Mocking component methods with jest - reactjs

Question

Say I have the following component:
export class ExampleComponent extends Component {
exampleMethod1 = () => {
console.log('in example 1')
}
exampleMethod2 = () => {
console.log('in example 2')
this.exampleMethod1()
}
render() {
return (
<TouchableOpacity id='touchable' onPress={exampleMethod2}><Text>Touchable</Text></TouchableOpacity>
)
}
}
This works exactly how you would expect. The button appears, and can be pressed. Both methods fire, and console log their text.
I now try to test this with jest:
describe('example tests', () => {
let wrapper
beforeEach(() => {
wrapper = shallow(<ExampleComponent/>)
})
it('this test fails. Interestingly both messages still print', () => {
const instance = wrapper.instance()
instance.exampleMethod2 = jest.fn()
wrapper.find('#touchable').simulate('press')
//wrapper.update() uncommenting this line has no effect.
expect(instance.exampleMethod2.mock.calls.length).toBe(1)
})
it('this test passes. Only the first message prints', () => {
const instance = wrapper.instnace()
instance.exampleMethod1 = jest.fn()
wrapper.find('#touchable').simulate('press')
expect(instance.exampleMethod1.mock.calls.length).toBe(1)
})
})
As annotated, the first test fails, and the original message prints, as if I had never mocked out the method. This happens irrespectively of whether wrapper.update() is run or not.
Interestingly, if we replace the onPress with a seemingly identical arrow function like so:
onPress={() => {exampleMethod2()}}
The test suddenly passes. This whole thing suggest some weird this binding shenanigans (I think?). Any explanation as to what is going on would be much appreciated!

Answer 1

If you want to test custom methods on component's prototype object, you should use mount function from enzyme and use spyOn to mock and trace the call to that method.
export class ExampleComponent extends Component {
exampleMethod1 = () => {
console.log('in example 1');
this.setState({ example1: true });
}
exampleMethod2 = () => {
console.log('in example 2')
this.exampleMethod1()
}
render() {
return (
<TouchableOpacity id='touchable' onPress={exampleMethod2}><Text>Touchable</Text></TouchableOpacity>
)
}
}
describe('example tests', () => {
let wrapper
beforeEach(() => {
wrapper = mount(<ExampleComponent/>)
})
afterAll(() => { wrapper = null; })
it('some desc here', () => {
const instance = wrapper.instance();
spyOn(instance, 'exampleMethod1').and.callThrough();
expect(instance.setState).toHaveBeenCalled();
});
})