How do you implement the Strategy pattern using NestJS DI?

Q: How do you implement the Strategy pattern using NestJS DI?

// Define the interface and implementations export interface PaymentStrategy { pay(amount: number): Promise ; } @Injectable() export class StripeStrategy implements PaymentStrategy { async pay(amount: number) { /* stripe logic */ } } @Injectable() export class PaypalStrategy implements PaymentStrategy { async pay(amount: number) { /* paypal logic */ } } // Expose all strategies as a Map via factory { provide: 'PAYMENT_STRATEGIES', useFactory: ( stripe: StripeStrategy, paypal: PaypalStrategy, ): Map => new Map([['stripe', stripe], ['paypal', paypal]]), inject: [StripeStrategy, PaypalStrategy], } // Inject and use @Injectable() export class PaymentService { constructor( @Inject('PAYMENT_STRATEGIES') private strategies: Map , ) {} async charge(provider: string, amount: number) { const strategy = this.strategies.get(provider); if (!strategy) throw new Error(`Unknown provider: ${provider}`); await strategy.pay(amount); } } Open/closed principle — add a new strategy by registering a new provider; no changes to PaymentService. All strategies are singleton instances managed by NestJS — no manual instantiation. Strategies can inject other NestJS providers (config, repositories, HTTP clients) normally. Easy to test — mock the PAYMENT_STRATEGIES token with a test map.

Difficulty: 6/10

Dependency Injection, Strategy Pattern, NestJS Providers

Register each strategy implementation as a provider, then expose them as a map or array via a factory provider. Inject the map or array into the orchestrating service and select the right strategy at runtime based on a key or condition.

Strategy pattern with a factory provider

Benefits of this approach:

1
Open/closed principle — add a new strategy by registering a new provider; no changes to PaymentService.
2

How do you implement the Strategy pattern using NestJS DI?

Difficulty: 6/10

Dependency Injection, Strategy Pattern, NestJS Providers

Register each strategy implementation as a provider, then expose them as a map or array via a factory provider. Inject the map or array into the orchestrating service and select the right strategy at runtime based on a key or condition.

Strategy pattern with a factory provider

Benefits of this approach:

1
Open/closed principle — add a new strategy by registering a new provider; no changes to PaymentService.
2

How do you implement the Strategy pattern using NestJS DI?

Register each strategy implementation as a provider, then expose them as a map or array via a factory provider. Inject the map or array into the orchestrating service and select the right strategy at runtime based on a key or condition.

How do you implement the Strategy pattern using NestJS DI?

Register each strategy implementation as a provider, then expose them as a map or array via a factory provider. Inject the map or array into the orchestrating service and select the right strategy at runtime based on a key or condition.

Scenario Questions

Follow-up Questions