feat: add is-server

Author: schiller-manuel

e2e/react-start/flamegraph-bench/package.json

@@ -19,7 +19,7 @@
   "devDependencies": {
     "@platformatic/flame": "latest",
     "@vitejs/plugin-react": "^4.3.4",
-    "nitro": "npm:nitro-nightly@latest",
+    "nitro": "file:/Users/caligano/source/nitro",
     "autocannon": "^8.0.0",
     "concurrently": "9.2.1",
     "typescript": "^5.7.2",

packages/react-router/src/Asset.tsx

@@ -1,4 +1,5 @@
 import * as React from 'react'
+import { isServer } from '@tanstack/router-core'
 import { useRouter } from './useRouter'
 import type { RouterManagedTag } from '@tanstack/router-core'
 
@@ -143,7 +144,7 @@ function Script({
     return undefined
   }, [attrs, children])
 
-  if (!router.isServer) {
+  if (!(isServer ?? router.isServer)) {
     const { src, ...rest } = attrs || {}
     // render an empty script on the client just to avoid hydration errors
     return (

packages/react-router/src/Match.tsx

@@ -6,6 +6,7 @@ import {
   getLocationChangeInfo,
   isNotFound,
   isRedirect,
+  isServer,
   rootRouteId,
 } from '@tanstack/router-core'
 import { CatchBoundary, ErrorComponent } from './CatchBoundary'
@@ -246,7 +247,7 @@ export const MatchInner = React.memo(function MatchInnerImpl({
       const routerMatch = router.getMatch(match.id)
       if (routerMatch && !routerMatch._nonReactive.minPendingPromise) {
         // Create a promise that will resolve after the minPendingMs
-        if (!router.isServer) {
+        if (!(isServer ?? router.isServer)) {
           const minPendingPromise = createControlledPromise<void>()
 
           routerMatch._nonReactive.minPendingPromise = minPendingPromise
@@ -285,7 +286,7 @@ export const MatchInner = React.memo(function MatchInnerImpl({
     // of a suspense boundary. This is the only way to get
     // renderToPipeableStream to not hang indefinitely.
     // We'll serialize the error and rethrow it on the client.
-    if (router.isServer) {
+    if (isServer ?? router.isServer) {
       const RouteErrorComponent =
         (route.options.errorComponent ??
           router.options.defaultErrorComponent) ||

packages/react-router/src/Matches.tsx

@@ -1,6 +1,6 @@
 import * as React from 'react'
 import warning from 'tiny-warning'
-import { rootRouteId } from '@tanstack/router-core'
+import { isServer, rootRouteId } from '@tanstack/router-core'
 import { CatchBoundary, ErrorComponent } from './CatchBoundary'
 import { useRouterState } from './useRouterState'
 import { useRouter } from './useRouter'
@@ -56,13 +56,14 @@ export function Matches() {
 
   // Do not render a root Suspense during SSR or hydrating from SSR
   const ResolvedSuspense =
-    router.isServer || (typeof document !== 'undefined' && router.ssr)
+    (isServer ?? router.isServer) ||
+    (typeof document !== 'undefined' && router.ssr)
       ? SafeFragment
       : React.Suspense
 
   const inner = (
     <ResolvedSuspense fallback={pendingElement}>
-      {!router.isServer && <Transitioner />}
+      {!(isServer ?? router.isServer) && <Transitioner />}
       <MatchesInner />
     </ResolvedSuspense>
   )

packages/react-router/src/ScriptOnce.tsx

@@ -1,11 +1,12 @@
+import { isServer } from '@tanstack/router-core'
 import { useRouter } from './useRouter'
 
 /**
  * Server-only helper to emit a script tag exactly once during SSR.
  */
 export function ScriptOnce({ children }: { children: string }) {
   const router = useRouter()
-  if (!router.isServer) {
+  if (!(isServer ?? router.isServer)) {
     return null
   }
 

packages/react-router/src/scroll-restoration.tsx

@@ -1,6 +1,7 @@
 import {
   defaultGetScrollRestorationKey,
   escapeHtml,
+  isServer,
   restoreScroll,
   storageKey,
 } from '@tanstack/router-core'
@@ -9,7 +10,7 @@ import { ScriptOnce } from './ScriptOnce'
 
 export function ScrollRestoration() {
   const router = useRouter()
-  if (!router.isScrollRestoring || !router.isServer) {
+  if (!router.isScrollRestoring || !(isServer ?? router.isServer)) {
     return null
   }
   if (typeof router.options.scrollRestoration === 'function') {

packages/react-router/vite.config.ts

@@ -5,6 +5,11 @@ import packageJson from './package.json'
 
 const config = defineConfig({
   plugins: [react()],
+  resolve: {
+    // Add 'development' condition for tests to resolve @tanstack/router-is-server
+    // to the development export (isServer = undefined) instead of node (isServer = true)
+    conditions: process.env.VITEST ? ['development'] : [],
+  },
   test: {
     name: packageJson.name,
     dir: './tests',

packages/router-core/package.json

@@ -80,6 +80,7 @@
   },
   "dependencies": {
     "@tanstack/history": "workspace:*",
+    "@tanstack/router-is-server": "workspace:*",
     "@tanstack/store": "^0.8.0",
     "cookie-es": "^2.0.0",
     "seroval": "^1.4.2",

packages/router-core/src/index.ts

@@ -105,6 +105,7 @@ export {
 export { encode, decode } from './qss'
 export { rootRouteId } from './root'
 export type { RootRouteId } from './root'
+export { isServer } from './isServer'
 
 export { BaseRoute, BaseRouteApi, BaseRootRoute } from './route'
 export type {

packages/router-core/src/isServer.ts

@@ -0,0 +1,24 @@
+/**
+ * Static server/client detection for tree-shaking support.
+ *
+ * This file re-exports `isServer` from `@tanstack/router-is-server` which uses
+ * conditional exports to provide different values based on the environment:
+ *
+ * - `browser` condition → `false` (client)
+ * - `node`/`worker`/`deno`/`bun` → `true` (server)
+ * - `development` condition → `undefined` (for tests, falls back to router.isServer)
+ *
+ * The bundler resolves the correct file at build time based on export conditions,
+ * and since the value is a literal constant, dead code can be eliminated.
+ *
+ * @example
+ * ```typescript
+ * import { isServer } from '@tanstack/router-core'
+ *
+ * // The ?? operator provides fallback for development/test mode
+ * if (isServer ?? router.isServer) {
+ *   // Server-only code - eliminated in client bundles
+ * }
+ * ```
+ */
+export { isServer } from '@tanstack/router-is-server'