How to Test a React Component Function with Jest?

Sometimes, we want to test React component methods with Jest.

In this article, we’ll look at how to test React component methods with Jest.

Test a React Component Function with Jest

We can mock functions that our component depend on so that we can do testing with it.

For instance, we can write:

import React, { Component } from 'react';

class Child extends Component {
   constructor (props) {
      super(props);
   };

  render() {
      const { onSubmit, label} = this.props;
      return(
        <form  onSubmit={onSubmit}>
          <Button type='submit'>{label}</Button>
        </form >
      );
   };
};

export default class App extends Component {
  constructor (props) {
    super(props);
    this.label = “foo”;
  };

  onSubmit = (option) => {
    console.log('submitted');
  };

  render () {
    return(
      <div className="container">
        <Child label={this.label} onSubmit={this.onSubmit} />
      </div>
    );
  };
};

We can then mock the onSubmit function when we test the child by writing:

import React from 'react';
import { shallow } from 'enzyme';
import Child from '../Child';

const onSubmitSpy = jest.fn();
const onSubmit = onSubmitSpy;

const wrapper = shallow(<Child onSubmit={onSubmitSpy} />);
let container, containerButton;

describe("Child", () => {
  beforeEach(() => {
    container = wrapper.find("div");
    containerButton = container.find(“Button”);
    onSumbitSpy.mockClear();
  });

  describe("<Button> behavior", () => {
     it("should call onSubmit", () => {
       expect(onSubmitSpy).not.toHaveBeenCalled();
       containerButton.simulate(‘click’);
       expect(onSubmitSpy).toHaveBeenCalled();
     });
  });
});

We mount the component with the onSubmit prop populated by the onSubmitSpy , which is a mock function.

We tested by simulating a click event on the button of the Child component.

Then we check if the mocked function is called with toHaveBeenCalled .

Conclusion

We can mock functions that our component depend on so that we can do testing with it.