fix: Move minus symbol in front of the currency symbol (#522)

* fix: Move minus symbol in front of the currency symbol

The minus symbol is put in front of the amount and currency symbol.
Before this change negative GBP and USD amounts were rendered
as £-1.71 and $-5.00 (basically every currency that appears before
the number amount) and after this change they are
rendered as -£1.71 and -$5.00.

Some examples using the same approach to negative currency amounts:

https://docs.microsoft.com/en-us/globalization/locale/currency-formatting
https://english.stackexchange.com/a/124804

* Match string interpolation style

* Make negative currency test case more intuitive and readable

src/util/currency.js

@@ -181,14 +181,18 @@ export function formatCurrency(amount, currency, locale) {
     get(currency)
   )(CURRENCY_SYMBOLS);
   const currencyFormat = getCurrencyFormat(currency, locale);
-  const formattedAmount = toCurrencyNumberFormat(amount, currencyFormat);
+
+  const absAmount = Math.abs(amount);
+  const sign = amount < 0 ? '-' : '';
+
+  const formattedAmount = toCurrencyNumberFormat(absAmount, currencyFormat);
   const currencyString = addSymbol(
     formattedAmount,
     currencySymbol,
     currencyFormat
   );
 
-  return currencyString;
+  return `${sign}${currencyString}`;
 }
 
 function toCurrencyNumberFormat(number, currencyFormat) {

src/util/currency.spec.js

@@ -273,6 +273,21 @@ describe('currency', () => {
         testCurrency(inputs, ccy, 'de-DE', outputs);
       });
     });
+
+    describe('handling negative amounts', () => {
+      it('should place a minus sign in front of the currency', () => {
+        const negativeInputs = [-11.23, '-1000', -0.98];
+
+        const eurOutputs = ['-11,23\xA0€', '-1.000,00\xA0€', '-0,98\xA0€'];
+        testCurrency(negativeInputs, 'EUR', 'de-DE', eurOutputs);
+
+        const usdOutputs = ['-$11.23', '-$1,000.00', '-$0.98'];
+        testCurrency(negativeInputs, 'USD', 'en-US', usdOutputs);
+
+        const gbpOutputs = ['-£11.23', '-£1,000.00', '-£0.98'];
+        testCurrency(negativeInputs, 'GBP', 'en-GB', gbpOutputs);
+      });
+    });
   });
 
   describe('formatAmountForLocale()', () => {