ExtractAndOverrideFactoryMethod.ts

class FileTransactionsRepository implements TransactionsRepository {
    public getAll(): Transaction[] {
        let transactions: Transaction[] = [];

        // read the transactions from some file...

        return transactions;
    }
}

class Account {
    private transactions: Transaction[];

    constructor() {
        const transactionsRepository = new FileTransactionsRepository();
        this.transactions = transactionsRepository.getAll(); // <- side effect in a constructor!! x(
    }

    // more code...
}

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

// After extracting the factory method, 
// you can apply Subclass & Override Method

class Account {
    private transactions: Transaction[];

    constructor() {
        // extracted a factory method
        this.transactions = this.getAllTransactions();
    }

    // factory method <- now apply Subclass & Override Method here
    protected getAllTransactions(): Transaction[] {
        return new FileTransactionsRepository().getAll();
    }

    // more code...
}

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

// After applying Subclass & Override Method

class Account {
    private transactions: Transaction[];

    constructor() {
        this.transactions = this.getAllTransactions();
    }

    // made protected for testing :(
    protected getAllTransactions(): Transaction[] {
        return new FileTransactionsRepository().getAll();
    }

    // more code...
}

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

// For some test we override the protected method to avoid the side effect in the constructor

class ForTestingAccount extends Account {
    private static transactions: Transaction[];

    public static create(someTransactions: Transaction[]): Account {
        // The field transactions and its initialization need to be static 
        // so that the field gets initialized before the super class constructor is called
        ForTestingAccount.transactions = someTransactions;
        return new ForTestingAccount();
    }

    private constructor() {
        super();
    }

    protected override getAllTransactions(): Transaction[] {
        return ForTestingAccount.transactions;
    }
}