Never return null

How to do it

Return [] for "no results". Use a Null Object, such as a mailer that sends nothing, when a default behaviour makes sense. A lookup that may find nothing returns the language's one absent value, such as undefined, Optional or None. Convert it to an error or a default where the result is used, never deeper.

Example

// Bad: every caller must remember the null check
function ordersFor(customerId: string): Order[] | null {
  return db.has(customerId) ? db.get(customerId) : null;
}

// Good: no results is an empty list
function ordersFor(customerId: CustomerId): Order[] {
  return db.get(customerId) ?? [];
}

// Good: a lookup has one absent value, handled where it is used
function findCustomer(id: CustomerId): Customer | undefined {
  return customers.get(id);
}
const customer = findCustomer(id);
if (!customer) throw new CustomerNotFound(id);

# Python: an empty list for no results, and None as the one absent value for a lookup
def orders_for(customer_id: str) -> list[Order]:
    return orders.get(customer_id, [])

def find_customer(customer_id: str) -> Customer | None:
    return customers.get(customer_id)

// Java: Optional makes the absent case part of the type
Optional<Customer> findCustomer(CustomerId id) {
    return Optional.ofNullable(customers.get(id));
}
Customer customer = findCustomer(id).orElseThrow(() -> new CustomerNotFound(id));

How to check

Strict null checks make every absent return visible, such as strictNullChecks in TypeScript, PHPStan level 8 or higher, or mypy --strict. Reviewers fail collections and services that can return null. A test calls each finder with no matches and expects an empty result.