Angular 2: How to call a function after get a response from subscribe http.post

Update your get_categories() method to return the total (wrapped in an observable):

// Note that .subscribe() is gone and I've added a return.
get_categories(number) {
  return this.http.post( url, body, {headers: headers, withCredentials:true})
    .map(response => response.json());
}

In search_categories(), you can subscribe the observable returned by get_categories() (or you could keep transforming it by chaining more RxJS operators):

// send_categories() is now called after get_categories().
search_categories() {
  this.get_categories(1)
    // The .subscribe() method accepts 3 callbacks
    .subscribe(
      // The 1st callback handles the data emitted by the observable.
      // In your case, it's the JSON data extracted from the response.
      // That's where you'll find your total property.
      (jsonData) => {
        this.send_categories(jsonData.total);
      },
      // The 2nd callback handles errors.
      (err) => console.error(err),
      // The 3rd callback handles the "complete" event.
      () => console.log("observable complete")
    );
}

Note that you only subscribe ONCE, at the end.

Like I said in the comments, the .subscribe() method of any observable accepts 3 callbacks like this:

obs.subscribe(
  nextCallback,
  errorCallback,
  completeCallback
);

They must be passed in this order. You don’t have to pass all three. Many times only the nextCallback is implemented:

obs.subscribe(nextCallback);