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Grokking Node.js Design Patterns for Interviews

Problem: Process Payments Through Configured Gateways

med
30 min
Build a payment service that can process transactions through different gateways—Stripe, PayPal, or crypto—selected dynamically at runtime.

Problem statement

Your e-commerce team is rolling out multi-provider payments. Some regions support Stripe (credit cards), while others use PayPal, and a few utilize crypto payments.

Checkout systems often hardcode payment gateway logic with conditionals like:

if (method === 'stripe') { /* Stripe logic */ }
else if (method === 'paypal') { /* PayPal logic */ }
else if (method === 'crypto') { /* Crypto logic */ }

Every time a new gateway is added, developers must modify this block and redeploy. You’ve decided to implement a Strategy Pattern so each gateway’s logic is encapsulated, and the payment workflow remains stable.

Goal

Implement both components of the pattern:

  1. Gateways (Strategies):

    1. StripeGateway: Logs a message like [Stripe] Processed payment of $100 successfully.

    2. PayPalGateway: Logs a message like [PayPal] Sent payment of $200 successfully.

    3. CryptoGateway: Logs a message like [Crypto] Confirmed transfer of 300 tokens.

    4. Each must implement .pay(amount) that returns a confirmation string.

  2. PaymentService (Context):

    1. Delegates all payment processing to the active gateway’s .pay() method.

    2. Supports runtime switching through .setGateway().

Constraints

  • No if or switch statements inside PaymentService.

  • Each gateway must define its own .pay() implementation.

  • Adding a new gateway must not require modifying PaymentService.

  • The design must simulate real external integrations via console logs.

Sample output

The examples below illustrate what the output should look like:

const service = new PaymentService(new StripeGateway());
console.log(service.pay(100));
/* Expected output:
[Stripe] Processed payment of $100 successfully.
*/
service.setGateway(new PayPalGateway());
console.log(service.pay(200));
/* Expected output:
[PayPal] Sent payment of $200 successfully.
*/
service.setGateway(new CryptoGateway());
console.log(service.pay(300));
/* Expected output:
[Crypto] Confirmed transfer of 300 tokens.
*/

Good luck trying the problem! If you’re unsure how to proceed, check the “Solution” tab above.

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Home
Courses
Grokking Node.js Design Patterns for Interviews

Problem: Process Payments Through Configured Gateways

med
30 min
Build a payment service that can process transactions through different gateways—Stripe, PayPal, or crypto—selected dynamically at runtime.

Problem statement

Your e-commerce team is rolling out multi-provider payments. Some regions support Stripe (credit cards), while others use PayPal, and a few utilize crypto payments.

Checkout systems often hardcode payment gateway logic with conditionals like:

if (method === 'stripe') { /* Stripe logic */ }
else if (method === 'paypal') { /* PayPal logic */ }
else if (method === 'crypto') { /* Crypto logic */ }

Every time a new gateway is added, developers must modify this block and redeploy. You’ve decided to implement a Strategy Pattern so each gateway’s logic is encapsulated, and the payment workflow remains stable.

Goal

Implement both components of the pattern:

  1. Gateways (Strategies):

    1. StripeGateway: Logs a message like [Stripe] Processed payment of $100 successfully.

    2. PayPalGateway: Logs a message like [PayPal] Sent payment of $200 successfully.

    3. CryptoGateway: Logs a message like [Crypto] Confirmed transfer of 300 tokens.

    4. Each must implement .pay(amount) that returns a confirmation string.

  2. PaymentService (Context):

    1. Delegates all payment processing to the active gateway’s .pay() method.

    2. Supports runtime switching through .setGateway().

Constraints

  • No if or switch statements inside PaymentService.

  • Each gateway must define its own .pay() implementation.

  • Adding a new gateway must not require modifying PaymentService.

  • The design must simulate real external integrations via console logs.

Sample output

The examples below illustrate what the output should look like:

const service = new PaymentService(new StripeGateway());
console.log(service.pay(100));
/* Expected output:
[Stripe] Processed payment of $100 successfully.
*/
service.setGateway(new PayPalGateway());
console.log(service.pay(200));
/* Expected output:
[PayPal] Sent payment of $200 successfully.
*/
service.setGateway(new CryptoGateway());
console.log(service.pay(300));
/* Expected output:
[Crypto] Confirmed transfer of 300 tokens.
*/

Good luck trying the problem! If you’re unsure how to proceed, check the “Solution” tab above.

Similar Problems
Sort Reports with Pluggable Algorithms
Format Greetings in Multiple Styles
Apply Flexible Discounts for Marketing Campaigns
Cache Data Using Selectable Backends
Compress Files with Switchable Encoders
Calculate Prices for Experiment Groups
Authenticate Users with Configurable Login and Tokens
Retry Failed Requests with Custom Policies
Node.js
// --- Strategies (implement these) ---
class StripeGateway {}
class PayPalGateway {}
class CryptoGateway {}
// --- Context (implement this too) ---
class PaymentService {
  constructor(gateway) {}
  setGateway(gateway) {}
  pay(amount) {}
}
// Example usage
const service = new PaymentService(new StripeGateway());
console.log(service.pay(100));
/* Expected output:
[Stripe] Processed payment of $100 successfully.
*/
service.setGateway(new PayPalGateway());
console.log(service.pay(200));
/* Expected output:
[PayPal] Sent payment of $200 successfully.
*/
service.setGateway(new CryptoGateway());
console.log(service.pay(300));
/* Expected output:
[Crypto] Confirmed transfer of 300 tokens.
*/