TypeScript for financial calculations

Building international transfer systems at Mercury required precision in financial calculations. JavaScript's floating-point arithmetic is notoriously problematic for money, but TypeScript gives us the type safety we need while working with proper decimal libraries.

The Problem with JavaScript Numbers

0.1 + 0.2 === 0.3 // false!

This classic JavaScript gotcha becomes catastrophic when dealing with currency. We needed a solution that caught these errors at compile time.

Our Approach

We used TypeScript with decimal.js for all monetary calculations. The type system ensures we never accidentally use native JavaScript numbers for money:

import Decimal from 'decimal.js';

type Money = Decimal;

function calculateInterest(principal: Money, rate: Money): Money {
  return principal.mul(rate);
}

Type Safety Benefits

TypeScript's type system caught dozens of potential bugs during development. When you're dealing with thousands of transactions daily, catching a bug at compile time versus production can save millions.

Lessons from Production

After launching international transfers through Wise, we learned that type safety alone isn't enough. You also need:

• Comprehensive test coverage
• Monitoring and alerting
• Clear error handling
• Audit trails for every calculation