next.js/packages/next/src/shared/lib/router/utils/relativize-url.ts

/**
 * The result of parsing a URL relative to a base URL.
 */
export type RelativeURL = {
  /**
   * The relative URL. Either a URL including the origin or a relative URL.
   */
  url: string

  /**
   * Whether the URL is relative to the base URL.
   */
  isRelative: boolean
}

export function parseRelativeURL(
  url: string | URL,
  base: string | URL
): RelativeURL {
  const baseURL = typeof base === 'string' ? new URL(base) : base
  const relative = new URL(url, base)

  // The URL is relative if the origin is the same as the base URL.
  const isRelative = relative.origin === baseURL.origin

  return {
    url: isRelative
      ? relative.toString().slice(baseURL.origin.length)
      : relative.toString(),
    isRelative,
  }
}

/**
 * Given a URL as a string and a base URL it will make the URL relative
 * if the parsed protocol and host is the same as the one in the base
 * URL. Otherwise it returns the same URL string.
 */
export function getRelativeURL(url: string | URL, base: string | URL): string {
  const relative = parseRelativeURL(url, base)
  return relative.url
}