SSR Common Issues

Each issue follows the same structure: symptom → cause → fix → prevention.

`window is not defined` / `document is not defined`

Symptom

ReferenceError: window is not defined
    at UserComponent.ngOnInit (main.server.mjs:1:2345)

Cause Node.js has no DOM. window, document, localStorage, sessionStorage, navigator and location are browser globals.

Fix 1 — Move the code to afterNextRender (preferred)

import { afterNextRender } from '@angular/core';

export class ThemeComponent {
  constructor() {
    afterNextRender(() => {
      this.theme = localStorage.getItem('theme') ?? 'light';
    });
  }
}

Fix 2 — Platform guard when you genuinely need a branch

import { PLATFORM_ID, inject } from '@angular/core';
import { isPlatformBrowser } from '@angular/common';

export class GeoService {
  private readonly isBrowser = isPlatformBrowser(inject(PLATFORM_ID));

  /**
   * Returns null on the server, where the Geolocation API is unavailable.
   */
  getPosition(): Promise<GeolocationPosition> | null {
    return this.isBrowser ? this.requestPosition() : null;
  }
}

Fix 3 — Inject DOCUMENT instead of using the global

import { DOCUMENT } from '@angular/common';

private readonly doc = inject(DOCUMENT);
// Works on both platforms: on the server it is a Domino document.

Prevention Add an ESLint rule banning direct use of browser globals, and always run ng build && node dist/.../server.mjs in CI, not only ng serve.

Every API call happens twice

Symptom The Network tab shows the same GET /api/products executed on the server and again in the browser. The list flickers.

Cause TransferState deduplication is not active.

Fix

providers: [
  provideClientHydration(),
  provideHttpClient(withFetch()), // ← mandatory
]

Only GET and HEAD are cached automatically. For POST you must opt in per request:

this.http.post<Result>('/api/search', body, {
  transferCache: { includeHeaders: [] },
});

Prevention Add an assertion in your smoke tests: after loading a page, no request should be repeated.

Hydration mismatch `NG0500` / `NG0505`

See the dedicated section in Hydration.

Quick triage:

Warning	Most likely cause
`NG0500`	Node count differs — invalid HTML nesting or manual DOM insertion
`NG0501`	Text content differs — non-deterministic value (date, random)
`NG0502`	Node type differs — conditional rendering based on browser API
`NG0505`	Hydration enabled but no server-rendered markup found

Authentication and cookies do not work on the server

Symptom Server-rendered pages always show the logged-out state, then flip to logged-in after hydration.

Cause The server-side fetch has no browser cookie jar. The incoming request cookies are never forwarded to your API.

Fix Forward the request headers using the REQUEST token and a functional interceptor:

import { REQUEST } from '@angular/core';
import { HttpInterceptorFn } from '@angular/common/http';

/**
 * Without this, authenticated pages render logged-out on the server.
 */
export const serverAuthInterceptor: HttpInterceptorFn = (req, next) => {
  const request = inject(REQUEST, { optional: true });
  const cookie = request?.headers.get('cookie');

  return cookie
    ? next(req.clone({ setHeaders: { cookie } }))
    : next(req);
};

Prevention For highly personalised routes, consider RenderMode.Client instead: SSR adds complexity with no SEO benefit behind a login.

Third-party library crashes on the server

Symptom

TypeError: Cannot read properties of undefined (reading 'documentElement')

from a charting, map or editor library.

Cause The library touches the DOM at import time.

Fix — Lazy import inside afterNextRender

constructor() {
  afterNextRender(async () => {
    const { Chart } = await import('chart.js/auto');
    new Chart(this.canvas.nativeElement, this.config);
  });
}

Combine with @defer so the chunk never reaches the server bundle:

@defer (on viewport) {
  <app-sales-chart [data]="data()" />
} @placeholder {
  <div class="chart-skeleton"></div>
}

Prevention Prefer libraries that declare SSR support. Check that they do not access window at module scope.

Node.js native addons break the SSR build

Symptom The server build fails with Cannot find module './build/Release/xxx.node', or the bundle tries to inline a .node binary.

Cause The esbuild-based builder tries to bundle a native dependency.

Fix Mark it as external in angular.json:

{
  "server": "src/main.server.ts",
  "externalDependencies": ["sharp", "better-sqlite3"]
}

Prevention Keep native dependencies out of the Angular server bundle entirely: put them behind a separate API process whenever possible.

Memory grows on every request

Symptom The Node process memory climbs steadily under load and eventually OOMs.

Cause Module-level mutable state. On the server, module scope is shared between all requests — it is not per-user.

// ❌ Shared by every visitor, leaks and cross-contaminates data
let currentUser: User | null = null;

Fix Keep all state inside Angular services scoped to the request injector. Never store per-user data in module-level variables or static fields.

Prevention Code review rule: no let or mutable export const at module scope in code reachable from the server bundle.

Routes return 404 after deployment

Symptom / works, /products/42 returns 404 from the reverse proxy.

Cause The proxy is serving static files only and never reaches the Node server.

Fix Route all non-asset traffic to the SSR process:

location / {
  try_files $uri @ssr;
}

location @ssr {
  proxy_pass http://127.0.0.1:4000;
  proxy_set_header Host $host;
  proxy_set_header X-Forwarded-Proto $scheme;
}

Prevention Include a deep-link smoke test (curl /products/42) in the deployment pipeline.

SSR is slower than CSR

Symptom TTFB jumps from 50 ms to 900 ms.

Cause (in order of likelihood)

Slow upstream API calls executed synchronously during rendering.
No caching layer in front of the SSR server.
Rendering routes that should be prerendered.
Zone.js waiting for a never-completing pending task (long-poll, setInterval).