Wrap Class example

Employee.ts

//////////////////////////////////////////////////
// Step 1: Identify the change point

export class Employee {
    public timeCards: TimeCard[];
    public payPeriod: Date[];
    public date: Date;
    public payRate: number;
    public payDispatcher: PayDispatcher;

    public pay(): void {
        const amount = new Money();
        for (const card of this.timeCards) {
            if (this.payPeriod.includes(this.date)) {
                amount.add(card.hours * this.payRate);
            }
        }
        this.payDispatcher.pay(this, this.date, amount);
        // <-- this is the change point
    }
}

//////////////////////////////////////////////////
// Step 2: Create a class that accepts the class we are going to wrap as a constructor argument

export class LoggingEmployee {
    private readonly employee: Employee;

    constructor(employee: Employee) {
        this.employee = employee;
    }
}

//////////////////////////////////////////////////
// Step 3: Create a method on the wrapper class with tests, that does the new behaviour and calls the old behaviour on the wrapper class

export class LoggingEmployee {
    private readonly employee: Employee;
    private readonly logger: Logger;

    constructor(employee: Employee, logger: Logger) {
        this.employee = employee;
        this.logger = logger;
    }

    public pay(): void { // <-- pay with logging
        this.employee.pay(); // <-- delegates to original method
        this.logPayment(); // <-- logs the payment
    }

    private logPayment(): void {
        // log employee data
    }
}

//////////////////////////////////////////////////
// Step 4: Instantiate the wrapper class in your code in the place where you need to enable the new behavior

export class Client {
    
    public someMethod(): void {
        const employee = new LoggingEmployee(new Employee(), new AcmeLogger());
        employee.pay();
        // rest of the code omitted for brevity...
    }
}