next.js/packages/next/src/shared/lib/router/utils/remove-trailing-slash.ts

/**
 * Removes the trailing slash for a given route or page path. Preserves the
 * root page. Examples:
 *   - `/foo/bar/` -> `/foo/bar`
 *   - `/foo/bar` -> `/foo/bar`
 *   - `/` -> `/`
 */
export function removeTrailingSlash(route: string) {
  return route.replace(/\/$/, '') || '/'
}