sercret_service.ts

import { Injectable, Inject } from '@angular/core';

@Injectable()
export class SecretService {
  
 constructor( @Inject('ENcRYPTI0n_k3y') private key: string, private features: FeatureFlags) {}
 //public api
 encode(secret: string) {
   return SecretService._encodeSecret(this.key, secret);
 }
 
 //private api
 static _mapToKeyChar(key: string, char: str, index: number) {
   const len = key.length;
   const dist = index > len-1 ? index - len : len - index;
   return key[dist];
 }
 
 static _encodeSecret(key: string, secret: string) {
   return secret
   .split('')
   .reverse()
   .map((char, index, str) => {
      return SecretService._mapToKeyChar(key, char, index); 
   })
   .join('');
 }
 
}

function createService(key, flags) {
  const featureFlags = new FeatureFlags(flags) //imaging flags is an object {"feature1": false ...
  return new SecretService(key, featureFlags);
}

describe('Secret Service', () => {
    it('uses better encryption when its on', () => {
      const svc = createService("key", {'more_entropy': true}); //Create a 'context'
      const result = SecretService.prototype.encode.call(svc); //Test the function at that 'context'
      expect(result).toBe(blahblhanblah);
    });
  
  ...
});