fix(): prevent unexpected process env stringification

lib/config.module.ts

@@ -192,13 +192,18 @@ export class ConfigModule {
     return config;
   }
 
-  private static assignVariablesToProcess(config: Record<string, any>) {
+  private static assignVariablesToProcess(config: Record<string, unknown>) {
     if (!isObject(config)) {
       return;
     }
     const keys = Object.keys(config).filter(key => !(key in process.env));
     keys.forEach(
-      key => (process.env[key] = (config as Record<string, any>)[key]),
+      key => {
+        const value = config[key];
+        if (typeof value === 'string') {
+          process.env[key] = value;
+        }
+      },
     );
   }
 

tests/e2e/optional.spec.ts

@@ -0,0 +1,33 @@
+import { Test } from '@nestjs/testing';
+import { AppModule } from '../src/app.module';
+import { ConfigService } from '../../lib';
+
+describe('Optional environment variables', () => {
+  it('should return undefined for optional variables', async () => {
+    const module = await Test.createTestingModule({
+      imports: [AppModule.withValidateFunction(() => ({
+        optional: undefined,
+      }))],
+    }).compile();
+
+    const app = module.createNestApplication();
+    await app.init();
+
+    const optional = module.get(ConfigService).get('optional')
+
+    expect(optional).toEqual(undefined)
+  });
+
+  it('should not assign complex objects back to process.env', async () => {
+    const module = await Test.createTestingModule({
+      imports: [AppModule.withValidateFunction(() => ({
+        complex: {hello: 'there'},
+      }))],
+    }).compile();
+
+    const app = module.createNestApplication();
+    await app.init();
+
+    expect(process.env.complex).toEqual(undefined)
+  });
+});