srp_code_migration.ts

# step #1 - Inline code inside a consumer 
# code is simple enough, consumer is simple enough 

class OrderService {
    public processOrder(Order order) {
        // validate input
        let totalPrice = orderData.items.reduce( (totalPrice, orderItem) => totalPrice + (orderItem.price *  orderItem.quantity));
        totalPrice = totalPrice + totalPrice * 0.05;    // add service commission
        // process payment
        // update order state in DB
        // notify warehouse
    }
}

# step #2 - move code to consumer's private function 
# the consumer's code has increased and we do not want to extend it further
# 

class OrderService {
    public processOrder(Order order) {
        // validate input
        ......
        ......
        ......
        ......
        ......
        const total = this.calculateTotal(orderData);
        // process payment
        ......
        ......
        ......
        ......
        ......
        ......
        // update order state in DB
        ......
        ......
        ......
        ......
        ......
        // notify warehouse
        ......
        ......
        ......
        ......
        ......
        ......
        ...lines & lines of code
    }
    
    private calculateTotal(orderData): number {
        let totalPrice = orderData.items.reduce( (totalPrice, orderItem) => totalPrice + (orderItem.price *  orderItem.quantity));
        totalPrice = totalPrice + totalPrice * 0.05; 
    }    
}

# step #3
# we receive requests to change the calculation algorithm, which do not affect changes within the OrderService
class OrderService {
    public processOrder(Order order) {
        ......
        const total = (new OrderCalculator()).calculateTotal(orderData);
        ......
    }     
}

class OrderCalculator {

    public calculate(orderData: any) {
        let totalPrice = orderData.items.reduce( (totalPrice, orderItem) => totalPrice + (orderItem.price *  orderItem.quantity));
        let applicationFee = 1; 
        if (orderData.customer.plan === Plan.Basic) {
            applicationFee = 0.05
        } else if (orderData.customer.plan === Plan.Basic) {
            applicationFee = 0.04
        }
        let paymentFee = 0; 
        if (orderData.user.country === Country.US) {
            paymentFee = 5;
        }
        if (orderData.user.country === Country.GB) {
            paymentFee = 5.5;
        }
        // etc
        return totalPrice + totalPrice * applicationFee + paymentFee;
    }
}