Spying on React functional component method with jest and enzyme; Cannot spyOn on a primitive value

The error means, the function sampleMethod you defined inside the functional component SampleComponent is not defined in SampleComponent.prototype. So SampleComponent.prototype.sampleMethod is undefined, jest can't spy on a undefined value.

So the correct way to test sampleMethod event handler is like this:

index.spec.tsx:

import React from 'react';import SampleComponent from './';import { shallow } from 'enzyme';describe('SampleComponent', () => {  test('should handle click correctly', () => {    const logSpy = jest.spyOn(console, 'log');    const wrapper = shallow(<SampleComponent></SampleComponent>);    const button = wrapper.find('button');    expect(button.text()).toBe('Click Me');    button.simulate('click');    expect(logSpy).toBeCalledWith('hello world');  });});

We can spy on console.log, to assert it is to be called or not.

Unit test result with 100% coverage:

 PASS  src/react-enzyme-examples/02-react-hooks/index.spec.tsx  SampleComponent    ✓ should handle click correctly (19ms)  console.log node_modules/jest-mock/build/index.js:860    hello world-----------|----------|----------|----------|----------|-------------------|File       |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |-----------|----------|----------|----------|----------|-------------------|All files  |      100 |      100 |      100 |      100 |                   | index.tsx |      100 |      100 |      100 |      100 |                   |-----------|----------|----------|----------|----------|-------------------|Test Suites: 1 passed, 1 totalTests:       1 passed, 1 totalSnapshots:   0 totalTime:        5.036s

Dependencies version:

"react": "^16.11.0","enzyme": "^3.10.0","enzyme-adapter-react-16": "^1.15.1","jest": "^24.9.0","jest-environment-enzyme": "^7.1.1","jest-enzyme": "^7.1.1",

sample.js

import * as React from 'react';export let util = {sampleMethod: null };const SampleComponent = () => {  util.sampleMethod = () => {    console.log('hello world');  };  return <button onClick={sampleMethod} type="button">Click Me</button>;};export default SampleComponent;

sample.test.js

import { shallow } from 'enzyme';import SampleComponent, {util} from './sample';test('testing spy', () => {  const spy = jest.spyOn( util, 'sampleMethod' );  const wrapper = shallow(<SampleComponent />);  wrapper.find('button').simulate('click');  expect(spy).toHaveBeenCalled(1);});

I know I'm late to answer but I think this would help some other developers also