Strategy and factory patterns in TypeScript

typescript-strategy-and-factory-patterns.ts

// see article with examples in JAVA here: https://dzone.com/articles/design-patterns-the-strategy-and-factory-patterns
// example for educational purposes shownig close and mature syntax of modern TypeScript

enum AccountTypes {CURRENT, SAVINGS, HIGH_ROLLER_MONEY_MARKET, STANDARD_MONEY_MARKET}

////////////////////////////////////////
/// the interface that is used by the strategy
////////////////////////////////////////

interface InterestCalculationStrategy {
    canHandle(accountType: AccountTypes): boolean;
    calculateInterest(accountBalance: number): number;
}
////////////////////////////////////////
/// Null object implementation and 4 account type related calculation stategies
////////////////////////////////////////

class NoInterestCalculation implements InterestCalculationStrategy {
    canHandle(accountType: AccountTypes): boolean {
        return true;
    }
    calculateInterest(accountBalance: number): number {
        return 0;
    }
}

class CurrentAccountInterestCalculation implements InterestCalculationStrategy {
    canHandle(accountType: AccountTypes): boolean {
        return accountType === AccountTypes.CURRENT;
    }
    calculateInterest(accountBalance: number): number {
        return +accountBalance * (0.02 / 12);
    }
}
class SavingsAccountInterestCalculation implements InterestCalculationStrategy {
    canHandle(accountType: AccountTypes): boolean {
        return accountType === AccountTypes.SAVINGS;
    }
    calculateInterest(accountBalance: number): number {
        return +accountBalance * (0.04 / 12);
    }
}
class MoneyMarketInterestCalculation implements InterestCalculationStrategy {
    canHandle(accountType: AccountTypes): boolean {
        return accountType === AccountTypes.STANDARD_MONEY_MARKET;
    }
    calculateInterest(accountBalance: number): number {
        return +accountBalance * (0.06/12);
    }
}
class HighRollerMoneyMarketInterestCalculation implements InterestCalculationStrategy {
    canHandle(accountType: AccountTypes): boolean {
        return accountType === AccountTypes.HIGH_ROLLER_MONEY_MARKET;
    }
    calculateInterest(accountBalance: number): number {
        return accountBalance < 100000.00 ? 0 : (+accountBalance) * (0.075/12)
    }
}

////////////////////////////////////////
/// select and apply the correct strategy
////////////////////////////////////////



class InterestCalculationStrategyFactory {
    public getInterestCalculationStrategy(accountType: AccountTypes): InterestCalculationStrategy {
        return [
            new CurrentAccountInterestCalculation(),
            new SavingsAccountInterestCalculation(),
            new MoneyMarketInterestCalculation(),
            new HighRollerMoneyMarketInterestCalculation(),
            new NoInterestCalculation()
        ].find((strategy) => strategy.canHandle(accountType));
    }
}