How to mock functions, and test that they're called, when passed as props in react components?

Question

I'm following the example from this stackoverflow answer - Test a React Component function with Jest. I have an example component and test set up. The component works correctly when loaded into App.js.

Component -

import React, { PropTypes, Component } from 'react';

export default class ExampleModule extends Component {
  static propTypes = {
    onAction: PropTypes.func,
  }

  static defaultProps = {
  onAction: () => { console.log("In onAction"); }
}

doAction = () => {
  // do something else
  console.log('In do action');
  this.props.onAction();
}

render() {
  return(
    <div>
      <button className='action-btn' onClick=  {this.doAction.bind(this)}>Do action</button>
    </div>
  )
}
}

And here's the test -

import React from 'react';
import ReactDOM from 'react-dom';
import TestUtils from 'react-addons-test-utils';
import ExampleComponent from './ExampleModule.js';

let Example;

describe('Example component', function() {
  beforeEach(function() {
    Example = TestUtils.renderIntoDocument(<ExampleComponent />);
  })

  it('calls props functions', function() {
    Example.doAction = jest.genMockFunction();
    let actionBtn = TestUtils.findRenderedDOMComponentWithClass(Example, 'action-btn');
    TestUtils.Simulate.click(actionBtn);
    expect(Example.doAction).toBeCalled();
  })

  it('doAction calls onAction', function() {
    expect(Example.props.onAction).not.toBeCalled();
    Example.doAction();
    expect(Example.props.onAction).toBeCalled();
  })
})

However, I get the following error -

FAIL  src/App/components/Example/ExampleModule.test.js
  Console

    console.log src/App/components/Example/ExampleModule.js:14
      In do action
    console.log src/App/components/Example/ExampleModule.js:24
      In onAction

  Example component › calls props functions

    Expected the mock function to be called.

      at Object.<anonymous> (src/App/components/Example/ExampleModule.test.js:17:30)
      at process._tickCallback (node.js:369:9)

  Example component › doAction calls onAction

    toBeCalled matcher can only be used on a spy or mock function.

      at Object.<anonymous> (src/App/components/Example/ExampleModule.test.js:21:40)
      at process._tickCallback (node.js:369:9)

I can see the console.logs in the doAction and onAction are being called even when I want to mock out doAction. Also, I'm unable to mock out onAction. I get this error -

TypeError: Cannot assign to read only property 'onAction' of #<Object>

I've tried jest.fn() but got the same errors.

How do I mock these functions and test them?

EDIT:

I was able to mock doAction by using jest.fn() in the following way -

let mockFn = jest.fn();
Example.doAction = mockFn()

However, I'm still unable to mock Example.props.onAction.

Answer 1

In your test when you render document, you need to pass a mock function which you can later watch if it was being called, something like this -

let onActionMock = jest.fn();

beforeAll(function() {
  Example = TestUtils.renderIntoDocument(<ExampleComponent onAction={onActionMock}/>);
});

beforeEach(function() {
  onActionMock.mockClear();
});

it('doAction calls onAction', () => {
  Example.doAction();
  expect(onActionMock).toBeCalled();
});

You can further test the function with what it has been called with -

expect(onActionMock).toBeCalledWith('Some argument');

Hope this helps.