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| # HTTP communication with `HttpClient` and `httpResource` | ||
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| Use Angular's HTTP APIs for backend communication so requests participate in dependency injection, interceptors, transfer cache, and security features. | ||
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| ## Setup | ||
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| In Angular v21 and later, `HttpClient` is available for injection by default. Add `provideHttpClient(...)` only when an app needs to configure HTTP features for a specific injector: | ||
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| ```ts | ||
| import {provideHttpClient, withInterceptors} from '@angular/common/http'; | ||
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| export const appConfig = { | ||
| providers: [provideHttpClient(withInterceptors([authInterceptor]))], | ||
| }; | ||
| ``` | ||
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| - `HttpClient` uses the fetch backend by default. | ||
| - Use `withXhr()` only when upload progress events are required. Do not use `withXhr()` for server-side rendering. | ||
| - Use `provideHttpClient(...)` for feature configuration such as interceptors, XSRF options, XHR, or parent-request delegation. | ||
| - Calling `provideHttpClient()` with no features is not required for basic HTTP requests, but it configures the default HTTP feature set for that injector, including Angular's XSRF interceptor. | ||
| - Prefer `provideHttpClient(...)` over `HttpClientModule` for feature configuration, especially with multiple injectors. | ||
| - Use `withRequestsMadeViaParent()` when a child injector should add interceptors while still delegating to the parent HTTP chain. | ||
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| ## `HttpClient` | ||
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| Encapsulate backend calls in injectable services, not components: | ||
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| ```ts | ||
| import {HttpClient} from '@angular/common/http'; | ||
| import {Service, inject} from '@angular/core'; | ||
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| @Service() | ||
| export class UserService { | ||
| private readonly http = inject(HttpClient); | ||
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| getUser(id: string) { | ||
| return this.http.get<User>(`/api/users/${id}`); | ||
| } | ||
| } | ||
| ``` | ||
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| Important rules: | ||
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| - `HttpClient` requests are cold `Observable`s. No request is sent until the `Observable` is subscribed to. Multiple subscriptions send multiple backend requests. | ||
| - Subscribe to mutation requests (`post`, `put`, `patch`, `delete`) so they execute. | ||
| - The generic type parameter is a type assertion only. Validate unknown backend data at runtime when the shape is not trusted. | ||
| - Use literal values for `responseType` and `observe`; if options are extracted, write values like `responseType: 'text' as const`. | ||
| - `HttpHeaders` and `HttpParams` are immutable; use the returned instance from `.set()` or `.append()`. | ||
| - Fetch options such as `timeout`, `cache`, `priority`, `mode`, `redirect`, `credentials`, `keepalive`, `referrer`, `referrerPolicy`, and `integrity` are supported where the backend supports them. `withCredentials: true` overrides `credentials`. | ||
| - Handle failures through `HttpErrorResponse`. Network and timeout failures use status `0`; backend failures use the server status code. | ||
| - Prefer the `async` pipe or `toSignal` for component reads so subscriptions are cleaned up. | ||
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| ## Interceptors | ||
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| Prefer functional interceptors configured with `withInterceptors`. | ||
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| ```ts | ||
| import { | ||
| HttpHandlerFn, | ||
| HttpRequest, | ||
| provideHttpClient, | ||
| withInterceptors, | ||
| } from '@angular/common/http'; | ||
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| export function authInterceptor(req: HttpRequest<unknown>, next: HttpHandlerFn) { | ||
| return next(req.clone({setHeaders: {Authorization: 'Bearer token'}})); | ||
| } | ||
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| export const appConfig = { | ||
| providers: [provideHttpClient(withInterceptors([authInterceptor]))], | ||
| }; | ||
| ``` | ||
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| - Interceptors run in the order listed. | ||
| - Request and response objects are mostly immutable; clone before changing them. | ||
| - Request and response bodies are not deeply immutable. Avoid in-place body mutation because retries can run the same interceptor again. | ||
| - Use `inject()` inside functional interceptors for services. | ||
| - Use `HttpContextToken` for per-request metadata that interceptors need but the backend should not receive. | ||
| - Use DI-based interceptors only for existing code, and enable them with `withInterceptorsFromDi()`. | ||
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| ## Security | ||
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| - `HttpClient` strips the XSSI prefix from JSON responses when present. | ||
| - `provideHttpClient()` configures XSRF protection by default for mutating relative and same-origin requests. It reads the `XSRF-TOKEN` cookie and sends the `X-XSRF-TOKEN` header. | ||
| - The backend must set the XSRF cookie and verify the header. Customize names with `withXsrfConfiguration(...)`; disable only deliberately with `withNoXsrfProtection()`. | ||
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| ## `httpResource` | ||
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| Use `httpResource` to create an asynchronous derivation that fetches data over HTTP and exposes the result as reactive signals. | ||
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| ```ts | ||
| import {httpResource} from '@angular/common/http'; | ||
| import {input} from '@angular/core'; | ||
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| export class UserProfile { | ||
| readonly userId = input.required<string>(); | ||
| readonly user = httpResource(() => `/api/users/${this.userId()}`); | ||
| } | ||
| ``` | ||
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| - `httpResource` is eager. It sends a request when its reactive request computation runs, not when an `Observable` is subscribed. | ||
| - When a dependency changes, it cancels the pending request and sends the next one. | ||
| - Return `undefined` from the request function to skip a backend request. | ||
| - Prefer `httpResource` for reads. Use `HttpClient` directly for mutations such as `POST`, `PUT`, `PATCH`, and `DELETE`. | ||
| - Guard `value()` reads with `hasValue()`; reading `value()` while the resource is in an error state throws. | ||
| - Use `httpResource.text`, `httpResource.blob`, or `httpResource.arrayBuffer` for non-JSON responses. | ||
| - Use the `parse` option to validate or transform responses with a runtime schema. | ||
| - Read `headers()`, `statusCode()`, and `progress()` when response metadata or download progress is needed. Set `reportProgress: true` for progress events. | ||
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