SubclassAndOverride.ts

class Account {
    private balance: number;
    private readonly logger: AcmeLogger;

    constructor(balance: number) {
        this.balance = balance;
        this.logger = new AcmeLogger();
    }

    public withdraw(value: number): void {
        this.balance += value;
        this.logWithdraw(value);
    }

    private logWithdraw(value: number): void {
        // side effect that impedes unit testing
        this.logger.logTransaction(new Date(), -value);
    }
}

class AcmeLogger {
    public logTransaction(date: Date, value: number): void {
        // Logger implementation
    }
}

//////////////////////////////////////////////

class AcmeLogger {
    public logTransaction(date: Date, value: number): void {
        // Logger implementation
    }
}

class Account {
    private balance: number;
    private readonly logger: AcmeLogger;

    constructor(balance: number) {
        this.balance = balance;
        this.logger = new AcmeLogger();
    }

    public withdraw(value: number): void {
        this.balance += value;
        this.logWithdraw(value);
    }

    //!! made it protected for testing
    protected logWithdraw(value: number): void {
        this.logger.logTransaction(new Date(), -value);
    }
}

// And in some test class you will use something like:
class ForTestingAccount extends Account {
    public loggedWithdrawals: number[];

    constructor(balance: number) {
        super(balance);
        this.loggedWithdrawals = [];
    }

    // overridden to sense or separate (sensing in this case)
    protected override logWithdraw(value: number): void { // keyword override supported for TypeScript 4.3 or later
        // do whatever we want, in this case we are spying 
        this.loggedWithdrawals.push(value);
    }
}