enzyme-instance-method.js

// Consider a class component with this method that's called when a button is pressed:  
onMultiplierChanged(multiplier) {
  this.setState({product: (parseInt(multiplier) * 12).toString()});
}

// A test utilizing wrapper.instance might look like this:
it('Should update state when onMultiplierChanged method is called', () => {
  const instance = wrapper.instance();

  instance.onMultiplierChanged("12");

  expect(wrapper.state('product')).toEqual("144");
});