“rxjs” observable.throw is not a function - Angular4

Question

I've been learning Angular 4 and everything was going smoothly until I tried to implement catch handling in a service. I'm trying to use "rxjs" catch and throw but I've got an undefined function error in my console.

import { Injectable } from '@angular/core';
import { Http } from "@angular/http";
import { Observable } from 'rxjs/observable';
import 'rxjs/add/operator/catch';
import 'rxjs/add/observable/throw';
import { AppError } from "../app/common/app.error";
import { NotFoundError } from "../app/common/not-found-error";
import { BadInput } from "../app/common/bad-input";

@Injectable()
export class PostService {
  private url = "https://jsonplaceholder.typicode.com/posts";

  constructor(private http: Http) { }

 deletepost(post){
      // return this.http.delete(this.url + '/' + post.id)
      // Hard-coded id to test 404
      return this.http.delete(this.url + '/' + 93498)
        .catch((error: Response) => {
          console.log('error within catch is ' + Response)
          if(error.status === 404)
            return Observable.throw(new NotFoundError(error));

          return Observable.throw(new AppError(error));
        });
    }
}

This is the error message:

TypeError: __WEBPACK_IMPORTED_MODULE_2_rxjs_observable__["Observable"].throw is not a function. 
(In '__WEBPACK_IMPORTED_MODULE_2_rxjs_observable__["Observable"].throw(new 
__WEBPACK_IMPORTED_MODULE_6__app_common_not_found_error__["a" /* NotFoundError 
*/](error))', 
'__WEBPACK_IMPORTED_MODULE_2_rxjs_observable__["Observable"].throw' is 
undefined) — post.service.ts:42

I also have this warning in my browser:

./~/rxjs/Observable.js
There are multiple modules with names that only differ in casing.
This can lead to unexpected behavior when compiling on a filesystem with other case-semantic.
Use equal casing. Compare these module identifiers:
* /Users/nickgowdy/Desktop/Angular2/angular4 source code/hello-world/node_modules/rxjs/Observable.js
    Used by 14 module(s), i. e.
    /Users/nickgowdy/Desktop/Angular2/angular4 source code/hello-world/node_modules/@angular/core/@angular/core.es5.js
* /Users/nickgowdy/Desktop/Angular2/angular4 source code/hello-world/node_modules/rxjs/observable.js
    Used by 1 module(s), i. e.
    /Users/nickgowdy/Desktop/Angular2/angular4 source code/hello-world/node_modules/@ngtools/webpack/src/index.js!/Users/nickgowdy/Desktop/Angular2/angular4 source code/hello-world/src/services/post.service.ts

Answer 1

The error There are multiple modules with names that only differ in casing. is indicating the wrong import is being targeted with how you are trying to use Observable.

The import should be with a capital "O" like:

import { Observable } from 'rxjs/Observable';

This will import the individual Observable operator, which be used in combination with operators such as catch or throw on created Observables.

import 'rxjs/add/operator/catch';
import 'rxjs/add/observable/throw';

To import the full Observable object you'd import it like this:

import { Observable } from 'rxjs/Rx'

Hopefully that helps!

Update:

With newer versions of RxJS (5.5+) operators such as map() and filter() can used as pipeable operators in combination with pipe() rather than chaining. They are imported such as:

import { filter, map, catchError } from 'rxjs/operators';

Keep in mind terms such as throw are reserved/key words in JavaScript so the RxJS throw operator is imported as:

import { _throw } from 'rxjs/observable/throw';

Update:

For newer versions of RxJS (6+), use this:

import { throwError } from 'rxjs';

and throw an error like this:

if (error.status === 404)
    return throwError( new NotFoundError(error) )

Answer 2

In RxJS 6, Observable.throw() is replaced by throwError() which operates very similarly to its predecessor. So, you can replace from Observable.throw(error) to only throwError(error) by importing:

import { throwError } from 'rxjs';

Checkout this link for further reference: https://www.metaltoad.com/blog/angular-6-upgrading-api-calls-rxjs-6

Answer 3

I was facing the same issue in my angular 5 application. What I did is, adding a new package.

import { throwError } from 'rxjs';
import { filter, map, catchError } from 'rxjs/operators';

And from my http service call I return a function.

return this.http.request(request)
      .pipe(map((res: Response) => res.json()),
        catchError((res: Response) => this.onError(res)));

And in onError function I am returning the error with throwError(error).

onError(res: Response) {
    const statusCode = res.status;
    const body = res.json();
    const error = {
      statusCode: statusCode,
      error: body.error
    };
    return throwError(error);
  }