Exploring the HttpClientModule in Angular

December 30, 2019 · 4023 words · 19 min

angular

Table of Contents

This article has been published on indepth.dev.

Introduction

In this post, we are going to understand how the HttpClientModule actually works behind the scenes and find answers to some questions that might have arisen while using this module.

My favorite way to understand how things really work is by using the debugger while having the source code in my text editor so that I can explore and make assumptions easily.

Installing Angular on your machine

git clone -b 8.2.x --single-branch https://github.com/angular/angular.git

StackBlitz

You can find a StackBlitz demo here.

We are going to use it throughout the article in order to get a better understanding of how entities are connecting with each other.

What is HttpClientModule?

The HttpClientModule is a service module provided by Angular that allows us to perform HTTP requests and easily manipulate those requests and their responses. It is called a service module because it only instantiates services and does not export any components, directives or pipes.

Let’s start exploring

Once in the StackBlitz project:

open the dev tools
head over to token.interceptor.ts(CTRL + P) and put a breakpoint next to the console.warn()
refresh the StackBlitz browser

Now, you should see something like this:

Content image

By clicking on the anonymous function from client.ts, you are now in the HttpClient class, which is the one you usually inject in your services.

As you might have expected, this class comprises the methods for the well-known HTTP verbs.

export class HttpClient {
  constructor (private handler: HttpHandler) { }

  /* ... Method overloads ... */
  request(first: string | HttpRequest<any>, url?: string, options: {/* ... */}): Observable<any> {
      /* ... */
  }

  /* ... Method overloads ... */
  delete(url: string, options: {/* ... */}): Observable<any> {
      return this.request<any>('DELETE', url, options as any);
  }

  /* ... Method overloads ... */
  get(url: string, options: {/* ... */}): Observable<any> {
      return this.request<any>('GET', url, options as any);
  }

  /* ... Method overloads ... */
  post(url: string, body: any | null, options: {/* ... */}): Observable<any> {
      return this.request<any>('POST', url, addBody(options, body));
  }

  /* ... Method overloads ... */
  put(url: string, body: any | null, options: {/* ... */}): Observable<any> {
      return this.request<any>('PUT', url, addBody(options, body));
  }
}

I’d kindly recommend switching over to your text editor and start exploring this HttpClient.request method a little.

Continuing on, put a breakpoint on the line 492 and refresh the browser. The most interesting part is just about to begin!

Content image

At this point, we can’t step into this.handler.handle() because the observable is just being built and has no subscribers yet. So, we must manually set a breakpoint inside the handle method.

To do so, switch over to your text editor and scroll up to the constructor.
The HttpHandler is a DI token that maps to HttpInterceptingHandler.

Here’s a list of all providers:

@NgModule({
  /* ... */
  
  providers: [
      HttpClient,
      { provide: HttpHandler, useClass: HttpInterceptingHandler },
      HttpXhrBackend,
      { provide: HttpBackend, useExisting: HttpXhrBackend },
      BrowserXhr,
      { provide: XhrFactory, useExisting: BrowserXhr },
  ],
})
export class HttpClientModule {
}

What’s left to do is to go into HttpInterceptingHandler class and set a breakpoint inside the handle method.

After successufully identifying its location, switch back to your dev tools, add your breakpoint and resume the execution!

Content image

BarInterceptor is provided in app.module

Here we are able to grab all the interceptors by injecting the HTTP_INTERCEPTOR(a multi-provider token) inside our method.

The next step consists of creating the injectors chain.
But first, let’s have a quick look at HttpInterceptorHandler:

export class HttpInterceptorHandler implements HttpHandler {
  constructor(private next: HttpHandler, private interceptor: HttpInterceptor) { }

  handle(req: HttpRequest<any>): Observable<HttpEvent<any>> {
      return this.interceptor.intercept(req, this.next);
  }
}

I like to think of this chain as a linked list that is built starting off from the tail node.

In order to get a better overview of this, I’d suggest that you keep resuming the execution until you reach line 42, while paying attention to what’s going on in the Scope tab.

Now, after the chain has been built, we can go through the list starting off from the head node by stepping into the handle function from line 42.

Here’s how this linked list could look like:

Content image

Judging by the image above, we can tell that every next.handle() returns an observable.
What this means is that every interceptor can add custom behavior to the returned observable. Those changes will propagate in the precedent interceptors in the chain.

Before going any further, let’s focus our attention on this.backend. Where does it come from? If you take a look at the constructor, you should see that is provided by HttpBackend, which maps to HttpXhrBackend(if not sure why, check what this module provides).

Let’s explore `HttpXhrBackend`

Setting some breakpoints here and there will definitely lead to a better understanding! :)

export class HttpXhrBackend implements HttpBackend {
  constructor(private xhrFactory: XhrFactory) {}

  handle(req: HttpRequest<any>): Observable<HttpEvent<any>> {
    // Everything happens on Observable subscription.
    return new Observable((observer: Observer<HttpEvent<any>>) => {
      const xhr = this.xhrFactory.build();
      
        /* ... Setting up the headers ... */
        /* ... Setting up the response type & serializing the body ... */

      // partialFromXhr extracts the HttpHeaderResponse from the current XMLHttpRequest
      // state, and memoizes it into headerResponse.
      const partialFromXhr = (): HttpHeaderResponse => { /* ... */ };

      // First up is the load event, which represents a response being fully available.
      const onLoad = () => { /* ... */ };

      const onError = (error: ProgressEvent) => { /* ... */ };

      xhr.addEventListener('load', onLoad);
      xhr.addEventListener('error', onError);

      // Fire the request, and notify the event stream that it was fired.
      xhr.send(reqBody !);
      observer.next({type: HttpEventType.Sent});

      // This is the return from the Observable function, which is the
      // request cancellation handler.
      return () => {
        xhr.removeEventListener('error', onError);
        xhr.removeEventListener('load', onLoad);
        xhr.abort();
      };
    });
  }
}

The first thing that leaps to the eye is the handle() method, which is also the last method called in the interceptor chain because it sits in the tail node. It is also responsible for dispatching the request to the backend.

partialFromXhr - extracts the HttpHeaderResponse from the current XMLHttpRequest and memoizes it; this object needs to be computed only once can be used in multiple places. For example, it used in the onLoad and onError events

onLoad - the callback function triggered when the response is fully available; it also parses and validates the body of the response

const onLoad = () => {
  // Read response state from the memoized partial data.
  let { headers, status, statusText, url } = partialFromXhr();

  // The body will be read out if present.
  let body: any | null = null;

  let ok = status >= 200 && status < 300;

  /* ... Parse body and check its validity ... */

  if (ok) {
      // A successful response is delivered on the event stream.
      observer.next(new HttpResponse({
          body,
          headers,
          status,
          statusText,
          url: url || undefined,
      }));
      // The full body has been received and delivered, no further events
      // are possible. This request is complete.
      observer.complete();
  } else {
      // An unsuccessful request is delivered on the error channel.
      observer.error(new HttpErrorResponse({
          // The error in this case is the response body (error from the server).
          error: body,
          headers,
          status,
          statusText,
          url: url || undefined,
      }));
  }
}

onError - the callback function called when a network error occurred during the request

const onError = (error: ProgressEvent) => {
  const {url} = partialFromXhr();
  const res = new HttpErrorResponse({
    error,
    status: xhr.status || 0,
    statusText: xhr.statusText || 'Unknown Error',
    url: url || undefined,
  });
  observer.error(res);
};

Lastly, it is important to mention that the returned observable from HttpXhrBackend.handle() will dispatch the request when we subscribe to one of the HttpClient’s methods(get, post etc). This means that HttpXhrBackend.handle() returns a cold observable which can be subscribed to by using concatMap:

this.httpClient.get(url).subscribe() -> of(req).pipe(concatMap(req => this.handler.handle))

The callback returned from the observable

return () => {
  xhr.removeEventListener('error', onError);
  xhr.removeEventListener('load', onLoad);
  xhr.abort();
};

will be invoked when the observable stops emitting values. That is, when an error or a complete notification occurs.

onComplete

const obsBE$ = new Observable(obs => {
  timer(1000)
    .subscribe(() => {
      obs.next({ response: { data: ['foo', 'bar'] } });

      // Stop receiving values!
      obs.complete();
    })

    return () => {
      console.warn("I've had enough values!");
    }
});

obsBE$.subscribe(console.log)
/* 
-->
response
I've had enough values!
*/

onError

const be$ = new Observable(o => {
  o.next('foo');

  return () => {
    console.warn('NO MORE VALUES!');
  }
});

be$
 .pipe(
    flatMap(v => throwError('foo')),
 )
  .subscribe(null, console.error)
/* 
-->
foo
NO MORE VALUES
*/

How can a request be canceled?

A common case is the typeahead feature:

this.keyPressed
  .pipe(
      debounceTime(300),
      switchMap(v => this.http.get(url + '?key=' + v))
  )

It is advised to do things this way. The reason is due to switchMap’s magic, which will unsubscribe from the inner observable to handle the next emitted value.

const src = new Observable(obs => {
  obs.next('src 1');
  obs.next('src 2');
  
  setTimeout(() => {
    obs.next('src 3');
    obs.complete(); 
  }, 1000);

  return () => {
    console.log('called on unsubscription')
  };
});

of(1, 2)
  .pipe(
    switchMap(() => src)
  )
  .subscribe(console.log)

/* 
src 1
src 2
called on unsubscription ---> unsubscribed from because the next value(`2`) kicked in
src 1
src 2
src 3
called on unsubscription ---> completion
*/

1 is emitted and while we are waiting for the inner observable to complete, another value, 2, comes straight away and will make switchMap unsubscribe from the current inner observable which in turn will invoke the returned function from the observable.

Here is what is going on inside the function returned from the observable that dispatches the request(found in HttpXhrBackend.handle):

return () => {
  /* Skipped some lines for brevity... */

  xhr.removeEventListener('error', onError);
  xhr.removeEventListener('load', onLoad);
  
  // Finally, abort the in-flight request.
  xhr.abort();
}

Thus, we can infer that if the observable that made the request is unsubscribed from, the above callback will be invoked.

How can interceptors retry requests?

A token interceptor might look like this:

intercept (req: HttpRequest<any>, next: HttpHandler) {
  /* ... Attach token and all that good stuff ... */

  return next.handle()
    .pipe(
      catchError(err => {
        if (err instanceof HttpErrorResponse && err.status === 401) {
          return this.handle401Error(req, next)
        }

        // Simply propagate the error to other interceptors or to the consumer
        return throwError(err);
      })
    )
}

private handle401Error (req: HttpRequest<any>, next: HttpHandler) {
  return this.authService.refreshToken()
    .pipe(
      tap(token => this.authService.setToken(token)),
      map(token => this.attachToken(req, token))
      switchMap(req => next.handle(req))
    )
}

private attachToken(req: HttpRequest<any>, token: string): HttpRequest<any> {
  return req.clone({ setHeaders: { 'x-access-token': token } })
}

The retry logic can be achieved with switchMap(() => next.handle(req)).

If we reach the code inside catchError, it means that the consumer will unsubscribe from the observable(the one that is returned from HttpXhrBackend.handle).
This will allow us to re-subscribe to that observable, which will cause the request to be sent again as well as the interceptors that follow this interceptor to run their intercept function again.

Let’s narrow it down to a simpler example:

const obsBE$ = new Observable(obs => {
  timer(1000)
    .subscribe(() => {
      // console.log('%c [OBSERVABLE]', 'color: red;');

      obs.next({ response: { data: ['foo', 'bar'] } });

      // Stop receiving values!
      obs.complete();
    })

    return () => {
      console.warn("I've had enough values!");
    }
});

// Composing interceptors the chain
const obsI1$ = obsBE$
  .pipe(
    tap(() => console.log('%c [i1]', 'color: blue;')),
    map(r => ({ ...r, i1: 'intercepted by i1!' }))
  );

let retryCnt = 0;
const obsI2$ = obsI1$
  .pipe(
    tap(() => console.log('%c [i2]', 'color: green;')),
    map(r => { 
      if (++retryCnt <=3) {
        throw new Error('err!') 
      }

      return r;
    }),
    catchError((err, caught) => {
      return getRefreshToken()
        .pipe(
          switchMap(() => /* obsI2$ */caught),
        )
    })
  );

const obsI3$ = obsI2$
  .pipe(
    tap(() => console.log('%c [i3]', 'color: orange;')),
    map(r => ({ ...r, i3: 'intercepted by i3!' }))
  );

function getRefreshToken () {
  return timer(1500)
    .pipe(q
      map(() => ({ token: 'TOKEN HERE' })),
    );
}

function get () {
  return obsI3$
}

get()
  .subscribe(console.log)

/* 
-->
[i1]
[i2]
I've had enough values!
[i1]
[i2]
I've had enough values!
[i1]
[i2]
I've had enough values!
[i1]
[i2]
[i3]
{
  "response": {
    "data": [
      "foo",
      "bar"
    ]
  },
  "i1": "intercepted by i1!",
  "i3": "intercepted by i3!"
}
I've had enough values!
*/

StackBlitz.

This is, in my view, the effect of next.handle() inside each interceptor(Image here). Imagine that instead of const obsI3$ = obsI2$ we would have something like this:

// Interceptor Nr.2
const next = {
  handle(req) {
    /* ... Some logic here ... */

    return of({ response: '' })
  }
}

const obsI3$ = next.handle(req)
  .pipe(
    map(r => ({ ...r, i3: 'this is interceptor 3!!' })),
    /* ... */
  )

obsI3$ will now be the observable returned by next.handle() which means it can add now its own custom behavior and if something goes wrong, it can reinvoke the source observable.

When using interceptors you would want to retry the request by using switchMap(() => next.handle(req)(as it is done in the first code snippet), because, besides the each interceptor’s returned observable, you would also want to run their logic that sits inside their intercept() function.

From this line switchMap(() => /* obsI2$ */caught) we can see that catchError can have a second argument, caught, which is the source observable.(More on this here)

Why is it sometimes necessary to clone the request object inside an interceptor?

The process of adding the JWT token to your request might look like this:

if (token) {
  request = request.clone({
    setHeaders: { [this.AuthHeader]: token },
  });
}

return next.handle(request)

The most important reason would be immutability. You wouldn’t want to mutate the request object from multiple places. Thus, every interceptor should configure the request independently.
The cloned request would eventually be passed to the next interceptor in the chain.

Why is it recommended to load the HttpClientModule only once in AppModule or CoreModule?

A lazy-loaded module A will create its own child injector which will resolve the providers from that module. This means that the providers provided inside A and those provided by modules imported by A will be scoped to module A.

Importing HttpClientModule in A will result in only applying the interceptors that are provided inside A, excluding anything that is up in the injector tree. This is because HttpClientModule comes with its own providers which, as mentioned above, will be scoped to A.

  { provide: HttpHandler, useClass: ... }
  AppModule {    /
    imports: [  /
      HttpClientModule
    ]
  }
                  { provide: HttpHandler, useClass: HttpInterceptingHandler } <- where interceptors are gathered
  FeatureModule { /  <- lazy-loaded                  |
    imports: [   /                                   |
      HttpClientModule <------------------           |
    ]                                     |          |
                                          |          |
    declarations: [FeatureComponent]       <------------------------
    providers: [                                     |              |
                                                    /               |
      { provide: HTTP_INTERCEPTORS, useClass: FeatInterceptor_1 },  |
      { provide: HTTP_INTERCEPTORS, useClass: FeatInterceptor_2 }   |
    ]                                      ------------------------>
  }                                       |
                                          | httpClient.get()
  FeatureComponent {                      |
    constructor (private httpClient: HttpClient) { }
  }

If HttpClientModule was not imported in A, it would look up the injector tree until it finds the needed providers(in this case, it would be in AppModule). This also means that any interceptors provided in A will be excluded.

How can interceptors be completely bypassed?

TLDR;

Make sure that HttpHandler maps to HttpXhrBackend:

@NgModule({
  imports: [
    /* ... */
    HttpClientModule,
    /* ... */
  ],
  declarations: [ /* ... */ ],
  providers: [
    /* ... */
    {
      provide: HttpHandler,
      useExisting: HttpXhrBackend,
    },
    /* ... */
  ]
})
export class AppModule { }

Detailed Explanation

Whenever you do something like HttpClient.get()(or any other HTTP verb), the HttpClient.request() method will be eventually called. In this method, this line will be reached:

const events$: Observable<HttpEvent<any>> =
        of(req).pipe(concatMap((req: HttpRequest<any>) => this.handler.handle(req)));

Let’s see how this.handler is retrieved:

@Injectable()
export class HttpClient {
  constructor(private handler: HttpHandler) {}

  /* ... */
}

If we take a look at HttpClientModule’s providers,

@NgModule({
  /* ... */
  
  providers: [
      HttpClient,
      { provide: HttpHandler, useClass: HttpInterceptingHandler },
      HttpXhrBackend,
      { provide: HttpBackend, useExisting: HttpXhrBackend },
      BrowserXhr,
      { provide: XhrFactory, useExisting: BrowserXhr },
  ],
})
export class HttpClientModule {
}

we can tell that HttpHandler maps to HttpInterceptingHandler:

@Injectable()
export class HttpInterceptingHandler implements HttpHandler {
  private chain: HttpHandler|null = null;

  constructor(private backend: HttpBackend, private injector: Injector) {}

  handle(req: HttpRequest<any>): Observable<HttpEvent<any>> {
    if (this.chain === null) {
      const interceptors = this.injector.get(HTTP_INTERCEPTORS, []);
      this.chain = interceptors.reduceRight(
          (next, interceptor) => new HttpInterceptorHandler(next, interceptor), this.backend);
    }
    return this.chain.handle(req);
  }
}

HttpInterceptingHandler constructs the interceptor chain, which will in the end allow us to apply all the registered interceptors to the request.

We can also see that HttpInterceptingHandler implements HttpHandler:

export abstract class HttpHandler {
  abstract handle(req: HttpRequest<any>): Observable<HttpEvent<any>>;
}

HttpHandler is implemented by HttpBackend

export abstract class HttpBackend implements HttpHandler {
  abstract handle(req: HttpRequest<any>): Observable<HttpEvent<any>>;
}

HttpBackend is implemented by HttpXhrBackend, which will eventually send the request to the server(More on this here).

@Injectable()
export class HttpXhrBackend implements HttpBackend {
  constructor(private xhrFactory: XhrFactory) {}

  handle(req: HttpRequest<any>): Observable<HttpEvent<any>> {
    /* ... */
  }
}

As you can see, both HttpInterceptingHandler and HttpXhrBackend will have to implement the handle() method.
Consequently, the solution to this problem would be to make HttpHandler map to HttpXhrBackend.

@NgModule({
  imports: [
    /* ... */
    HttpClientModule,
    /* ... */
  ],
  declarations: [ /* ... */ ],
  providers: [
    /* ... */
    {
      provide: HttpHandler,
      useExisting: HttpXhrBackend,
    },
    /* ... */
  ]
})
export class AppModule { }

What is the difference between `setHeaders` and `headers`?

`setHeaders`

req = req.clone({
  setHeaders: { foo: 'bar' },
})

With setHeaders, we can append the provided headers to the existing ones.

`headers`

req = req.clone({
  setHeaders: { foo: 'bar' },
})

With headers(an instance of HttpHeaders), we can override the existing headers.

Here is the excerpt from the sources:

// Headers and params may be appended to if `setHeaders` or
// `setParams` are used.
let headers = update.headers || this.headers;
let params = update.params || this.params;

// Check whether the caller has asked to add headers.
if (update.setHeaders !== undefined) {
  // Set every requested header.
  headers =
      Object.keys(update.setHeaders)
          .reduce((headers, name) => headers.set(name, update.setHeaders ![name]), headers);
}

The same goes for setParams & params.

What’s the magic behind `HttpHeaders`?

HttpHeaders is a class that allows us to manipulate(perform CRUD operations on) headers for our requests.

Have a look at this example:

const headers = new HttpHeaders({
  foo: 'foo',
  bar: 'bar',
});

const newHeaders = headers
  .append('name', 'andrei')
  .set('city', 'tgv')
  .delete('foo')
  .has('abcd');

The magic about this is that it will only initialize the headers(key-value pairs) when they are actually needed. That is, when you want to query for their current state(HttpHeaders.forEach(), HttpHeaders.get() etc…).

Here’s how the constructor looks like:

constructor(headers?: string|{[name: string]: string | string[]}) {
  if (!headers) {
    this.headers = new Map<string, string[]>();
  } else if (typeof headers === 'string') {
    this.lazyInit = () => { /* ... */ }
  } else {
    this.lazyInit = () => { /* ... */ }
  }
}

As we can see, the lazyInit function is initialized in the constructor of HttpHeaders.
As a result, in order to perform actions such as HttpHeaders.append, HttpHeaders.set or HttpHeaders.delete, which would eventually mutate the initial state that was provided to the constructor, there will be a clone created that will store the new actions(create -> set, update -> append, delete -> delete).
These stored actions will be merged with the initial state in the end.

Here’s how the HttpHeaders.clone looks like:

// action
interface Update {
  name: string;
  value?: string|string[];
  op: 'a'|'s'|'d';
}

private clone(update: Update): HttpHeaders {
  const clone = new HttpHeaders();
  // Preserve the initialization across multiple clones
  clone.lazyInit =
      (!!this.lazyInit && this.lazyInit instanceof HttpHeaders) ? this.lazyInit : this;
  // Accumulate actions 
  clone.lazyUpdate = (this.lazyUpdate || []).concat([update]);
  return clone;
}

Let’s understand this logic by using our initial example:

const headers = new HttpHeaders({
  foo: 'foo',
  bar: 'bar',
});
/* 
-->
h1.lazyInit = () => {
  // Initialize headers
}
*/

const newHeaders = headers
  .append('name', 'andrei')
  /* 
  -->
  // Creating a clone
  h2.lazyInit = h1 // Preserving the first `instance` across multiple clones
  h2.lazyUpdate = { "name": "name", "value": "andrei", "op": "a" }
  */
  .set('city', 'tgv')
  /* 
  -->
  // Creating a clone
  // h2.lazyInit = h1
  h3.lazyInit = h2.lazyInit // Preserving the first `instance` across multiple clones
  h3.lazyUpdate = [
    { "name": "name", "value": "andrei", "op": "a" }, // append
    { "name": "city", "value": "tgv", "op": "s" } // set
  ]
  */
  .delete('foo')
  /* 
  -->
  // Creating a clone
  // h3.lazyInit = h2.lazyInit
  h4.lazyInit = h3.lazyInit // Preserving the first `instance` across multiple clones
  h4.lazyUpdate = [
    { "name": "name", "value": "andrei", "op": "a" },
    { "name": "city", "value": "tgv", "op": "s" },
    { "name": "foo", "op": "d" }
  ]
  */
  .has('abcd');
  /* 
  -->
  Here is where the initialization takes place
  */

The merging process would look like this:

private init(): void {
  if (!!this.lazyInit) {
    if (this.lazyInit instanceof HttpHeaders) {
      this.copyFrom(this.lazyInit);
    } else {
      this.lazyInit();
    }
    this.lazyInit = null;
    if (!!this.lazyUpdate) {
      this.lazyUpdate.forEach(update => this.applyUpdate(update));
      this.lazyUpdate = null;
    }
  }
}

private copyFrom(other: HttpHeaders) {
  other.init();
  Array.from(other.headers.keys()).forEach(key => {
    this.headers.set(key, other.headers.get(key) !);
    this.normalizedNames.set(key, other.normalizedNames.get(key) !);
  });
}

The HttpHeaders.init() method is called when querying for the state of the headers(by using methods like HttpHeaders.get(), HttpHeaders.has())

In HttpHeaders.copyFrom(), other will be the first instance of HttpHeaders, which contains the initialization logic: lazyInit. Calling other.init() will eventually reach this part of the HttpHeaders.init(): this.lazyInit();. Here the initial state is created into the first instance.

We are then left with two other things to do:

copy the state of the first instance into this current instance(last clone); this is achieved in these lines from HttpHeaders.copyFrom():

Array.from(other.headers.keys()).forEach(key => {
  this.headers.set(key, other.headers.get(key) !);
  this.normalizedNames.set(key, other.normalizedNames.get(key) !);
});

apply the collected actions onto the copied state:

// HttpHeaders.init()
if (!!this.lazyUpdate) {
  this.lazyUpdate.forEach(update => this.applyUpdate(update));
  this.lazyUpdate = null;
}

What about `HttpClientJsonpModule`?

JSONP is a way to solve the well-known CORS issue. It does so by treating the resource as a script file.

When we are requesting a resource with the script tag we can pass along a defined callback which the resource will eventually wrap the json response in. The callback will be called while the script is loading.

This module provides as a way to use JSONP without worrying too much about the aforementioned details.

Let’s quickly explore it and see why it is awesome!

@NgModule({
  providers: [
    JsonpClientBackend,
    {provide: JsonpCallbackContext, useFactory: jsonpCallbackContext},
    {provide: HTTP_INTERCEPTORS, useClass: JsonpInterceptor, multi: true},
  ],
})
export class HttpClientJsonpModule {
}

JsonpCallbackContext maps to jsonpCallbackContext which will return either the window object or an empty object(used in a testing environment). The returned object is used to store the callback which will in the end be called by script.

It also provides an interceptor, namely JsonpInterceptor. What this interceptor does is that it makes sure that our request never reaches the HttpBackend(which will contain completely different logic) when the request method is JSONP.

@Injectable()
export class JsonpInterceptor {
  constructor(private jsonp: JsonpClientBackend) {}

  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    if (req.method === 'JSONP') {
      return this.jsonp.handle(req as HttpRequest<never>);
    }

    // Fall through for normal HTTP requests.
    return next.handle(req); // Next interceptor in the chain
  }
}

JsonpClientBackend is where the magic happens. It automatically generates a callback that will be called later by the script. It does so by replacing the JSONP_CALLBACK parameter value in the url with the newly generated callback name.

export class JsonpClientBackend implements HttpBackend {
  private nextCallback(): string { return `ng_jsonp_callback_${nextRequestId++}`; }

  /* ... */

  handle (/* ... */) {
    return new Observable<HttpEvent<any>>((observer: Observer<HttpEvent<any>>) => {
      /* ... */
      const callback = this.nextCallback();
      const url = req.urlWithParams.replace(/=JSONP_CALLBACK(&|$)/, `=${callback}$1`);
    });
  }
}

Then, it stores the callback function in the object returned by jsonpCallbackContext using the generated callback name.

this.callbackMap[callback] = (data?: any) => { delete this.callbackMap[callback]; if (cancelled) { return; } body = data; finished = true; };

It is important to notice once again that the above callback should be called before the script finished downloading. This way, we can determine whether the provided callback was called or not in case we decide to provide the callback name ourselves.
This is done when the script has finished loading:

// Inside `JsonpClientBackend.handle`
const onLoad = (event: Event) => {
    // Maybe due to `switchMap`
    if (cancelled) {
      return;
    }

    cleanup();

    // Was the callback called with the response?
    if (!finished) {
      // If not, send the error response to the stream
      
      return;
    }

    // If yes, sent the response to the stream - everything was successful
}

Conclusion

I hope exploring this module was a fun adventure!

Thanks for reading!

A thorough exploration of Angular Forms

Introduction

Installing Angular on your machine

StackBlitz

What is HttpClientModule?

Let’s start exploring

Let’s explore HttpXhrBackend

onComplete

onError

How can a request be canceled?

How can interceptors retry requests?

Why is it sometimes necessary to clone the request object inside an interceptor?

Why is it recommended to load the HttpClientModule only once in AppModule or CoreModule?

How can interceptors be completely bypassed?

TLDR;

Detailed Explanation

What is the difference between setHeaders and headers?

setHeaders

headers

What’s the magic behind HttpHeaders?

What about HttpClientJsonpModule?

Conclusion

Let’s explore `HttpXhrBackend`

What is the difference between `setHeaders` and `headers`?

`setHeaders`

`headers`

What’s the magic behind `HttpHeaders`?

What about `HttpClientJsonpModule`?