Angular4 - How to detect if a response is JSON or plain text with observable

Question

I'm wondering if there is a way to detect the format of a response in an observable, when using a generic api service to call a variety of endpoints. Some send back JSON, others plain text.

this.http.get(endpoint).map(response => {
    // if JSON
    return response.json();
    // else if plain text
    return response.text();
})...

Answer 1

A ResponseContentType exists in the angular doc but I never found this enum in the response object so I use a "trick" :

this.http.get(endpoint).map(response => {
    const contentType = res.headers.get('Content-type');
   if (contentType == 'application/json') {
    return response.json();   
   } else if (contentType == 'application/text') {
    return response.text();
   }
})...

if someone know where we can find the ResponseContentType enum please told us !

Answer 2

I was never able to find a built in way with Angular to verify JSON, so I just check the content type in the response header.

Here's the function I use:

/**
 * True of the response content type is JSON.
 */
private static isJson(value: Response): boolean {
    return /\bapplication\/json\b/.test(value.headers.get('Content-Type'));
}

Content-Type can have extra stuff in the string so I use a regex to be safe.

I would recommend failing if it's not JSON.

 this.http.get().map((value:Response)=>{
      if(this.isJson(value)) {
         return value;
      }
      throw value;
 });

You can then catch the known JSON responses later in the subscriber. Now you know all responses in the subscriber are getting JSON.