Troubleshooting problem with TypeScript observables in Angular 5

Having trouble with a messaging app, specifically an error related to TS.

The syntax checker in the Editor is flagging this issue: Type 'Observable<{}>' is not compatible with type 'Observable'. Type '{}' cannot be assigned to type 'MessageThread[]'. Property 'includes' is missing in type '{}'.

The CODE

import { Injectable } from '@angular/core';
import { Http, Response, Headers, RequestOptions } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import * as io from 'socket.io-client';

import { MessageThread } from './message-thread.model';

@Injectable()
export class MessageThreadService {

    private getMessageThreadsUrl = 'api/message-thread/get';  
    private postMessageThreadsUrl = 'api/message-thread/post';  

    constructor(private http: Http) { }

    private socket;
    private url = window.location.origin;

    /*
    * Get MessageThreads from server
    */
    getMessageThreads(): Observable<MessageThread[]> {
        const observable = new Observable(observer => {
            console.log('Socket:', this.url);
            this.socket = io(this.url);
            this.socket.on('refreshMessageThreads', (data) => {
                observer.next(data);
            });

            return () => {
                this.socket.disconnect();
            };
        });

        return observable; 
    }

...

An error is being thrown by the angular transpiler:

ERROR in src/app/components/messages/message-thread.service.ts(35,3): error TS2322: Type 'Observable<{}>' is not assignable to type 'Observable'. Type '{}' is not assignable to type 'MessageThread[]'. Property 'includes' is missing in type '{}'.

This is how the Message Thread Model is structured:

export class MessageThread {
    constructor(
        public messageThread: string
    ) { }

}

Any help would be greatly appreciated.

Answer №1

The main issue lies in the error message section below:

Type 'Observable<{}>' is not compatible with type 'Observable'.
Type '{}' cannot be assigned to type 'MessageThread[]'.

When creating an instance of a class that requires a generic, TypeScript will default that generic to {} if it's not inferred elsewhere. In this scenario, both instances look similar:

const observable = new Observable(observer => {
const observable = new Observable<{}>(observer => {

The issue arises when attempting to return Observable<{}> to a function expecting

Observable<MessageThread[]>
. To resolve this problem, manually specify the generic type:

const observable = new Observable<MessageThread[]>(observer => {

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