Testing React Native

It's a list of snippets, that will help you test your component properly.

What is worth testing

1. Proper values
2. Passing undefined or null value
3. In case you are calculating something:

• proper numbers
• zero values (helps to test dividing)
• null/undefined (worth to check NaN or infinite values)

react-native-testing-library

Snapshot

import React from 'react';
import { render } from 'react-native-testing-library';

import { Button } from 'components';

describe('components/Button', () => {
  const defaultProps = {
    text: 'Hello world',
  };

  test('snapshot', () => {
    const tree = render(<Button {...defaultProps} />).toJSON();
    expect(tree).toMatchSnapshot();
  });
});

Button press

import React from 'react';
import { render, fireEvent } from 'react-native-testing-library';

 import { Button } from 'components';

 describe('components/Button', () => {
  const defaultProps = {
    text: 'Hello World'
    onPress: jest.fn(),
  };

  test('Should handle click', () => {
    const { getByTestId } = render(
      <ButtonCard testID="button" {...defaultProps} />,
    );
    fireEvent.press(getByTestId('button'));

    expect(defaultProps.onPress).toHaveBeenCalledTimes(1);
  });
});

Enzyme

Snapshot

Simple test to check changes in snapshots. Pretty useful, but not really testing logic, but just props and style. Easy to ignore.

It should be only your base. You will start here and add more tests to it.

describe('Component: EXAMPLECOMPONENT', () => {
  describe('with basic props', () => {
    const props = {
      ...defaultProps
    }

    const wrapper = shallow(
      <EXAMPLECOMPONENT {...props} />
    )

    it('Should render snapshot properly', () => {
      expect(wrapper).toMatchSnapshot()
    })
  })
})

Testing components

To make sure some components are on screen you can easily add test to find it.

Count number of elements

it('Should render properly', () => {
  expect(wrapper.find(View).length).toBe(1)
})

Find specific element

<View testID="COMPONENT_TO_TEST">

it('Should has component', () => {
  const component = wrapper.findWhere(node => node.prop('testID') === 'COMPONENT_TO_TEST')
  expect(component).toBeDefined()
})

Test styles

Sometimes we need to has some specific styles, e.g. mandatory style for button.

it('Should has mandatory style', () => {
  const component = wrapper.findWhere(node => node.prop('testID') === 'COMPONENT_TO_TEST')
  expect(component.props().style).toHaveProperty('color', 'red')
})

it('Should has mandatory style', () => {
  const expectedStyle = {
    color: 'red'
  }
  const component = wrapper.findWhere(node => node.prop('testID') === 'COMPONENT_TO_TEST')
  expect(component.props().style).toMatch(expectedStyle)
})

Test component logic

Sometimes we have some helpers functions inside our components, e.g. getIcon, getValue or anything.

import { icons } from '../assets'

it('Should return default icon', () => {
  const icon = wrapper.instance().getIcon()
  expect(icon).toEqual(icons.default)
})

it('Should return 42', () => {
  wrapper.instance().makeCalculation()
  wrapper.update()
  const value = wrapper.instance().getValue()
  expect(value).toEqual(42)
})

Test buttons

Sometimes we pass function through props like this:

  <MyComponent onPress={() => amazing()} />

and inside MyComponent we use it like this to call this amazing function

  <TouchableOpacity onPress={() => this.props.onPress()}>

So we can check does this prop was called and how many times

it('Should handle item click', () => {
  wrapper.find(TouchableOpacity).simulate('press')
  expect(props.onPress).toHaveBeenCalledTimes(1)
})