fix(toDecimal): preserve negative sign for leading zeros

Change #690 fixed the general case of formatting negative unit values,
but was one remaining case where formatting was still incorrect: when
the first unit is -0, the resulting string didn't include the leading
negative sign.

This change identifies that exact case (negative value, leading zero)
and pads the resulting string with a leading negative sign.

Fixes #692

packages/core/src/api/toDecimal.ts

@@ -1,7 +1,7 @@
 import { NON_DECIMAL_CURRENCY_MESSAGE } from '../checks';
 import { assert } from '../helpers';
 import type { Calculator, Dinero, Formatter, Transformer } from '../types';
-import { absolute, computeBase, equal, isArray } from '../utils';
+import { absolute, computeBase, equal, isArray, lessThan } from '../utils';
 
 import { toUnits } from './toUnits';
 
@@ -49,12 +49,17 @@ function getDecimal<TAmount>(
   formatter: Formatter<TAmount>
 ) {
   const absoluteFn = absolute(calculator);
+  const equalFn = equal(calculator);
+  const lessThanFn = lessThan(calculator);
+  const zero = calculator.zero();
 
   return (units: readonly TAmount[], scale: TAmount) => {
-    return units
+    const lastIndex = units.length - 1;
+
+    const decimalString = units
       .map((unit, index) => {
         const isFirst = index === 0;
-        const isLast = units.length - 1 === index;
+        const isLast = lastIndex === index;
 
         const unitAsString = formatter.toString(
           isFirst ? unit : absoluteFn(unit)
@@ -67,5 +72,11 @@ function getDecimal<TAmount>(
         return unitAsString;
       })
       .join('.');
+
+    // A leading negative zero is a special case because the toString
+    // formatter (generally String) won't preserve its negative sign.
+    return equalFn(units[0], zero) && lessThanFn(units[lastIndex], zero)
+      ? `-${decimalString}`
+      : decimalString;
   };
 }

packages/dinero.js/src/api/__tests__/toDecimal.test.ts

@@ -45,6 +45,16 @@ describe('toDecimal', () => {
 
         expect(toDecimal(d)).toEqual('-10.50');
       });
+      it('returns the negative amount with a leading zero in decimal format', () => {
+        const d = dinero({ amount: -1, currency: USD });
+
+        expect(toDecimal(d)).toEqual('-0.01');
+      });
+      it('returns negative zero amount as a positive value in decimal format', () => {
+        const d = dinero({ amount: -0, currency: USD });
+
+        expect(toDecimal(d)).toEqual('0.00');
+      });
       it('uses a custom transformer', () => {
         const d = dinero({ amount: 1050, currency: USD });
 
@@ -118,6 +128,16 @@ describe('toDecimal', () => {
 
         expect(toDecimal(d)).toEqual('-10.50');
       });
+      it('returns the negative amount with a leading zero in decimal format', () => {
+        const d = dinero({ amount: -1n, currency: bigintUSD });
+
+        expect(toDecimal(d)).toEqual('-0.01');
+      });
+      it('returns negative zero amount as a positive value in decimal format', () => {
+        const d = dinero({ amount: -0n, currency: bigintUSD });
+
+        expect(toDecimal(d)).toEqual('0.00');
+      });
       it('uses a custom transformer', () => {
         const d = dinero({ amount: 1050n, currency: bigintUSD });
 
@@ -198,6 +218,16 @@ describe('toDecimal', () => {
 
         expect(toDecimal(d)).toEqual('-10.05');
       });
+      it('returns the negative amount with a leading zero in decimal format', () => {
+        const d = dinero({ amount: new Big(-1), currency: bigjsUSD });
+
+        expect(toDecimal(d)).toEqual('-0.01');
+      });
+      it('returns negative zero amount as a positive value in decimal format', () => {
+        const d = dinero({ amount: new Big(-0), currency: bigjsUSD });
+
+        expect(toDecimal(d)).toEqual('0.00');
+      });
       it('uses a custom transformer', () => {
         const d = dinero({ amount: new Big(1050), currency: bigjsUSD });