Jest / sinon async componentWillMount test utility function

async.willmount.test.js

import { spy } from 'sinon';
import { shallow } from 'enzyme';

// Utility method to asynchronously wait for component's willMount function to
// complete. Requires the componentWillMount method *returns* its async promise
// to wait for
export const willMount = async unmountedComponent => {
  // Spy on the willmount method
  const lifecycleMethod = spy(unmountedComponent.type.prototype, 'componentWillMount');
  const wrapper = shallow(unmountedComponent);

  // Wait for the return value of the spy (the promise returned in
  // componentWillMount) and then undo the mock
  await lifecycleMethod.returnValues[0];
  lifecycleMethod.restore();

  // Update component so async changes take place
  return wrapper.update();
};

usage.js

class MyComponent extends Component {
  // Note you have to *return* the promise from your lifecycle method
  componentWillMount() {
   return new Promise(function(resolve) { 
       setTimeout(resolve, 100);
   });
  }
}

it('waits for the mount', async () => {
  const wrapper = await willMount(<MyComponent ... />);
});