ExtractInterface.ts

// We'd like to test PaydayTransaction which 
// depends on a concrete class: TransactionLog

class TransactionLog {
    public saveTransaction(paydayTransaction: PaydayTransaction): void {
        // persist the transaction
        // ...
    }
}

class PaydayTransaction {
    private readonly payRollRepository: PayRollRepository;
    private readonly transactionLog: TransactionLog;

    constructor(payRollRepository: PayRollRepository, 
      transactionLog: TransactionLog) { // <- injecting a concrete class
        this.payRollRepository = payRollRepository;
        this.transactionLog = transactionLog;
    }

    public run(): void {
        // does important stuff, like paying the employees :)
        // ...

        this.transactionLog.saveTransaction(this);
    }
}

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

// !! we extracted an interface
interface TransactionRecorder {
    saveTransaction(paydayTransaction: PaydayTransaction): void;
}

class TransactionLog implements TransactionRecorder {
    saveTransaction(paydayTransaction: PaydayTransaction): void {
        // persist the transaction
        // ...
    }
}

// Now PaydayTransaction depends on the interface
class PaydayTransaction {
    private readonly payRollRepository: PayRollRepository;
    private readonly transactionRecorder: TransactionRecorder;

    constructor(payRollRepository: PayRollRepository, transactionRecorder: TransactionRecorder) {
        this.payRollRepository = payRollRepository;
        this.transactionRecorder = transactionRecorder;
    }

    run(): void {
        // does important stuff, like paying the employees :)
        // ...

        this.transactionRecorder.saveTransaction(this);
    }
}