Performance.js

const hasMark = window.performance && !!window.performance.mark;

const Performance = () => {
  const defaultMark = {};

  const getTiming = () => {
    const timing = performance && performance.timing ? performance.timing.toJSON() : {};
    const defaultTiming = {ipsyPageStart: __PAGE_START__, ipsyPageEnd: __PAGE_END__};

     return {...timing, ...defaultTiming};
  };

  const getNavigationTiming = (): NavigationTimingType | void => {
    return new Promise(resolve => {
      document.readyState === 'complete'
        ? resolve(getTiming())
        : window.addEventListener('load', () => resolve(getTiming()));
    });
  };

  const basePerf = {
    getNavigationTiming,
    START_CHANGE_PAGE,
  }

  if (!hasMark) {
    return {
      getMarkFullRenderPage: () => ({fullRender: defaultMark}),
      getMarkPartialRenderPage: () => ({partialRender: defaultMark}),
      ...basePerf,
    }
  }

  // More Code

  const getPartialRender = () => { /* ... */}
  const getMarkPartialRenderPage = () => { /* ... */}

  return {
    getPartialRender,
    getMarkPartialRenderPage,
    ...basePerf,

  }
}