skipping Redux).

// MultiCityTravelEditContainer.js

// imports...

export class MultiCityTravelEditContainer extends...

Now we can import the Container with no issue.

5. Creating an instance

https://reactjs.org/docs/test-renderer.html

An instance consists of all the states, props and functions in the component/container. This allows us to run functions and check if the state has been modified.

// demo.test.js

...

describe('My Test Suite', () => {
    // renderer is imported from 'react-test-renderer
     const inst = renderer
        .create(<MultiCityTravelEditContainer />)
        .getInstance();

})

Instances are usually created in test suites so it can be used by all of the tests.

For Redux, this will show an error because travelReducer props needs be handled.

6. Mocking Reducer (Redux only)

Mocking reduce is just simply as passing in the reducer as an object.

let travelReducerMock = {
  travelReducer: 'test', // or anything to be tested
};

const inst = renderer
    .create(<MultiCityTravelEditContainer 
        travelReducer={travelReducerMock} />)
    .getInstance();

7. Using instances

To check that it is working, you can use console.log to show the states, functions and props of the instance. You can even use setState()

const inst = renderer
    .create(<MultiCityTravelEditContainer 
        travelReducer={travelReducerMock} />)
    .getInstance();

console.log(inst) // shows everything

console.log(inst.state) // shows all states
console.log(inst.onChangeValue('params')) // returns what the function should
inst.setState({ data: 'newData' })

The console should display all the data given but somehow the test fails

TypeError: this._picker.setNativeProps is not a function

This is because the container is dependent on setNativeProps used by react-native-datepicker, which is imported from an external library.

8. Mocking Functions

https://jestjs.io/docs/en/mock-functions.html

Mocking a library is easy when the library is supported by Jest, you can just add the following line below your imports.

// demo.test.js

// import..

jest.mock('react-native-datepicker');

jest.mock() automatically set all exports of a module to the Mock Function jest.fn(), which basically does the following

export const setNativeProps = jest.fn()

9. Mock functions manually

https://jestjs.io/docs/en/manual-mocks

root
 ├── __mocks__     // mocks
 └── src
      ├── components
      │      └── __test__    // tests components
      └── scene
             └── __test__    // tests scenes

Notice the __mocks__ folder at root directory. It needs to be adjacent to the libary in node_modules