The type 'Observable<Response | Observable<Response>>' cannot be assigned to the type 'Observable<Response>'

Question

The type 'Observable<Response | Observable<Response>>' cannot be assigned to the type 'Observable<Response>'

My service features a basic structure:

import { Injectable } from '@angular/core';
import { Http, Response } from '@angular/http';

import 'rxjs/add/observable/throw';
import 'rxjs/add/operator/catch';
import 'rxjs/add/operator/map';
import { Observable } from 'rxjs/Observable';

@Injectable()
export class AddressService {

  constructor(private http: Http) { }

  getAnything = (): Observable<Response> => {
    return this.http.get('https://my_api.com')
      .map(this.handleSuccess)
      .catch(this.handleError);
  }

  handleError = (error: Response): Observable<Response> => {
    return Observable.throw(error || 'Server Error');
  }

  handleSuccess = (response: Response): Observable<Response> => {
    let body;

    if (response.text()) {
      body = response.json();
    }

    return body || {};
  }
}

Everything was functioning flawlessly, up until the upgrade of Typescript from version 2.3.4 to 2.4.1.

Post-upgrade, I encounter a puzzling error:

Type 'Observable<Response | Observable<Response>>' is not assignable to type 'Observable<Response>'

What's causing this issue? What modifications in TS 2.4.x have impacted the functionality of my application?

angular typescript observable

Answer 1

Answer №1

TypeScript version 2.4 brought improvements in checking for generics. It now flags errors in your code that need to be addressed.

One example is the mismatch between the return type of handleSuccess and what it actually returns; it returns an anonymous object, but is incorrectly typed as returning Observable<Response>. This leads to a situation where, when used with map, you end up having a nested observable that's typed as

Observable<Response | Observable<Response>>

.

The errors indicated by TypeScript are legitimate and should not be ignored.

Answer 2

TypeScript version 2.4 brought improvements in checking for generics. It now flags errors in your code that need to be addressed.

One example is the mismatch between the return type of handleSuccess and what it actually returns; it returns an anonymous object, but is incorrectly typed as returning Observable<Response>. This leads to a situation where, when used with map, you end up having a nested observable that's typed as

Observable<Response | Observable<Response>>

.

The errors indicated by TypeScript are legitimate and should not be ignored.

Answer 3

Answer №2

The update brings about a more rigorous type-checking process.

When dealing with an Observable<Response>, it cannot be assigned to something defined as Response due to the mismatch in types. Let's refer to these types as X and Y for simplicity.

Observable<X | Y> cannot be directly assigned to something typed as Y because it could potentially contain both X and Y, which is restricted by the stricter checking introduced in version 2.4.1.

To resolve this issue, store the value of your expression in a let variable and return that variable. This way, you can examine the type of the variable and see that it is not compatible as per the compiler's warning.

To successfully compile your code, you need to verify the instance of the correct type before performing any casting.

If time constraints prevent you from cleaning up your codebase, you have the option to reduce the strictness of the type-checking using --noStrictGenericChecks.

Below is an example of TypeScript code demonstrating a similar problem. In this case, FieldInstance does not have a Form property while FormInstance does, allowing for differentiation and resolution of the type ambiguity. FieldInstanceRepeater can be assigned to FieldInstance since it is a subclass.

constructor(scope: FormInstance | FieldInstanceRepeater, field: IFormFieldRow) {
  this.FormInstance = ((function getFormInstance(fi: FormInstance | FieldInstance): FormInstance | FieldInstance {
    let isFormInstance = !!(fi as FormInstance).Form;
    return isFormInstance ? fi : getFormInstance((fi as FieldInstance).Scope);
  })(scope) as FormInstance);
  this.Scope = scope;
  ...
}

Answer 4

The update brings about a more rigorous type-checking process.

When dealing with an Observable<Response>, it cannot be assigned to something defined as Response due to the mismatch in types. Let's refer to these types as X and Y for simplicity.

Observable<X | Y> cannot be directly assigned to something typed as Y because it could potentially contain both X and Y, which is restricted by the stricter checking introduced in version 2.4.1.

To resolve this issue, store the value of your expression in a let variable and return that variable. This way, you can examine the type of the variable and see that it is not compatible as per the compiler's warning.

To successfully compile your code, you need to verify the instance of the correct type before performing any casting.

If time constraints prevent you from cleaning up your codebase, you have the option to reduce the strictness of the type-checking using --noStrictGenericChecks.

Below is an example of TypeScript code demonstrating a similar problem. In this case, FieldInstance does not have a Form property while FormInstance does, allowing for differentiation and resolution of the type ambiguity. FieldInstanceRepeater can be assigned to FieldInstance since it is a subclass.

constructor(scope: FormInstance | FieldInstanceRepeater, field: IFormFieldRow) {
  this.FormInstance = ((function getFormInstance(fi: FormInstance | FieldInstance): FormInstance | FieldInstance {
    let isFormInstance = !!(fi as FormInstance).Form;
    return isFormInstance ? fi : getFormInstance((fi as FieldInstance).Scope);
  })(scope) as FormInstance);
  this.Scope = scope;
  ...
}

Answer 5

Answer №3

Here are some suggested changes you might want to consider:

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

export interface Data{} // This interface defines the object in JSON format returned by your backend

@Injectable()
export class AddressService {

  constructor(private http: Http) { }

  getAnything = (): Observable<Data> => {
    return this.http.get('https://my_api.com')
      .map(this.handleSuccess)
      .catch(this.handleError);
  }

  handleError = (error: Response) : ErrorObservable { 
    return _throw(error || 'Server Error');
  }

  handleSuccess = (response: Response) {
    let body;

    if (response.text()) {
      body = response.json();
    }

    return body || {};
  }
}

The latest versions of TypeScript have improved type and generics detection, which is why the compiler may be flagging issues with your current methods.

In the most recent release of Angular, the HTTP client has been updated to avoid dealing with untyped POJOs when extracting response content. The new HTTP methods now support generics, allowing developers to map directly into a specific type. For more information, refer to this link.

Answer 6

Here are some suggested changes you might want to consider:

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

export interface Data{} // This interface defines the object in JSON format returned by your backend

@Injectable()
export class AddressService {

  constructor(private http: Http) { }

  getAnything = (): Observable<Data> => {
    return this.http.get('https://my_api.com')
      .map(this.handleSuccess)
      .catch(this.handleError);
  }

  handleError = (error: Response) : ErrorObservable { 
    return _throw(error || 'Server Error');
  }

  handleSuccess = (response: Response) {
    let body;

    if (response.text()) {
      body = response.json();
    }

    return body || {};
  }
}

The latest versions of TypeScript have improved type and generics detection, which is why the compiler may be flagging issues with your current methods.

In the most recent release of Angular, the HTTP client has been updated to avoid dealing with untyped POJOs when extracting response content. The new HTTP methods now support generics, allowing developers to map directly into a specific type. For more information, refer to this link.

The type 'Observable<Response | Observable<Response>>' cannot be assigned to the type 'Observable<Response>'

Answer №1

Answer №2

Answer №3

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