Difference between JSON.parse and .json() in Javasscript

Question

I have a simple angular2 http get call like below

import { Http, Response } from '@angular/http';
@Injectable()
export class ServiceBase {
  protected resource: string;
  constructor(private _http: Http) { }

  getTnCByCountryCodeNdType(countryCode: string, tncType: string) {
        let url = `${CONFIG.URL}/tnc?countryCode=${country}&tnc=${tnc}`;

        return this._http.get(url, this.getHeader())
                    .map((res: Response) => {
                        return res.json();
                    });
    }
}

The Error response from the above when received,

I tried

this.service
      .getTnCByCountryCodeNdType('MY', this.tnc.toLowerCase())
              .subscribe(x => {
                //
              },
              (err: Response) => {
                error = JSON.parse(err);
              });

where: err is the response error;

and it threw the usual json error .

EXCEPTION: Unexpected token R in JSON at position 0

To my surprise

error = err.json();

Works fine. What is the difference between the two and why does the first one failed? Any help is appreciated

err is not defined or even declared in the code you've provided — Quentin
– Quentin, Commented Apr 27, 2017 at 9:49
@Quentin its an angular 2 service. Please i update code above. — Nuru Salihu
– Nuru Salihu, Commented Apr 27, 2017 at 10:00

T.J. Crowder · Accepted Answer · 2017-04-27 10:15:17Z

3

JSON.parse is a JavaScript thing. It parses a string containing JSON into whatever that string represents. You don't pass it objects. Clearly, your err isn't a string.

The res and err that Angular 2's http gives you are Response objects, which says it derives from Body. I can't see anything in the Angular 2 docs saying what Body is, but clearly it has a json function that returns the result of parsing the response data as JSON. That's not a JavaScript thing, that's something from Angular. (As noted in the link above, it's inspired by — but not the same as — fetch.)

edited Apr 27, 2017 at 10:15

answered Apr 27, 2017 at 9:51

T.J. Crowder

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1 Comment

Nuru Salihu Over a year ago

Crowder thanks for the answer. Please i updated my code above.

Yakov Fain · Accepted Answer · 2017-04-27 11:05:45Z

2

JSON.parse expects a JSON-formatted string, but you give it a Response object, which is not JSON, hence the parsing error.

res.json() extracts the JSON-formatted data from the Response object and converts the data into a JavaScript object.

answered Apr 27, 2017 at 11:05

Yakov Fain

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