Learn TypeScript 3 by Building Web Applications

Creating the service implementation skeleton

Create a file called population-service.ts under src/services and add the skeleton to it:

import {PopulationService} from "./population-service.intf"; 
import {Country, DataPoint} from "../domain"; 
import { 
    WorldBankApiV2, 
} from "./world-bank-api"; 

export class PopulationServiceImpl implements PopulationService { 
    private readonly countriesApiBaseUrl: string;

    constructor(baseUrl: string) { 
        if (!baseUrl || baseUrl.trim().length === 0) { 
            throw new Error("The base URL must be provided!"); 
        } else if (!baseUrl.toLocaleLowerCase().startsWith('https://')         && !baseUrl.toLocaleLowerCase().startsWith('http://')) { 
            throw new Error("The URL looks invalid. It should start  with 'http://' or https://'"); } let ...