Yield example

yield.ts

// Run it live on: https://goo.gl/dgwE2D
class Example {
    public getCountries(fields: string[] = ['name', 'alpha2Code', 'translations', 'flag']): Observable<any[]> {
        const sep = (function* () {
            let call = 0;
            while (true) {
                yield (call++ === 0) ? '?' : '&';
            }
        })();

        const url = `https://restcountries.eu/rest/v2/all`
            +
            ((fields && fields.length > 0) ? sep.next().value + 'fields=' + fields.join(';')
                : '')
            ;

        return url;
    }
}

const y = new Example(); 
document.write('<a href="'+y.getCountries()+'">'+y.getCountries()+'</a>');