safe-money

An implementation of safe-money in typescript.

⚠️ WARNING: This implementation hasn't been battle-tested yet. Use with caution.

This software is provided by the copyright holders and contributors "as is" and any express or implied warranties, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose are disclaimed. In no event shall the copyright holder or contributors be liable for any direct, indirect, incidental, special, exemplary, or consequential damages (including, but not limited to, procurement of substitute goods or services; loss of use, data, or profits; or business interruption) however caused and on any theory of liability, whether in contract, strict liability, or tort (including negligence or otherwise) arising in any way out of the use of this software, even if advised of the possibility of such damage.

Documentation

• API Reference

Getting started

Install safe-money with npm or yarn:

yarn add safe-money

npm install safe-money

Examples

Dense values

Dense values can represent arbitrary fractions of currency units.

import { Dense, Rational } from "safe-money";

// exact arithmetic
Dense.fromDecimal("0.2", "EUR")
  .add(Dense.fromDecimal("0.1", "EUR"))
  .toDecimal(); // "0.3"

// based on rationals
const x = Dense.of(Rational.of(1, 3));
x.toDecimal(); // "0.333333333333"
x.mul(Rational.nat(3)).toDecimal(); // "1.0"

// mixing currencies raises a type error
Dense.fromDecimal("10", "EUR").add(Dense.fromDecimal("10", "USD")); // USD is not assignable to EUR

Discrete values

A discrete value can only represent an integer multiple of a currency unit.

A discrete value is based on a scale. The scale is the ratio between the chosen unit and the main currency unit.

import { Discrete, Rational } from "safe-money";

const euroCent = Discrete.scale("EUR", "cent", Rational.of(100, 1)); // 100 cents in 1 euro
const euro = Discrete.scale("EUR", "euro", Rational.of(1, 1)); // 1 euro in 1 euro
const usdCent = Discrete.scale("USD", "cent", Rational.nat(100)); // 100 cents in 1 dollar

Discrete.of(1337, euroCent).dense().toDecimal(); // "13.37"

Discrete.of(42, euroCent).add(Discrete.of(1, usdCent)); // type error
Discrete.of(1137, euroCent).add(Discrete.of(23, euro)); // type error

Conversions

To convert between currencies you can use an ExchangeRate:

import * as Money from "safe-money";

const a = Money.Dense.fromDecimal("0.2", "EUR");
const b = Money.Dense.fromDecimal("0.1", "USD");
const eurToUsd = Money.exchangeRate("EUR", "USD", Rational.fromDecimal("1.12"));

Money.exchange(eurToUsd, a).add(b); // ok
Money.exchange(eurToUsd, b); // type error

To convert a dense value to a discrete value you can use floor, ceil or round. Those operations will return the closest discrete value and a dense remainder.

const x = Dense.fromDecimal("1.337", "EUR");
const [value, remainder] = Money.round(x, euroCent);
value.toDecimal(); // "1.34"
remainder.toDecimal(); // "-0.003"

const [value, remainder] = Money.floor(x, euroCent);
value.toDecimal(); // "1.33"
remainder.toDecimal(); // "0.007"

// mismatched units raise a type error
Money.floor(Dense.fromDecimal("1", "EUR"), usdCent);

License

Copyright © 2020, Stefan Oestreicher and contributors.

Distributed under the terms of the BSD-3-Clause license.