typed-config-manager.ts

interface MyConfig {
  dbName: string;
  port: number;
  env: 'prod' | 'dev' | 'test';
}

class TypedConfigManager<T = any> {
  getConfig<K extends keyof T>(key: K, fallback: T[K]) {
    // do magic stuf here.
  }
}

const myConfig = new TypedConfigManager<MyConfig>();

// ❌ Error: Argument of type '"blah"' is not assignable to parameter of type '"prod" | "dev" | "test"'.
myConfig.getConfig('env', 'blah');
// ✔ Correct
myConfig.getConfig('dbName', 'lolo');
// ❌ Error: Argument of type '"1000"' is not assignable to parameter of type 'number'.
myConfig.getConfig('port', '1000');