Angular Knowledge Patch (v19–v21)

Baseline: Angular through v18.x (signals basics, new control flow, standalone components, zoneless experimental, SSR hydration basics, esbuild builder).

This patch covers features from Angular v19 through v21 (2024-11 to 2025-11).

Index

Topic	File	Key APIs
Signals & Reactivity	references/signals-and-reactivity.md	linkedSignal, resource, httpResource
Signal Forms	references/signal-forms.md	form(), FormField, validators
SSR & Hydration	references/ssr-and-hydration.md	RenderMode, ServerRoute, incremental hydration
Zoneless & Testing	references/zoneless-and-testing.md	provideZonelessChangeDetection, Vitest
Components & Templates	references/components-and-templates.md	Angular Aria, regex in templates, @defer viewport

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Quick Reference

Signal APIs at a glance

API	Status	Import	Purpose
linkedSignal	Stable (v20)	@angular/core	Writable signal that resets when source changes
resource	Experimental	@angular/core	Signal-based async data loading
httpResource	Experimental	@angular/common/http	Signal-based HTTP fetching (reads only)
form()	Experimental (v21+)	@angular/forms/signals	Signal-based forms with schema validation

linkedSignal — short form

const selected = linkedSignal(() => options()[0]);
selected.set(options()[2]); // writable
// Resets to options()[0] when options() changes

resource — basic pattern

import { resource, Signal } from '@angular/core';

const userId: Signal<string> = getUserId();
const userResource = resource({
  params: () => ({ id: userId() }),
  loader: ({ params, abortSignal }) =>
    fetch(`/users/${params.id}`, { signal: abortSignal }),
});
// userResource.value(), .isLoading(), .error(), .hasValue()
// IMPORTANT: .value() throws in error state — guard with .hasValue()

httpResource — basic pattern

import { httpResource } from '@angular/common/http';

// Reactive URL — re-fetches when userId() changes
const user = httpResource<User>(() => `/api/user/${userId()}`);
// user.value(), .isLoading(), .error(), .hasValue(), .headers()
// Only for reads — use HttpClient directly for mutations

Zoneless Angular (stable v20.2, default v21)

bootstrapApplication(AppComponent, {
  providers: [
    provideZonelessChangeDetection(),
    provideBrowserGlobalErrorListeners(),
  ],
});
// Remove zone.js polyfill from angular.json

Vitest (default in v21)

# Migrate existing Jasmine tests
ng g @schematics/angular:refactor-jasmine-vitest

Route-level render mode

import { RenderMode, ServerRoute } from '@angular/ssr';

export const serverRouteConfig: ServerRoute[] = [
  { path: '/login', mode: RenderMode.Server },
  { path: '/dashboard', mode: RenderMode.Client },
  {
    path: '/product/:id',
    mode: RenderMode.Prerender,
    async getPrerenderPaths() {
      return (await inject(ProductService).getIds()).map((id) => ({ id }));
    },
  },
  { path: '/**', mode: RenderMode.Prerender },
];

Incremental hydration

provideClientHydration(withIncrementalHydration())

// Template
@defer (hydrate on viewport) {
  <shopping-cart/>
}

Signal Forms — minimal example

import { form, FormField, required, email, submit } from '@angular/forms/signals';

loginModel = signal({ email: '', password: '' });
loginForm = form(this.loginModel, (schema) => {
  required(schema.email, { message: 'Email is required' });
  email(schema.email, { message: 'Invalid email' });
  required(schema.password, { message: 'Password is required' });
});

Template: <input [formField]="loginForm.email" />, access state via loginForm.email() → .value(), .touched(), .valid(), .errors(), .dirty(), .pending().

linkedSignal — long form (preserve selection across source changes)

const selected = linkedSignal<ShippingMethod[], ShippingMethod>({
  source: shippingOptions,
  computation: (newOptions, previous) =>
    newOptions.find(o => o.id === previous?.value.id) ?? newOptions[0],
});

httpResource — advanced request + Zod validation

const user = httpResource(() => ({
  url: `/api/user/${userId()}`,
  method: 'GET',
  headers: { 'X-Special': 'true' },
  params: { fast: 'yes' },
}));

// Response type variants
httpResource.text(() => url);
httpResource.blob(() => url);

// Zod validation
const res = httpResource(() => `/api/people/${id()}`, {
  parse: starWarsPersonSchema.parse,
});

Angular Aria (developer preview, v21)

Headless accessible components: npm i @angular/aria. Patterns: Accordion, Combobox, Grid, Listbox, Menu, Tabs, Toolbar, Tree. Unstyled — you provide all CSS.