Real World Example

Digital wallets keep a balance and a transaction history. A class groups the data and the rules (no negative balance, every change is recorded) in one place.

Example

type Txn = { type: "CREDIT" | "DEBIT"; amount: number; note: string };

class Wallet {
  private history: Txn[] = [];

  constructor(public readonly owner: string, private balance = 0) {}

  addMoney(amount: number, note = "Top-up"): void {
    this.balance += amount;
    this.history.push({ type: "CREDIT", amount, note });
  }

  pay(amount: number, note: string): boolean {
    if (amount > this.balance) {
      console.log(`Payment of Rs. ${amount} to ${note} failed: low balance`);
      return false;
    }
    this.balance -= amount;
    this.history.push({ type: "DEBIT", amount, note });
    return true;
  }

  statement(): void {
    for (const t of this.history) {
      console.log(t.type.padEnd(6), String(t.amount).padStart(6), t.note);
    }
    console.log("Balance:", this.balance);
  }
}

const w = new Wallet("Rohit");
w.addMoney(1000);
w.pay(249, "Swiggy");
w.pay(900, "Electricity bill");
w.pay(120, "Metro card");
w.statement();

Output

Payment of Rs. 900 to Electricity bill failed: low balance
CREDIT   1000 Top-up
DEBIT     249 Swiggy
DEBIT     120 Metro card
Balance: 631

A Wallet Like Paytm or PhonePe

Example