Exploring Angular (5) http client capabilities with the options to observe and specify the response type as 'blob'

Question

Exploring Angular (5) http client capabilities with the options to observe and specify the response type as 'blob'

Situation: I'm facing a challenge in downloading a binary file from a backend system that requires certain data to be posted as JSON-body. The goal is to save this file using File-Saver with the filename specified by the backend in the content-disposition header. In order to access the necessary headers, I believe I need the HttpResponse object.

However, I've encountered difficulties while trying to utilize Angular's

HttpClient.post<T>(...): Observable<HttpResponse<T>>;

method when dealing with a Blob type.

Whenever I make the call:

this.httpclient.post<Blob>('MyBackendUrl', 
        params, 
        {observe: 'response', responseType: 'blob'});
the compiler raises an error related to the 'blob' argument ('json' is accepted without any issues):

error TS2345: Argument of type '{ observe: "response"; responseType: "blob"; }' is not assignable to parameter of type '{ headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: "body"; params?: Ht...'.
  Types of property 'observe' are incompatible.
    Type '"response"' is not assignable to type '"body"'.

When I tried wrapping the options in a separate object following advice on (without using "as" ...) it resulted in the post(...):Observable being executed but impeded my ability to retrieve the headers.

In addition, even a basic example like

return this.http.get<Blob>('backendUrl', {responseType: 'blob'});

as shown in did not work for me.

Software versions currently in use:

Angular Version: 5.0.3 (scheduled to update to the latest version 5 in approximately a week)
Typescript: 2.4.2
Webpack: 3.8.1

angular typescript angular-httpclient

Answer 1

Answer №1

When utilizing observe:response, refrain from typing the call (post<Blob>(...)), as the resultant Observable will be of HttpResponse. Therefore, this code snippet should function correctly:

this.httpclient.post('MyBackendUrl', 
    params,
    {observe: 'response', responseType: 'blob'}
);

The reason for this behavior is that there are two versions of the post method, one with a generic type and one without:

/**
     * Construct a POST request which interprets the body as JSON and returns the full event stream.
     *
     * @return an `Observable` of all `HttpEvent`s for the request, with a body type of `T`.
     */
    post<T>(url: string, body: any | null, options: {
        headers?: HttpHeaders | {
            [header: string]: string | string[];
        };
        observe: 'events';
        params?: HttpParams | {
            [param: string]: string | string[];
        };
        reportProgress?: boolean;
        responseType?: 'json';
        withCredentials?: boolean;
    }): Observable<HttpEvent<T>>;
    /**
     * Construct a POST request which interprets the body as an `ArrayBuffer` and returns the full response.
     *
     * @return an `Observable` of the `HttpResponse` for the request, with a body type of `ArrayBuffer`.
     */
    post(url: string, body: any | null, options: {
        headers?: HttpHeaders | {
            [header: string]: string | string[];
        };
        observe: 'response';
        params?: HttpParams | {
            [param: string]: string | string[];
        };
        reportProgress?: boolean;
        responseType: 'arraybuffer';
        withCredentials?: boolean;
    }): Observable<HttpResponse<ArrayBuffer>>;

Answer 2

When utilizing observe:response, refrain from typing the call (post<Blob>(...)), as the resultant Observable will be of HttpResponse. Therefore, this code snippet should function correctly:

this.httpclient.post('MyBackendUrl', 
    params,
    {observe: 'response', responseType: 'blob'}
);

The reason for this behavior is that there are two versions of the post method, one with a generic type and one without:

/**
     * Construct a POST request which interprets the body as JSON and returns the full event stream.
     *
     * @return an `Observable` of all `HttpEvent`s for the request, with a body type of `T`.
     */
    post<T>(url: string, body: any | null, options: {
        headers?: HttpHeaders | {
            [header: string]: string | string[];
        };
        observe: 'events';
        params?: HttpParams | {
            [param: string]: string | string[];
        };
        reportProgress?: boolean;
        responseType?: 'json';
        withCredentials?: boolean;
    }): Observable<HttpEvent<T>>;
    /**
     * Construct a POST request which interprets the body as an `ArrayBuffer` and returns the full response.
     *
     * @return an `Observable` of the `HttpResponse` for the request, with a body type of `ArrayBuffer`.
     */
    post(url: string, body: any | null, options: {
        headers?: HttpHeaders | {
            [header: string]: string | string[];
        };
        observe: 'response';
        params?: HttpParams | {
            [param: string]: string | string[];
        };
        reportProgress?: boolean;
        responseType: 'arraybuffer';
        withCredentials?: boolean;
    }): Observable<HttpResponse<ArrayBuffer>>;

Answer 3

Answer №2

one possible method is to utilize

responseType: 'blob' converted to 'json'

Answer 4

one possible method is to utilize

responseType: 'blob' converted to 'json'

Answer 5

Answer №3

While the previous responses provide valuable information, they lack a concrete example.

The primary solution involves setting the responseType to Blob when receiving a response from an API. To achieve this, include the parameter observe: 'response', which will result in an HTTPResponse being returned.

For instance: I encountered a similar issue and devoted 6 hours to resolving it.

Thus, I offer an illustration on how to extract a filename from headers and download the file:

downloadPDF(url: string): Observable<any> {
return this.http.get<any>(url, { responseType: 'blob', observe: 'response' }).pipe(
  map((result:HttpResponse<Blob>) => {
    console.log(result);
    saveAs(result, "Quotation.pdf");
    return result;
  }));

In this code snippet, http refers to an instance of HttpClient, while saveAs() is a function within the FileSaver npm package, as mentioned by the original poster.

Another potential issue arises when you only receive standard headers (such as Cache-Control and Pragma) in the result.headers, excluding custom ones like x-filename.

This limitation stems from CORS restrictions, which prevent browsers from accessing more than a few predefined headers. To address this, the server/API should send the Access-Control-Expose-Headers header along with the request.

Answer 6

While the previous responses provide valuable information, they lack a concrete example.

The primary solution involves setting the responseType to Blob when receiving a response from an API. To achieve this, include the parameter observe: 'response', which will result in an HTTPResponse being returned.

For instance: I encountered a similar issue and devoted 6 hours to resolving it.

Thus, I offer an illustration on how to extract a filename from headers and download the file:

downloadPDF(url: string): Observable<any> {
return this.http.get<any>(url, { responseType: 'blob', observe: 'response' }).pipe(
  map((result:HttpResponse<Blob>) => {
    console.log(result);
    saveAs(result, "Quotation.pdf");
    return result;
  }));

In this code snippet, http refers to an instance of HttpClient, while saveAs() is a function within the FileSaver npm package, as mentioned by the original poster.

Another potential issue arises when you only receive standard headers (such as Cache-Control and Pragma) in the result.headers, excluding custom ones like x-filename.

This limitation stems from CORS restrictions, which prevent browsers from accessing more than a few predefined headers. To address this, the server/API should send the Access-Control-Expose-Headers header along with the request.

Exploring Angular (5) http client capabilities with the options to observe and specify the response type as 'blob'

Answer №1

Answer №2

Answer №3

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