# How to Make HTTP Requests ## About HttpClient Angular has the amazing [HttpClient](https://angular.io/guide/http) for communication with backend services. It is a layer on top and a simplified representation of [XMLHttpRequest Web API](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest). It also is the recommended agent by Angular for any HTTP request. There is nothing wrong with using the `HttpClient` in your ABP project. However, `HttpClient` leaves error handling to the caller (method). In other words, HTTP errors are handled manually and by hooking into the observer of the `Observable` returned. ```js getConfig() { this.http.get(this.configUrl).subscribe( config => this.updateConfig(config), error => { // Handle error here }, ); } ``` Although clear and flexible, handling errors this way is repetitive work, even when error processing is delegated to the store or any other injectable. An `HttpInterceptor` is able to catch `HttpErrorResponse`  and can be used for a centralized error handling. Nevertheless, cases where default error handler, therefore the interceptor, must be disabled require additional work and comprehension of Angular internals. Check [this issue](https://github.com/angular/angular/issues/20203) for details. ## RestService ABP core module has a utility service for HTTP requests: `RestService`. Unless explicitly configured otherwise, it catches HTTP errors and dispatches a `RestOccurError` action. This action is then captured by the `ErrorHandler` introduced by the `ThemeSharedModule`. Since you should already import this module in your app, when the `RestService` is used, all HTTP errors get automatically handled by deafult. ### Getting Started with RestService In order to use the `RestService`, you must inject it in your class as a dependency. ```js import { RestService } from '@abp/ng.core'; @Injectable({ /* class metadata here */ }) class DemoService { constructor(private rest: RestService) {} } ``` You do not have to provide the `RestService` at module or component/directive level, because it is already **provided in root**. ### How to Make a Request with RestService You can use the `request` method of the `RestService` is for HTTP requests. Here is an example: ```js getFoo(id: number) { const request: Rest.Request = { method: 'GET', url: '/api/some/path/to/foo/' + id, }; return this.rest.request(request); } ``` The `request` method always returns an `Observable`. Therefore you can do the following wherever you use `getFoo` method: ```js doSomethingWithFoo(id: number) { this.demoService.getFoo(id).subscribe( foo => { // Do something with foo. } ) } ``` **You do not have to worry about unsubscription.** The `RestService` uses `HttpClient` behind the scenes, so every observable it returns is a finite observable, i.e. it closes subscriptions automatically upon success or error. As you see, `request` method gets a request options object with `Rest.Request` type. This generic type expects the interface of the request body. You may pass `null` when there is no body, like in a `GET` or a `DELETE` request. Here is an example where there is one: ```js postFoo(body: Foo) { const request: Rest.Request = { method: 'POST', url: '/api/some/path/to/foo', body }; return this.rest.request(request); } ``` You may [check here](https://github.com/abpframework/abp/blob/dev/npm/ng-packs/packages/core/src/lib/models/rest.ts#L23) for complete `Rest.Request` type, which has only a few chages compared to [HttpRequest](https://angular.io/api/common/http/HttpRequest) class in Angular. ### How to Disable Default Error Handler of RestService The `request` method, used with defaults, always handles errors. Let's see how you can change that behavior and handle errors yourself: ```js deleteFoo(id: number) { const request: Rest.Request = { method: 'DELETE', url: '/api/some/path/to/foo/' + id, }; return this.rest.request(request, { skipHandleError: true }); } ``` `skipHandleError` config option, when set to `true`, disables the error handler and the returned observable starts throwing an error that you can catch in your subscription. ```js removeFooFromList(id: number) { this.demoService.deleteFoo(id).subscribe( foo => { // Do something with foo. }, error => { // Do something with error. } ) } ``` ### How to Get a Specific API Endpoint From Application Config Another nice config option that `request` method receives is `apiName` (available as of v2.4), which can be used to get a specific module endpoint from application configuration. ```js putFoo(body: Foo, id: string) { const request: Rest.Request = { method: 'PUT', url: '/' + id, body }; return this.rest.request(request, {apiName: 'foo'}); } ``` `putFoo` above will request `https://localhost:44305/api/some/path/to/foo/{id}` as long as the environment variables are as follows: ```js // environment.ts export const environment = { apis: { default: { url: 'https://localhost:44305', }, foo: { url: 'https://localhost:44305/api/some/path/to/foo', }, }, /* rest of the environment variables here */ } ``` ### How to Observe Response Object or HTTP Events Instead of Body `RestService` assumes you are generally interested in the body of a response and, by default, sets `observe` property as `'body'`. However, there may be times you are rather interested in something else, such as a custom proprietary header. For that, the `request` method receives `observe` property in its config object. ```js getSomeCustomHeaderValue() { const request: Rest.Request = { method: 'GET', url: '/api/some/path/that/sends/some-custom-header', }; return this.rest.request>( request, {observe: Rest.Observe.Response}, ).pipe( map(response => response.headers.get('Some-Custom-Header')) ); } ``` You may find `Rest.Observe` enum [here](https://github.com/abpframework/abp/blob/dev/npm/ng-packs/packages/core/src/lib/models/rest.ts#L10). ## What's Next? * [Localization](./Localization.md)