Example to extract type from argument with generic types

index.ts

// Given type definition below
interface IQueryType { params: any, result: any }
class Query<T extends IQueryType> {
  run: (params: T['params']) => T['result']
}

// If we need to have our DB acquire connection
// and then release the after query, it can
// then be done in this way
const createDbQuery = (dbPool: Pool) => <P, R> (
  query: Query<{ params: P, result: R}>,
  params: P
): Promise<R[]> => {
  const client = await dbPool.connect()
  try {
    return await query.run(params)
  } finally {
    client.release()
  }
}