Step 4: Calculator Math Engine

4. Calculator Math Engine

Write math calculation classes processing array lists of typed data.

Detailed Implementation Steps:

Create /src/calculator.ts. Write a class containing methods: calculateBalance(transactions: Transaction[]): number, getExpensesByCategory(transactions: Transaction[]): Record<string, number>, and estimateTax(income: number, rate: number): number.

Ensure all methods feature explicit parameter type signatures and return type declarations. Do not use the any type anywhere in the module.

Verification Checklist:

1. Verify methods compile with zero type errors.