How can I use Jest to spy on a method call?

Question

I recently wanted to test that some custom method gets conditionally called in the componentDidMount method of a React component.

componentDidMo         


        

Answer 1

I know its a bit late, but I came across this and would suggest that to test componentDidMount initiates the call to your nested method that your test should look something like:

Module

componentDidMount() {
  if (this.props.initOpen) {
    this.methodName();
  }
}

Test - Good

it('should call methodName during componentDidMount', () => {
    const methodNameFake = jest.spyOn(MyComponent.prototype, 'methodName');
    const wrapper = mount();
    expect(methodNameFake).toHaveBeenCalledTimes(1);
});

If you call componentDidMount then the assertion that methodName was called via componentDidMount is more valid.

Test - Bad

it('should call methodName during componentDidMount', () => {
    const spy = jest.spyOn(Component.prototype, 'methodName');
    const wrapper = mount();
    wrapper.instance().methodName();
    expect(spy).toHaveBeenCalled();
}

By writing the test like this - you call the method and then assert that it was called. Which of course it will have given you just called it.