import TestRenderer from 'react-test-renderer';class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.input = null;
  }
  componentDidMount() {
    this.input.focus();
  }
  render() {
    return <input type="text" ref={el => this.input = el} />
  }}let focused = false;TestRenderer.create(
  <MyComponent />,
  {
    createNodeMock: (element) => {
      if (element.type === 'input') {        // mock a focus function
        return {
          focus: () => {
            focused = true;
          }
        };
      }
      return null;
    }
  });expect(focused).toBe(true);