return promise from computed

controllers.application.js

import Ember from 'ember';
import {inject as service} from '@ember/service';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  companiesService: service('companies')
});

services.companies.js

import {computed}  from '@ember/object';
import PromiseProxyMixin from "@ember/object/promise-proxy-mixin";
import ArrayProxy from "@ember/array/proxy";
import Service from '@ember/service';
import RSVP from 'rsvp';

const PendingResponse = ArrayProxy.extend(PromiseProxyMixin);

export default Service.extend({
	companies: computed(function() {
    // Usual case with ember-data would be:
    // let promise = this.store.fetchAll("company"); 
    
    // Faking the data here for the sake of simplicity:
    let promise = new RSVP.Promise((resolve) => {
      setTimeout(() => resolve([{name: 'foo'}, {name: 'bar'}]), 3000)
    });
        
    let responseObject = PendingResponse.create({ promise });
    responseObject.catch(() => {});
    return responseObject;
  }),
  
  firstCompany: computed('companies.[]', function() {
  	return this.get('companies.firstObject');
  })
});

templates.application.hbs

<h1>List of my companies:</h1>

<ul>
	{{#each companiesService.companies as |company|}}
		<li>{{company.name}}</li>
	{{/each}}
</ul>

And first company is: {{companiesService.firstCompany.name}}

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}