Basic Concepts of CQS

CQS divides methods into two types:

1. Queries – retrieve data but do not change the state of the object.
2. Commands – change the state of the object but do not return anything.

Bad Example (Violation of CQS)

    class User {
      private balance: number = 100;
    
      getBalanceAndDeduct(amount: number): number {
        this.balance -= amount; // Simultaneously changes the state and returns a value
        return this.balance;
      }
    }

Problem: This method both performs a query and changes the state, which violates the CQS principle.

Good Example (Adhering to CQS)

    class User {
      private balance: number = 100;
    
      getBalance(): number {
        return this.balance; // Pure query, does not change anything
      }
    
      deductBalance(amount: number): void {
        this.balance -= amount; // Command, does not return a value
      }
    }

Now getBalance() simply returns a value, while deductBalance(amount) performs an action without returning anything.

Why Use CQS?

1. Code becomes more predictable – you always know which functions change the state and which do not.
2. Testing becomes easier – queries can be tested without side effects.
3. Code is easier to understand – it is clear when and what is being changed.
4. Efficiency in multi-threaded systems – simplifies conflict management.