added config service tests

test/services/config.service.test.ts

@@ -0,0 +1,40 @@
+import { expect } from 'chai';
+import crypto from 'crypto';
+import { ConfigService } from '../../src/services/config.service';
+
+import { config } from 'dotenv';
+config();
+
+const env = Object.assign({}, process.env);
+
+describe('Config service', () => {
+  beforeEach(() => {
+    process.env = env;
+  });
+
+  after(() => {
+    process.env = env;
+  });
+
+  it('When an env property is requested, then the get method return its value', async () => {
+    const envKey = 'APP_CRYPTO_SECRET';
+    const envValue = crypto.randomBytes(8).toString('hex');
+    process.env[envKey] = envValue;
+
+    const newEnvValue = ConfigService.instance.get(envKey);
+    expect(newEnvValue).to.equal(envValue);
+  });
+
+  it('When an env property that do not have value is requested, then an error is thrown', async () => {
+    const envKey = 'APP_CRYPTO_SECRET';
+    process.env = {};
+
+    try {
+      ConfigService.instance.get(envKey);
+      expect(false).to.be.true; //should throw error
+    } catch (err) {
+      const error = err as Error;
+      expect(error.message).to.equal(`Config key ${envKey} was not found in process.env`);
+    }
+  });
+});