GreetingsService.ts

import { ServiceKey } from "@microsoft/sp-core-library";

export interface IGreetingsService {
    sayHello(who: string) : string;
}

export class GreetingsService implements IGreetingsService {
    public sayHello(who: string) : string {
        return `Hello ${who}`;
    }
}

export const GreetingsServiceKey = ServiceKey.create<IGreetingsService>(
	'YPCODE:GreetingsService',
	GreetingsService
);