currency.js precise money math: working versions (tested 2026-10-16)

{
  "name": "currency-js-money-math",
  "private": true,
  "type": "module",
  "scripts": {
    "test": "node --import tsx --test test/*.test.ts"
  },
  "dependencies": {
    "currency.js": "2.0.4"
  },
  "devDependencies": {
    "@types/node": "26.6.5",
    "tsx": "4.23.15"
  }
}

import currency from "currency.js";

export type Money = ReturnType<typeof currency>;
export type MoneyOptions = NonNullable<Parameters<typeof currency>[1]>;

export function money(value: string | number, options?: MoneyOptions): Money {
  return currency(value, options);
}

export function add(a: string | number, b: string | number): Money {
  return currency(a).add(b);
}

export function subtract(a: string | number, b: string | number): Money {
  return currency(a).subtract(b);
}

export function multiply(a: string | number, factor: number): Money {
  return currency(a).multiply(factor);
}

export function distribute(total: string | number, parts: number): number[] {
  return currency(total)
    .distribute(parts)
    .map((part) => part.value);
}

export function format(amount: string | number, options?: MoneyOptions): string {
  return currency(amount, options).format();
}

import { test } from "node:test";
import assert from "node:assert/strict";
import { money, add, subtract, multiply, distribute, format } from "../src/money.ts";

test("constructs from string and number", () => {
  assert.equal(money("12.34").value, 12.34);
  assert.equal(money(12.34).value, 12.34);
  assert.equal(money("0.1").intValue, 10);
});

test("adds precise cents with no floating point drift", () => {
  assert.notEqual(0.1 + 0.2, 0.3);
  assert.equal(add("0.1", "0.2").value, 0.3);
  assert.equal(add(0.1, 0.2).value, 0.3);
  assert.equal(money(0.1).add(0.2).format(), "$0.30");
});

test("subtracts precise cents with no floating point drift", () => {
  assert.notEqual(0.3 - 0.1, 0.2);
  assert.equal(subtract("0.3", "0.1").value, 0.2);
  assert.equal(subtract(0.3, 0.1).value, 0.2);
});

test("multiplies to whole cents", () => {
  assert.notEqual(1.1 * 3, 3.3);
  assert.equal(multiply("19.99", 3).value, 59.97);
  assert.equal(multiply(1.1, 3).value, 3.3);
});

test("distributes a total into whole cents with no drift", () => {
  const parts = distribute("10", 3);
  assert.equal(parts.length, 3);
  for (const part of parts) {
    assert.equal(part, Number(part.toFixed(2)));
  }
  const total = Number(parts.reduce((sum, part) => sum + part, 0).toFixed(2));
  assert.equal(total, 10);
  assert.deepEqual(distribute(9, 3), [3, 3, 3]);
});

test("formats with symbol and separator options", () => {
  assert.equal(format(1234.5, { symbol: "$", separator: "," }), "$1,234.50");
  assert.equal(format(1234.5, { symbol: "USD ", separator: "," }), "USD 1,234.50");
  assert.equal(format("0.1", { precision: 2 }), "$0.10");
});