HTTP is the primary protocol for browser/server communication.

Modern browsers support two HTTP-based APIs: XMLHttpRequest (XHR) and JSONP. A few browsers also support Fetch.

The Angular HTTP library simplifies application programming with the XHR and JSONP APIs. This page covers:

• The Tour of Heroes HTTP client demo.
• Fetch data with http.get.
• RxJS library.
• Enable RxJS operators.
• Process the response object.
• Always handle errors.
• Send data to the server.
• Fall back to promises.
• Cross-Origin Requests: Wikipedia example.
  • Search parameters.
  • More fun with observables.
• Guarding against Cross-Site Request Forgery.
• Override default request headers (and other request options).
• Appendix: Tour of Heroes in-memory web api.

A live example illustrates these topics.

Demos

This page describes server communication with the help of the following demos:

• The Tour of Heroes HTTP client demo.
• Fall back to Promises.
• Cross-Origin Requests: Wikipedia example.
• More fun with observables.

The root AppComponent orchestrates these demos:

There is nothing remarkable here except for the import of RxJS operators, which is described later.

Providing HTTP services

First, configure the application to use server communication facilities.

The Angular Http client communicates with the server using a familiar HTTP request/response protocol. The Http client is one of a family of services in the Angular HTTP library.

Before you can use the Http client, you need to register it as a service provider with the dependency injection system.

Register providers by importing other NgModules to the root NgModule in app.module.ts.

Begin by importing the necessary members. The newcomers are the HttpModule and the JsonpModule from the Angular HTTP library. For more information about imports and related terminology, see the MDN reference on the import statement.

To add these modules to the application, pass them to the imports array in the root @NgModule.

The Tour of Heroes HTTP client demo

The first demo is a mini-version of the tutorial's "Tour of Heroes" (ToH) application. This version gets some heroes from the server, displays them in a list, lets the user add new heroes, and saves them to the server. The app uses the Angular Http client to communicate via XMLHttpRequest (XHR).

It works like this:

This demo has a single component, the HeroListComponent. Here's its template:

It presents the list of heroes with an ngFor. Below the list is an input box and an Add Hero button where you can enter the names of new heroes and add them to the database. A template reference variable, newHeroName, accesses the value of the input box in the (click) event binding. When the user clicks the button, that value passes to the component's addHero method and then the event binding clears it to make it ready for a new hero name.

Below the button is an area for an error message.

The HeroListComponent class

Here's the component class:

Angular injects a HeroService into the constructor and the component calls that service to fetch and save data.

The component does not talk directly to the Angular Http client. The component doesn't know or care how it gets the data. It delegates to the HeroService.

This is a golden rule: always delegate data access to a supporting service class.

Although at runtime the component requests heroes immediately after creation, you don't call the service's get method in the component's constructor. Instead, call it inside the ngOnInit lifecycle hook and rely on Angular to call ngOnInit when it instantiates this component.

The service's getHeroes() and addHero() methods return an Observable of hero data that the Angular Http client fetched from the server.

Think of an Observable as a stream of events published by some source. To listen for events in this stream, subscribe to the Observable. These subscriptions specify the actions to take when the web request produces a success event (with the hero data in the event payload) or a fail event (with the error in the payload).

With a basic understanding of the component, you're ready to look inside the HeroService.

Fetch data with http.get

In many of the previous samples the app faked the interaction with the server by returning mock heroes in a service like this one:

You can revise that HeroService to get the heroes from the server using the Angular Http client service:

Notice that the Angular Http client service is injected into the HeroService constructor.

Look closely at how to call http.get:

You pass the resource URL to get and it calls the server which returns heroes.

If you are familiar with asynchronous methods in modern JavaScript, you might expect the get method to return a promise. You'd expect to chain a call to then() and extract the heroes. Instead you're calling a map() method. Clearly this is not a promise.

In fact, the http.get method returns an Observable of HTTP Responses (Observable<Response>) from the RxJS library and map is one of the RxJS operators.

RxJS library

RxJS ("Reactive Extensions") is a 3rd party library, endorsed by Angular, that implements the asynchronous observable pattern.

All of the Developer Guide samples have installed the RxJS npm package and loaded via system.js because observables are used widely in Angular applications.

The app needs it when working with the HTTP client. Additionally, you must take a critical extra step to make RxJS observables usable.

Enable RxJS operators

The RxJS library is large. Size matters when building a production application and deploying it to mobile devices. You should include only necessary features.

Accordingly, Angular exposes a stripped down version of Observable in the rxjs/Observable module that lacks most of the operators such as the map method you called above in getHeroes.

It's up to you to add the operators you need.

You could add every RxJS operator with a single import statement. While that is the easiest thing to do, you'd pay a penalty in extended launch time and application size because the full library is so big.

Since this app only uses a few operators, it's better to import each Observable operator and static class method, one-by-one, for a custom Observable implementation tuned precisely to the app's requirements. Put the import statements in one app/rxjs-operators.ts file.

If you forget an operator, the TypeScript compiler warns that it's missing and you'll update this file.

Finally, import rxjs-operator into app.component.ts:

Now continue to the next section to return to the HeroService.

Process the response object

Remember that the getHeroes() method used an extractData helper method to map the http.get response object to heroes:

The response object doesn't hold the data in a form the app can use directly. You must parse the response data into a JSON object.

Parse to JSON

The response data are in JSON string form. The app must parse that string into JavaScript objects by calling response.json().

Do not return the response object

The getHeroes() method could have returned the HTTP response but this wouldn't be a best practice. The point of a data service is to hide the server interaction details from consumers. The component that calls the HeroService only wants heroes and is kept separate from getting them, the code dealing with where they come from, and the response object.

Always handle errors

An important part of dealing with I/O is anticipating errors by preparing to catch them and do something with them. One way to handle errors is to pass an error message back to the component for presentation to the user, but only if it says something that the user can understand and act upon.

This simple app conveys that idea, albeit imperfectly, in the way it handles a getHeroes error.

The catch operator passes the error object from http to the handleError method. The handleError method transforms the error into a developer-friendly message, logs it to the console, and returns the message in a new, failed observable via Observable.throw.

HeroListComponent error handling

Back in the HeroListComponent, in heroService.getHeroes(), the subscribe function has a second function parameter to handle the error message. It sets an errorMessage variable that's bound conditionally in the HeroListComponent template.

Send data to the server

So far you've seen how to retrieve data from a remote location using an HTTP service. Now you'll add the ability to create new heroes and save them in the backend.

You'll write a method for the HeroListComponent to call, an addHero() method, that takes just the name of a new hero and returns an Observable of Hero. It begins like this:

To implement it, you must know the server's API for creating heroes.

This sample's data server follows typical REST guidelines. It expects a POST request at the same endpoint as GET heroes. It expects the new hero data to arrive in the body of the request, structured like a Hero entity but without the id property. The body of the request should look like this:

The server generates the id and returns the entire JSON representation of the new hero including its generated id. The hero arrives tucked inside a response object with its own data property.

Now that you know how the API works, implement addHero()as follows:

Headers

In the headers object, the Content-Type specifies that the body represents JSON.

Next, the headers object is used to configure the options object. The options object is a new instance of RequestOptions, a class that allows you to specify certain settings when instantiating a request. In this way, headers is one of the RequestOptions.

In the return statement, options is the third argument of the post method, as shown above.

JSON results

As with getHeroes(), use the extractData() helper to extract the data from the response.

Back in the HeroListComponent, its addHero() method subscribes to the observable returned by the service's addHero() method. When the data arrive it pushes the new hero object into its heroes array for presentation to the user.

Fall back to promises

Although the Angular http client API returns an Observable<Response> you can turn it into a Promise<Response>. It's easy to do, and in simple cases, a promise-based version looks much like the observable-based version.

Here is a comparison of the HeroService using promises versus observables, highlighting just the parts that are different.

You can follow the promise then(this.extractData).catch(this.handleError) pattern as in this example.

Alternatively, you can call toPromise(success, fail). The observable's map callback moves to the first success parameter and its catch callback to the second fail parameter in this pattern: .toPromise(this.extractData, this.handleError).

The errorHandler forwards an error message as a failed promise instead of a failed observable.

The diagnostic log to console is just one more then in the promise chain.

You have to adjust the calling component to expect a Promise instead of an observable:

The only obvious difference is that you call then on the returned promise instead of subscribe. Both methods take the same functional arguments.

Cross-Origin Requests: Wikipedia example

You just learned how to make XMLHttpRequests using the Angular Http service. This is the most common approach for server communication, but it doesn't work in all scenarios.

For security reasons, web browsers block XHR calls to a remote server whose origin is different from the origin of the web page. The origin is the combination of URI scheme, hostname, and port number. This is called the same-origin policy.

Some servers do not support CORS but do support an older, read-only alternative called JSONP. Wikipedia is one such server.

Search wikipedia

Here is a simple search that shows suggestions from Wikipedia as the user types in a text box:

Wikipedia offers a modern CORS API and a legacy JSONP search API. This example uses the latter. The Angular Jsonp service both extends the Http service for JSONP and restricts you to GET requests. All other HTTP methods throw an error because JSONP is a read-only facility.

As always, wrap the interaction with an Angular data access client service inside a dedicated service, here called WikipediaService.

The constructor expects Angular to inject its jsonp service, which is available because JsonpModule is in the root @NgModule imports array in app.module.ts.

Search parameters

The Wikipedia "opensearch" API expects four parameters (key/value pairs) to arrive in the request URL's query string. The keys are search, action, format, and callback. The value of the search key is the user-supplied search term to find in Wikipedia. The other three are the fixed values "opensearch", "json", and "JSONP_CALLBACK" respectively.

If you're looking for articles with the word "Angular", you could construct the query string by hand and call jsonp like this:

In more parameterized examples you could build the query string with the Angular URLSearchParams helper:

This time you call jsonp with two arguments: the wikiUrl and an options object whose search property is the params object.

Jsonp flattens the params object into the same query string you saw earlier, putting the request on the wire.

The WikiComponent

Now that you have a service that can query the Wikipedia API turn to the component (template and class) that takes user input and displays search results.

The template presents an <input> element search box to gather search terms from the user, and calls a search(term) method after each keyup event.

The component's search(term) method delegates to the WikipediaService, which returns an observable array of string results (Observable<string[]>). Instead of subscribing to the observable inside the component, as in the HeroListComponent, the app forwards the observable result to the template (via items) where the async pipe in the ngFor handles the subscription. Read more about async pipes in the Pipes page.

A wasteful app

The wikipedia search makes too many calls to the server. It is inefficient, and potentially expensive on mobile devices with limited data plans.

1. Wait for the user to stop typing

Presently, the code calls the server after every key stroke. It should only make requests when the user stops typing . Here's how it will work after refactoring:

2. Search when the search term changes

Suppose a user enters the word angular in the search box and pauses for a while. The application issues a search request for angular.

Then the user backspaces over the last three letters, lar, and immediately re-types lar before pausing once more. The search term is still angular. The app shouldn't make another request.

3. Cope with out-of-order responses

The user enters angular, pauses, clears the search box, and enters http. The application issues two search requests, one for angular and one for http.

Which response arrives first? It's unpredictable. A load balancer could dispatch the requests to two different servers with different response times. The results from the first angular request might arrive after the later http results. The user will be confused if the angular results display to the http query.

When there are multiple requests in-flight, the app should present the responses in the original request order. That won't happen if angular results arrive last.

More fun with observables

You can address these problems and improve the app with the help of some nifty observable operators.

You could make changes to the WikipediaService, but for a better user experience, create a copy of the WikiComponent instead and make it smarter. Here's the WikiSmartComponent which uses the same template.

Create a stream of search terms

The template still binds to the search box keyup event and passes the complete search box value into the component's search method after every user keystroke.

The WikiSmartComponent turns the search box values into an observable stream of search terms with the help of a Subject which you import from the RxJS observable library:

The component creates a searchTermStream as a Subject of type string. The search method adds each new search box value to that stream via the subject's next method.

Listen for search terms

Earlier, you passed each search term directly to the service and bound the template to the service results.

Now you listen to the stream of search terms, manipulating the stream before it reaches the WikipediaService.

Wait for the user to stop typing for at least 300 milliseconds (debounceTime). Only changed search values make it through to the service (distinctUntilChanged).

The WikipediaService returns a separate observable of string arrays (Observable<string[]>) for each request. There could be multiple requests in-flight, all awaiting the server's reply, which means multiple observables-of-strings could arrive at any moment in any order.

The switchMap (formerly known as flatMapLatest) returns a new observable that combines these WikipediaService observables, re-arranges them in their original request order, and delivers to subscribers only the most recent search results.

The displayed list of search results stays in sync with the user's sequence of search terms.

Guarding against Cross-Site Request Forgery

In a cross-site request forgery (CSRF or XSRF), an attacker tricks the user into visiting a different web page with malignant code that secretly sends a malicious request to your application's web server,

The server and client application must work together to thwart this attack. Angular's Http client does its part by applying a default CookieXSRFStrategy automatically to all requests.

The CookieXSRFStrategy supports a common anti-XSRF technique in which the server sends a randomly generated authentication token in a cookie named XSRF-TOKEN. The HTTP client adds an X-XSRF-TOKEN header with that token value to subsequent requests. The server receives both the cookie and the header, compares them, and processes the request only if the cookie and header match.

See the XSRF topic on the Security page for more information about XSRF and Angular's XSRFStrategy counter measures.

Override default request headers (and other request options)

Request options (such as headers) are merged into the default RequestOptions before the request is processed. The HttpModule provides these default options via the RequestOptions token.

You can override these defaults to suit your application needs. by creating a custom sub-class of RequestOptions that sets the default options for the application.

This sample creates a class that sets the default Content-Type header to JSON. It exports a constant with the necessary RequestOptions provider to simplify registration in AppModule.

Then it registers the provider in the root AppModule.

After this change, the header option setting in HeroService.addHero is no longer necessary,

You can confirm that DefaultRequestOptions is working by examing HTTP requests in the browser developer tools' network tab. If you're short-circuiting the server call with something like the in-memory web api, try commenting-out the addHero header option, set a breakpoint on the POST call, and step through the request processing to verify the header is there.

Individual requests options, like this one, take precedence over the default RequestOptions. It might be wise to keep the addHero request header setting for extra safety.

Appendix: Tour of Heroes in-memory web api

If the app only needed to retrieve data, you could get the heroes from a heroes.json file:

You'd set the endpoint to the JSON file like this:

The get heroes scenario would work, but since the app can't save changes to a JSON file, it needs a web API server. Because there isn't a real server for this demo, it substitutes the Angular in-memory web api simulator for the actual XHR backend service.

The in-memory web API gets its data from a custom application class with a createDb() method that returns a map whose keys are collection names and whose values are arrays of objects in those collections.

Here's the class for this sample, based on the JSON data:

Ensure that the HeroService endpoint refers to the web API:

Finally, redirect client HTTP requests to the in-memory web API by adding the InMemoryWebApiModule to the AppModule.imports list. At the same time, call its forRoot configuration method with the HeroData class.

How it works

Angular's http service delegates the client/server communication tasks to a helper service called the XHRBackend.

Using standard Angular provider registration techniques, the InMemoryWebApiModule replaces the default XHRBackend service with its own in-memory alternative. At the same time, the forRoot method initializes the in-memory web API with the seed data from the mock hero dataset.

Here is the final, revised version of app/app.module.ts, demonstrating these steps.

See the full source code in the .