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 {
    calculateInterest(accountBalance:Number):Number;
}
////////////////////////////////////////
/// Null object implementation and 4 account type related calculation stategies
////////////////////////////////////////

class NoInterestCalculation implements InterestCalculationStrategy {
    calculateInterest(accountBalance:Number):Number {
        return 0;
    }
}

class CurrentAccountInterestCalculation implements InterestCalculationStrategy {
    calculateInterest(accountBalance:Number):Number {
        return +accountBalance * (0.02 / 12);
    }
}
class SavingsAccountInterestCalculation implements InterestCalculationStrategy {
    calculateInterest(accountBalance:Number):Number {
        return +accountBalance * (0.04 / 12);
    }
}
class MoneyMarketInterestCalculation implements InterestCalculationStrategy {
    calculateInterest(accountBalance:Number):Number {
        return +accountBalance * (0.06/12);
    }
}
class HighRollerMoneyMarketInterestCalculation implements InterestCalculationStrategy {
    calculateInterest(accountBalance:Number):Number {
        return accountBalance < 100000.00 ? 0 : (+accountBalance) * (0.075/12)
    }
}

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



class InterestCalculationStrategyFactory {
    //Strategies for calculating interest.
    private currentAccountInterestCalculationStrategy:InterestCalculationStrategy  = new CurrentAccountInterestCalculation();
    private savingsAccountInterestCalculationStrategy:InterestCalculationStrategy  = new SavingsAccountInterestCalculation();
    private moneyMarketAccountInterestCalculationStrategy:InterestCalculationStrategy  = new MoneyMarketInterestCalculation();
    private highRollerMoneyMarketAccountInterestCalculationStrategy:InterestCalculationStrategy  = new HighRollerMoneyMarketInterestCalculation();
    private noInterestCalculationStrategy:InterestCalculationStrategy = new NoInterestCalculation();

    public getInterestCalculationStrategy(accountType:AccountTypes):InterestCalculationStrategy {
        switch (accountType) {
            case AccountTypes.CURRENT: return this.currentAccountInterestCalculationStrategy;
            case AccountTypes.SAVINGS: return this.savingsAccountInterestCalculationStrategy;
            case AccountTypes.STANDARD_MONEY_MARKET: return this.moneyMarketAccountInterestCalculationStrategy;
            case AccountTypes.HIGH_ROLLER_MONEY_MARKET: return this.highRollerMoneyMarketAccountInterestCalculationStrategy;
            default: return this.noInterestCalculationStrategy;
        }
    }
}

What solution would be better to get rid of that switch block?

getInterestCalc(type) : void {
eval("new InterestStrat"+type+"()");
}
class InteresStratTypeA implements Interface {}
class InteresStratTypeB implements Interface {}

Im working out another solution due to eval deprecation.

Did you ever figure out a better solution? I tried using an Enum and creating an object mapping each value to functions, but I could not get it to work in TS.

@shershen08 @joshrizzo

interface InterestCalculationStrategy {
  calculateInterest(accountBalance:Number):Number;
  test(type: AccountTypes): boolean;
}

// and then you implement test function for every strategy
class SavingsAccountInterestCalculation implements InterestCalculationStrategy {
    test(type: AccountType) {
        return type === AccountType.SAVING
    }
    
    calculateInterest(accountBalance:Number):Number {
        return +accountBalance * (0.04 / 12);
    }
}

class InterestCalculationStrategyFactory {
   strategies = [
     new SavingsAccountInterestCalculation(),
     // ...all other strategies
  ]

  getInterestCalculationStrategy(accountType:AccountTypes):InterestCalculationStrategy {
    return this.strategies.find(strategy => strategy.test(accountType));
  }
}

it could be like that too...

interface InterestCalculationStrategy {
  calculateInterest(accountBalance:Number):Number;
}

// and then you implement test function for every strategy
class SavingsAccountInterestCalculation implements InterestCalculationStrategy {
    calculateInterest(accountBalance:Number):Number {
        return +accountBalance * (0.04 / 12);
    }
}

class InterestCalculationStrategyFactory {
  private static strategies = {
     [AccountTypes.SAVING]: new SavingsAccountInterestCalculation(),
     // ...all other strategies
  }

  static getInterestCalculationStrategy(accountType:AccountTypes):InterestCalculationStrategy {
    return InterestCalculationStrategyFactory.strategies[accountType];
  }
}