@segment/analytics-next/src/core/environment/index.ts

Analytics Next (aka Analytics 2.0) is the latest version of Segment’s JavaScript SDK - enabling you to send your data to any tool without having to learn, test, or use a new API every time.

export function isBrowser(): boolean {
  return typeof window !== 'undefined'
}

export function isServer(): boolean {
  return !isBrowser()
}