mock-with-jest.mo.md

[MO] How to use mocks with jest (~2minutes)

Owner: Pierre-Louis Le Portz

Why

Most teams at BAM use jest for testing their react components. One of the most frequent andon is about mocks.

Typical use case: "My component's snapshot test was passing but then I imported a module and now the test is broken :'("

Prerequisites

Have jest installed ;)

Steps

In this MO you will learn to:

• mock some method on an imported module
• mock an imported component
• mock a class that is used for both rendering a component AND using static methods

Example 1: mock some method on an imported module

File to be tested:

import React, { PureComponent } from 'react';
import { View } from 'react-native';
import Permissions from 'react-native-permissions';

const checkPushNotifsPermission = () => {
  return Permissions.check('notification').then(status => {
    // do something depending on status
  });
};

export default class Home extends PureComponent {
  // do something with checkPushNotifsPermission ...
  render() {
    return (
      <View>
        // render something...
      </View>
    );
  }
}

Test file with snapshot test:

import React from 'react';
import renderer from 'react-test-renderer';
import { Home } from './Home';

jest.mock('react-native-permissions', () => ({
  check: _ => Promise.resolve(true),
}));

describe('<Home />', () => {
  it('renders correctly', () => {
    const props = {
      // props to give to the Home component
    };
    const tree = renderer.create(<Home {...props} />);
    expect(tree.toJSON()).toMatchSnapshot();
  });
});

Even better

In the example above we are mocking a native module (react-native-permissions). Since you always need to mock a native module, you should centralize the mock definition in order to avoid redefining it in numerous test files. Here is how to do it:

//project_root/__mocks__/react-native-permissions.js

jest.mock('react-native-permissions', () => ({
  check: _ => Promise.resolve(true),
}));

Example 2: Mock one of your own components

In the example below, we chose to mock the Votes component when we added a container with graphql logic around the original Votes component

File to be tested:

import React, { PureComponent } from 'react';
import { View } from 'react-native';
import { Votes } from '../../components/Votes/Votes.container'

export default class Recipe extends PureComponent {
  render() {
    return (
      <View>
        // render things and then use:
        <Votes />
      </View>
    );
  }
}

Test file with snapshot test:

import React from 'react';
import renderer from 'react-test-renderer';
import Recipe from './Recipe';

jest.mock('../../components/Votes/Votes.container', () => props => <votes {...props} />);

  // Here we use <votes {...props} /> WITHOUT a capital letter

describe('<Recipe />', () => {
  it('renders correctly', () => {
    const props = {
      // props to give to the Recipe component
    };
    const tree = renderer.create(<Recipe {...props} />);
    expect(tree.toJSON()).toMatchSnapshot();
  });
});

Example 3: Mock a class that is used for both rendering a component AND using static methods

In the example below we use the react-rte/lib/RichTextEditor module to build our own state-controlled markdown input component.

File to be tested:

import React, { Component } from 'react';
import RichTextEditor from 'react-rte/lib/RichTextEditor';

export default class MarkdownInput extends Component{

  // use RichTextEditor.createValueFromString() and RichTextEditor.createEmptyValue() in some lifecycle methods

  render() {
    return <RichTextEditor
      //pass some props
      />;
  }
}

Centralized mock (see Example 1, section "Even better"):

jest.mock('path_from_root_to_node_modules/node_modules/react-rte/lib/RichTextEditor', () => {
  const RichTextEditor = props => <richTextEditor {...props} />;
  RichTextEditor.createValueFromString = string => ({ content: string });
  RichTextEditor.createEmptyValue = () => ({ content: '' });
  return RichTextEditor;
});