Testing with Jest and Enzyme in React — Part 5 (Dive with Jest and Enzyme)

In the previous tutorial, I discussed shallow vs. mount in Enzyme. In this tutorial let us dive more into testing with Jest and Enzyme by writing tests for rendering, interactions, and lifecycle method calls.

First, you need to have that project named “testing-demo-app”, which I used in the previous tutorials. You can find it in the below link.

Let us write some tests for rendering, interactions, and lifecycle method calls.

Step 1 — Writing tests for Rendering

  • First, create a new file named Form.test.js inside the testing-demo-app/test folder with the following content.
import Form from '../src/components/Form';

let wrapper;

beforeEach(() => {
wrapper = shallow(<Form/>);
});

describe('<Form /> rendering', () => {
it('should render one <form>', () => {
expect(wrapper.find('form')).toHaveLength(1);
});

it('should not render any <button> when operator is not passed in props', () => {
expect(wrapper.find('button')).toHaveLength(0);
});

it('should render 2 <label>s', () => {
expect(wrapper.find('label')).toHaveLength(2);
});

it('should render 2 <input>s', () => {
expect(wrapper.find('input')).toHaveLength(2);
});

});
  • Open up a terminal inside the project directory and run the below command in order to run the tests in Form.test.js file.
npm test Form.test.js
  • The tests should pass as follows.
  • The first test checks whether there is one <form> element when the <Form> is rendered.
  • The second test is special. If you check Form.js file in testing-demo-app/src/components, you can see that if the operator is not passed, a button would be rendered. In this test we check that. We do not pass the operator as a prop when we are initializing the wrapper. Because of that, no button will be rendered.
  • The third test checks there are 2 <label>s. (Those are to label the FIRST NUMBER and the SECOND_NUMBER in UI)
  • The fourth test checks whether there are 2 <inputs>, which are to enter the numbers to be added.
  • Let us add another 2 tests as follows and run it again.
it('should render one <button> to Add when operator \'+\' is passed in props', () => {
wrapper.setProps({ operator: '+' } );
expect(wrapper.find('#formButtonAdd')).toHaveLength(1);
expect(wrapper.find('#formButtonSubtract')).toHaveLength(0);
});

it('should render one <button> to Subtract when operator \'-\' is passed in props', () => {
wrapper.setProps({ operator: '-' } );
expect(wrapper.find('#formButtonAdd')).toHaveLength(0);
expect(wrapper.find('#formButtonSubtract')).toHaveLength(1);
});
  • In the newly added fifth test, we set the prop operator to ‘+’. According to what we have discussed in the second test, now the <Form> must have a <button>. In here we are searching that <button> using its id ‘formButtonAdd’.
  • Similarly, in the sixth test, we set the prop operator to ‘-’ and check whether there is a <button> with id ‘formButtonSubtract’.

Step 2 — Writing tests for Interactions

  • Add another set of tests under ‘<Form> interactions’ in Form.test.js like below and run the file again.
describe('<Form /> interactions', () => {
it('should change the state firstNumber when onChange function of the #number1 input is invoked', () => {
wrapper.find('#number1').simulate('change',
{ target: { value: 50 } }
);
expect(wrapper.state('firstNumber')).toEqual(50);
expect(wrapper.state('secondNumber')).toEqual('');
});

it('should change the state secondNumber when onChange function of the #number2 input is invoked', () => {
wrapper.find('#number2').simulate('change',
{ target: { value: 60 } }
);
expect(wrapper.state('secondNumber')).toEqual(60);
expect(wrapper.state('firstNumber')).toEqual('');
});
});
  • In the first test under the ‘<Form> interactions’, we are simulating an event in an <input> field. We are simulating the onChange method in the <input> which has the id ‘number1’. We are assigning a value to number1 <input> as 50. If you check the onChange function of number1 in Form.js file, it calls handleFirstNumber() method, which is like below.
handleFirstNumber(event) {
this.setState({ firstNumber: event.target.value });
}
  • So according to our test, it sets 50 as the value of number1 and expects this function to automatically change the value of the state.firstNumber in Form.js to 50. This test is passed, which means that we have successfully simulated the onChange method of number1.
  • In the second test also, we are doing the similar thing to simulate the onChange method in number2 <input>.
  • Let us add 2 more test as below and run the file.
it('should call the onClick function when \'Add\' button is clicked when the operator is \'+\'', () => {
wrapper.setProps({ operator: '+' } );
const mockedHandleClickAdd = jest.fn();
wrapper.instance().handleAdd = mockedHandleClickAdd;
wrapper.find('#formButtonAdd').props().onClick();
expect(mockedHandleClickAdd).toHaveBeenCalledTimes(1);
});

it('should call the onClick function when \'Subtract\' button is clicked when the operator is \'-\'', () => {
wrapper.setProps({ operator: '-' } );
const mockedHandleClickSubtract = jest.fn();
wrapper.instance().handleSubtract = mockedHandleClickSubtract;
wrapper.find('#formButtonSubtract').props().onClick();
expect(mockedHandleClickSubtract).toHaveBeenCalledTimes(1);
});
  • In the third test, under ‘<Form> interactions’, is mocking the onClick function of a <button> with the id ‘formButtonAdd’. In Form.js, when this is clicked the function named handleAdd() should be called. Since we have mocked it using a jest function named mockedHandleClickAdd(), this must be called instead. So our test has passed means, we have successfully mocked onClick function of formButtonAdd.
  • Similarly, in the fourth test, we are mocking the onClick function of ‘formButtonSubtract’.

Step 3 — Writing tests for Lifecycle Method calls

  • Add another test under ‘<Form> lifecycle method invocations’ in Form.test.js like below and run the file again.
describe('<Form /> lifecycle method invocations', () => {
it('should change the state componentState componentDidMount method is invoked', () => {
expect(wrapper.state('componentState')).toEqual('mounted');
});
});
  • In here we are checking the state named componentState which is defined in Form.js file. According to Form.js, the state componentState will be changed to ‘mounted’ when componentDidMount method is called. Our test is passed, that means the componentMount mehod is called successfully.

Complete test file — Form.test.js

import Form from '../src/components/Form';

let wrapper;

beforeEach(() => {
wrapper = shallow(<Form/>);
});

describe('<Form /> rendering', () => {
it('should render one <form>', () => {
expect(wrapper.find('form')).toHaveLength(1);
});

it('should not render any <button> when operator is not passed in props', () => {
expect(wrapper.find('button')).toHaveLength(0);
});

it('should render 2 <label>s', () => {
expect(wrapper.find('label')).toHaveLength(2);
});

it('should render 2 <input>s', () => {
expect(wrapper.find('input')).toHaveLength(2);
});

it('should render one <button> to Add when operator \'+\' is passed in props', () => {
wrapper.setProps({ operator: '+' } );
expect(wrapper.find('#formButtonAdd')).toHaveLength(1);
expect(wrapper.find('#formButtonSubtract')).toHaveLength(0);
});

it('should render one <button> to Subtract when operator \'-\' is passed in props', () => {
wrapper.setProps({ operator: '-' } );
expect(wrapper.find('#formButtonAdd')).toHaveLength(0);
expect(wrapper.find('#formButtonSubtract')).toHaveLength(1);
});
});

describe('<Form /> interactions', () => {
it('should change the state firstNumber when onChange function of the #number1 input is invoked', () => {
wrapper.find('#number1').simulate('change',
{ target: { value: 50 } }
);
expect(wrapper.state('firstNumber')).toEqual(50);
expect(wrapper.state('secondNumber')).toEqual('');
});

it('should change the state secondNumber when onChange function of the #number2 input is invoked', () => {
wrapper.find('#number2').simulate('change',
{ target: { value: 60 } }
);
expect(wrapper.state('secondNumber')).toEqual(60);
expect(wrapper.state('firstNumber')).toEqual('');
});

it('should call the onClick function when \'Add\' button is clicked when the operator is \'+\'', () => {
wrapper.setProps({ operator: '+' } );
const mockedHandleClickAdd = jest.fn();
wrapper.instance().handleAdd = mockedHandleClickAdd;
wrapper.find('#formButtonAdd').props().onClick();
expect(mockedHandleClickAdd).toHaveBeenCalledTimes(1);
});

it('should call the onClick function when \'Subtract\' button is clicked when the operator is \'-\'', () => {
wrapper.setProps({ operator: '-' } );
const mockedHandleClickSubtract = jest.fn();
wrapper.instance().handleSubtract = mockedHandleClickSubtract;
wrapper.find('#formButtonSubtract').props().onClick();
expect(mockedHandleClickSubtract).toHaveBeenCalledTimes(1);
});
});

describe('<Form /> lifecycle method invocations', () => {
it('should change the state componentState componentDidMount method is invoked', () => {
expect(wrapper.state('componentState')).toEqual('mounted');
});
});

Special Note on ‘componentDidMount’

Assume you have a database call in one of your lifecycle methods (componentDidMount or any other). If you call it from the tests, it will definitely fail because you do not have the database connection when you are testing. So, the solution for this is to use a prototype function of componentDidMount which is defined in your test file like below.

beforeAll(() => {
Form.prototype.componentDidMount = () => {
console.log('componentDidMount method is called');
};
});

This story is published in Noteworthy, where 10,000+ readers come every day to learn about the people & ideas shaping the products we love.

Follow our publication to see more product & design stories featured by the Journal team.