Strategy Pattern

The Problem

Multiple pricing models (percentage discount, flat discount, buy-one-get-one, volume-based tiers) are embedded in a single service with conditional logic. Adding a new pricing strategy requires modifying the existing service, violating the Open/Closed Principle.

The Solution

The Strategy pattern extracts each pricing algorithm into its own class implementing PricingStrategy. The PricingService holds a reference to the current strategy and delegates calculations. Strategies are interchangeable at runtime.

Structure

Strategy (PricingStrategy interface) — Declares calculate(basePrice, quantity).
Concrete Strategies — PercentageDiscountStrategy, FlatDiscountStrategy, BogoStrategy, TieredPricingStrategy.
Context — PricingService delegates to the current strategy.
Client — StrategyController selects the strategy from the request.

Implementation

Four interchangeable pricing strategies:

PercentageDiscountStrategy — Configurable percentage discount (e.g., 15% off).
FlatDiscountStrategy — Fixed dollar amount off (capped at the subtotal).
BogoStrategy — Buy One Get One Free.
TieredPricingStrategy — Volume-based: 0% for 1-10 items, 10% for 11-50, 20% for 50+.

export interface PricingResult {
  finalPrice: number;
  discount: number;
  strategy: string;
}

export interface PricingStrategy {
  readonly name: string;
  calculate(basePrice: number, quantity: number): PricingResult;
}

export const PRICING_STRATEGY = Symbol('PRICING_STRATEGY');

import { Injectable } from '@nestjs/common';
import { PricingStrategy, PricingResult } from './pricing-strategy.interface';

@Injectable()
export class PercentageDiscountStrategy implements PricingStrategy {
  readonly name = 'percentage';

  constructor(private readonly percentage: number) {}

  calculate(basePrice: number, quantity: number): PricingResult {
    const subtotal = basePrice * quantity;
    const discount = subtotal * this.percentage / 100;
    const finalPrice = subtotal - discount;

    return {
      finalPrice: Math.round(finalPrice * 100) / 100,
      discount: Math.round(discount * 100) / 100,
      strategy: `${this.percentage}% discount`,
    };
  }
}

import { Injectable } from '@nestjs/common';
import { PricingStrategy, PricingResult } from './pricing-strategy.interface';

@Injectable()
export class FlatDiscountStrategy implements PricingStrategy {
  readonly name = 'flat';

  constructor(private readonly flatAmount: number) {}

  calculate(basePrice: number, quantity: number): PricingResult {
    const subtotal = basePrice * quantity;
    const discount = Math.min(this.flatAmount, subtotal);
    const finalPrice = subtotal - discount;

    return {
      finalPrice: Math.round(finalPrice * 100) / 100,
      discount: Math.round(discount * 100) / 100,
      strategy: `$${this.flatAmount} flat discount`,
    };
  }
}

import { Injectable } from '@nestjs/common';
import { PricingStrategy, PricingResult } from './pricing-strategy.interface';

@Injectable()
export class BogoStrategy implements PricingStrategy {
  readonly name = 'bogo';

  calculate(basePrice: number, quantity: number): PricingResult {
    const freeItems = Math.floor(quantity / 2);
    const paidItems = quantity - freeItems;
    const finalPrice = basePrice * paidItems;
    const discount = basePrice * freeItems;

    return {
      finalPrice: Math.round(finalPrice * 100) / 100,
      discount: Math.round(discount * 100) / 100,
      strategy: `Buy one get one free (${freeItems} free item${freeItems !== 1 ? 's' : ''})`,
    };
  }
}

import { Injectable } from '@nestjs/common';
import { PricingStrategy, PricingResult } from './pricing-strategy.interface';

@Injectable()
export class TieredPricingStrategy implements PricingStrategy {
  readonly name = 'tiered';

  calculate(basePrice: number, quantity: number): PricingResult {
    let discountPercentage: number;
    let tierName: string;

    if (quantity > 50) {
      discountPercentage = 20;
      tierName = '50+ items (20% off)';
    } else if (quantity >= 11) {
      discountPercentage = 10;
      tierName = '11-50 items (10% off)';
    } else {
      discountPercentage = 0;
      tierName = '1-10 items (base price)';
    }

    const subtotal = basePrice * quantity;
    const discount = subtotal * discountPercentage / 100;
    const finalPrice = subtotal - discount;

    return {
      finalPrice: Math.round(finalPrice * 100) / 100,
      discount: Math.round(discount * 100) / 100,
      strategy: `Tiered pricing: ${tierName}`,
    };
  }
}

import { Inject, Injectable, BadRequestException } from '@nestjs/common';
import type { PricingStrategy, PricingResult } from './pricing-strategy.interface';
import { PRICING_STRATEGY } from './pricing-strategy.interface';

@Injectable()
export class PricingService {
  private readonly strategies: Map<string, PricingStrategy>;

  constructor(
    @Inject(PRICING_STRATEGY)
    private readonly defaultStrategy: PricingStrategy,
    @Inject('ALL_PRICING_STRATEGIES')
    strategies: PricingStrategy[],
  ) {
    this.strategies = new Map(strategies.map((s) => [s.name, s]));
  }

  getDefaultStrategyName(): string {
    return this.defaultStrategy.name;
  }

  getAvailableStrategies(): string[] {
    return [...this.strategies.keys()];
  }

  calculatePrice(basePrice: number, quantity: number, strategyName?: string): PricingResult {
    const strategy = strategyName
      ? this.resolveStrategy(strategyName)
      : this.defaultStrategy;

    return strategy.calculate(basePrice, quantity);
  }

  private resolveStrategy(name: string): PricingStrategy {
    const strategy = this.strategies.get(name);
    if (!strategy) {
      throw new BadRequestException(
        `Unknown strategy "${name}". Available: ${this.getAvailableStrategies().join(', ')}`,
      );
    }
    return strategy;
  }
}

NestJS Integration

All strategies are decorated with @Injectable() and registered as providers. The PricingService receives all strategies via constructor injection and stores them in a map. The controller specifies the strategy name in the request, and the service selects and applies it. This leverages NestJS’s DI to manage strategy lifecycle.

When to Use

You have a family of related algorithms and want to switch between them at runtime.
You want to isolate the algorithm logic from the code that uses it.
Multiple classes differ only in their behavior — Strategy lets you extract the varying behavior.
You want to eliminate conditional statements for selecting behaviors.

When NOT to Use

You only have one or two algorithms that rarely change — the pattern adds unnecessary abstraction.
The algorithms share significant state with the context, making separation awkward.

State

Allow an object to alter its behavior when its internal state changes.

Template Method

Define the skeleton of an algorithm in an operation, deferring some steps to subclasses.

Factory Method

Define an interface for creating an object, but let subclasses decide which class to instantiate.