@atlaskit/editor-common/dist/esm/core-utils/is-ssr.js

A package that contains common classes and components for editor and renderer

export function isSSR() {
  try {
    return Boolean(
    // In most places there is no document when running on server-side
    typeof document === 'undefined' ||
    // When SSRing editor with full cycle mode we define the document
    // Check Confluence specific environment variable
    typeof process !== 'undefined' && process.env.REACT_SSR);
  } catch (e) {
    // Catch possible error that might occur and just return false
    return false;
  }
}