Use suitable decimals while calculating tax

src/components/MoneyRequestAmountInput.tsx

@@ -103,7 +103,7 @@ const getNewSelection = (oldSelection: Selection, prevLength: number, newLength:
     return {start: cursorPosition, end: cursorPosition};
 };
 
-const defaultOnFormatAmount = (amount: number) => CurrencyUtils.convertToFrontendAmountAsString(amount);
+const defaultOnFormatAmount = (amount: number, currency?: string) => CurrencyUtils.convertToFrontendAmountAsString(amount, currency ?? CONST.CURRENCY.USD);
 
 function MoneyRequestAmountInput(
     {

src/components/MoneyRequestConfirmationList.tsx

@@ -329,10 +329,10 @@ function MoneyRequestConfirmationList({
             taxCode = transaction?.taxCode ?? TransactionUtils.getDefaultTaxCode(policy, transaction) ?? '';
         }
         const taxPercentage = TransactionUtils.getTaxValue(policy, transaction, taxCode) ?? '';
-        const taxAmount = TransactionUtils.calculateTaxAmount(taxPercentage, taxableAmount);
+        const taxAmount = TransactionUtils.calculateTaxAmount(taxPercentage, taxableAmount, currency);
         const taxAmountInSmallestCurrencyUnits = CurrencyUtils.convertToBackendAmount(Number.parseFloat(taxAmount.toString()));
         IOU.setMoneyRequestTaxAmount(transaction?.transactionID ?? '', taxAmountInSmallestCurrencyUnits);
-    }, [policy, shouldShowTax, previousTransactionAmount, previousTransactionCurrency, transaction, isDistanceRequest, customUnitRateID]);
+    }, [policy, shouldShowTax, previousTransactionAmount, previousTransactionCurrency, transaction, isDistanceRequest, customUnitRateID, currency]);
 
     // If completing a split expense fails, set didConfirm to false to allow the user to edit the fields again
     if (isEditingSplitBill && didConfirm) {

src/libs/CurrencyUtils.ts

@@ -87,20 +87,22 @@ function convertToBackendAmount(amountAsFloat: number): number {
  *
  * @note we do not support any currencies with more than two decimal places.
  */
-function convertToFrontendAmountAsInteger(amountAsInt: number): number {
-    return Math.trunc(amountAsInt) / 100.0;
+function convertToFrontendAmountAsInteger(amountAsInt: number, currency: string = CONST.CURRENCY.USD): number {
+    const decimals = getCurrencyDecimals(currency);
+    return Number((Math.trunc(amountAsInt) / 100.0).toFixed(decimals));
 }
 
 /**
  * Takes an amount in "cents" as an integer and converts it to a string amount used in the frontend.
  *
  * @note we do not support any currencies with more than two decimal places.
  */
-function convertToFrontendAmountAsString(amountAsInt: number | null | undefined): string {
+function convertToFrontendAmountAsString(amountAsInt: number | null | undefined, currency: string = CONST.CURRENCY.USD): string {
     if (amountAsInt === null || amountAsInt === undefined) {
         return '';
     }
-    return convertToFrontendAmountAsInteger(amountAsInt).toFixed(2);
+    const decimals = getCurrencyDecimals(currency);
+    return convertToFrontendAmountAsInteger(amountAsInt, currency).toFixed(decimals);
 }
 
 /**
@@ -111,7 +113,7 @@ function convertToFrontendAmountAsString(amountAsInt: number | null | undefined)
  * @param currency - IOU currency
  */
 function convertToDisplayString(amountInCents = 0, currency: string = CONST.CURRENCY.USD): string {
-    const convertedAmount = convertToFrontendAmountAsInteger(amountInCents);
+    const convertedAmount = convertToFrontendAmountAsInteger(amountInCents, currency);
     /**
      * Fallback currency to USD if it empty string or undefined
      */
@@ -137,7 +139,7 @@ function convertToDisplayString(amountInCents = 0, currency: string = CONST.CURR
  * @param currency - IOU currency
  */
 function convertToShortDisplayString(amountInCents = 0, currency: string = CONST.CURRENCY.USD): string {
-    const convertedAmount = convertToFrontendAmountAsInteger(amountInCents);
+    const convertedAmount = convertToFrontendAmountAsInteger(amountInCents, currency);
 
     return NumberFormatUtils.format(BaseLocaleListener.getPreferredLocale(), convertedAmount, {
         style: 'currency',
@@ -168,7 +170,7 @@ function convertAmountToDisplayString(amount = 0, currency: string = CONST.CURRE
  * Acts the same as `convertAmountToDisplayString` but the result string does not contain currency
  */
 function convertToDisplayStringWithoutCurrency(amountInCents: number, currency: string = CONST.CURRENCY.USD) {
-    const convertedAmount = convertToFrontendAmountAsInteger(amountInCents);
+    const convertedAmount = convertToFrontendAmountAsInteger(amountInCents, currency);
     return NumberFormatUtils.formatToParts(BaseLocaleListener.getPreferredLocale(), convertedAmount, {
         style: 'currency',
         currency,

src/libs/TransactionUtils.ts

@@ -9,6 +9,7 @@ import type {Comment, Receipt, TransactionChanges, TransactionPendingFieldsKey,
 import {isEmptyObject} from '@src/types/utils/EmptyObject';
 import type {IOURequestType} from './actions/IOU';
 import {isCorporateCard, isExpensifyCard} from './CardUtils';
+import {getCurrencyDecimals} from './CurrencyUtils';
 import DateUtils from './DateUtils';
 import * as Localize from './Localize';
 import * as NumberUtils from './NumberUtils';
@@ -714,9 +715,11 @@ function hasWarningTypeViolation(transactionID: string, transactionViolations: O
 /**
  * Calculates tax amount from the given expense amount and tax percentage
  */
-function calculateTaxAmount(percentage: string, amount: number) {
+function calculateTaxAmount(percentage: string, amount: number, currency: string) {
     const divisor = Number(percentage.slice(0, -1)) / 100 + 1;
-    return Math.round(amount - amount / divisor) / 100;
+    const taxAmount = (amount - amount / divisor) / 100;
+    const decimals = getCurrencyDecimals(currency);
+    return parseFloat(taxAmount.toFixed(decimals));
 }
 
 /**

src/pages/iou/MoneyRequestAmountForm.tsx

@@ -70,8 +70,8 @@ type MoneyRequestAmountFormProps = {
 };
 
 const isAmountInvalid = (amount: string) => !amount.length || parseFloat(amount) < 0.01;
-const isTaxAmountInvalid = (currentAmount: string, taxAmount: number, isTaxAmountForm: boolean) =>
-    isTaxAmountForm && Number.parseFloat(currentAmount) > CurrencyUtils.convertToFrontendAmountAsInteger(Math.abs(taxAmount));
+const isTaxAmountInvalid = (currentAmount: string, taxAmount: number, isTaxAmountForm: boolean, currency: string) =>
+    isTaxAmountForm && Number.parseFloat(currentAmount) > CurrencyUtils.convertToFrontendAmountAsInteger(Math.abs(taxAmount), currency);
 
 const AMOUNT_VIEW_ID = 'amountView';
 const NUM_PAD_CONTAINER_VIEW_ID = 'numPadContainerView';
@@ -148,14 +148,17 @@ function MoneyRequestAmountForm(
         });
     }, [isFocused, wasFocused]);
 
-    const initializeAmount = useCallback((newAmount: number) => {
-        const frontendAmount = newAmount ? CurrencyUtils.convertToFrontendAmountAsString(newAmount) : '';
-        moneyRequestAmountInput.current?.changeAmount(frontendAmount);
-        moneyRequestAmountInput.current?.changeSelection({
-            start: frontendAmount.length,
-            end: frontendAmount.length,
-        });
-    }, []);
+    const initializeAmount = useCallback(
+        (newAmount: number) => {
+            const frontendAmount = newAmount ? CurrencyUtils.convertToFrontendAmountAsString(newAmount, currency) : '';
+            moneyRequestAmountInput.current?.changeAmount(frontendAmount);
+            moneyRequestAmountInput.current?.changeSelection({
+                start: frontendAmount.length,
+                end: frontendAmount.length,
+            });
+        },
+        [currency],
+    );
 
     useEffect(() => {
         if (!currency || typeof amount !== 'number') {
@@ -218,7 +221,7 @@ function MoneyRequestAmountForm(
                 return;
             }
 
-            if (isTaxAmountInvalid(currentAmount, taxAmount, isTaxAmountForm)) {
+            if (isTaxAmountInvalid(currentAmount, taxAmount, isTaxAmountForm, currency)) {
                 setFormError(translate('iou.error.invalidTaxAmount', {amount: formattedTaxAmount}));
                 return;
             }

src/pages/iou/request/step/IOURequestStepAmount.tsx

@@ -294,7 +294,7 @@ function IOURequestStepAmount({
         const defaultTaxCode = TransactionUtils.getDefaultTaxCode(policy, currentTransaction, currency) ?? '';
         const taxCode = (currency !== transactionCurrency ? defaultTaxCode : transactionTaxCode) ?? defaultTaxCode;
         const taxPercentage = TransactionUtils.getTaxValue(policy, currentTransaction, taxCode) ?? '';
-        const taxAmount = CurrencyUtils.convertToBackendAmount(TransactionUtils.calculateTaxAmount(taxPercentage, newAmount));
+        const taxAmount = CurrencyUtils.convertToBackendAmount(TransactionUtils.calculateTaxAmount(taxPercentage, newAmount, currency ?? CONST.CURRENCY.USD));
 
         if (isSplitBill) {
             IOU.setDraftSplitTransaction(transactionID, {amount: newAmount, currency, taxCode, taxAmount});

src/pages/iou/request/step/IOURequestStepDistanceRate.tsx

@@ -81,7 +81,7 @@ function IOURequestStepDistanceRate({
             const taxRateExternalID = policyCustomUnitRate?.attributes?.taxRateExternalID ?? '-1';
             const taxableAmount = DistanceRequestUtils.getTaxableAmount(policy, customUnitRateID, TransactionUtils.getDistance(transaction));
             const taxPercentage = TransactionUtils.getTaxValue(policy, transaction, taxRateExternalID) ?? '';
-            const taxAmount = CurrencyUtils.convertToBackendAmount(TransactionUtils.calculateTaxAmount(taxPercentage, taxableAmount));
+            const taxAmount = CurrencyUtils.convertToBackendAmount(TransactionUtils.calculateTaxAmount(taxPercentage, taxableAmount, rates[customUnitRateID].currency ?? CONST.CURRENCY.USD));
             IOU.setMoneyRequestTaxAmount(transactionID, taxAmount);
             IOU.setMoneyRequestTaxRate(transactionID, taxRateExternalID);
         }

src/pages/iou/request/step/IOURequestStepTaxAmountPage.tsx

@@ -50,7 +50,7 @@ function getTaxAmount(transaction: OnyxEntry<Transaction>, policy: OnyxEntry<Pol
     const moneyRequestTaxPercentage = (transactionTaxCode ? getTaxValue(transactionTaxCode) : defaultTaxValue) ?? '';
     const editingTaxPercentage = (transactionTaxCode ? getTaxValue(transactionTaxCode) : moneyRequestTaxPercentage) ?? '';
     const taxPercentage = isEditing ? editingTaxPercentage : moneyRequestTaxPercentage;
-    return CurrencyUtils.convertToBackendAmount(TransactionUtils.calculateTaxAmount(taxPercentage, transactionTaxAmount));
+    return CurrencyUtils.convertToBackendAmount(TransactionUtils.calculateTaxAmount(taxPercentage, transactionTaxAmount, currency ?? CONST.CURRENCY.USD));
 }
 
 function IOURequestStepTaxAmountPage({

src/pages/iou/request/step/IOURequestStepTaxRatePage.tsx

@@ -7,6 +7,7 @@ import * as CurrencyUtils from '@libs/CurrencyUtils';
 import Navigation from '@libs/Navigation/Navigation';
 import type {TaxRatesOption} from '@libs/OptionsListUtils';
 import * as TransactionUtils from '@libs/TransactionUtils';
+import {getCurrency} from '@libs/TransactionUtils';
 import * as IOU from '@userActions/IOU';
 import CONST from '@src/CONST';
 import ONYXKEYS from '@src/ONYXKEYS';
@@ -38,7 +39,7 @@ function getTaxAmount(policy: OnyxEntry<Policy>, transaction: OnyxEntry<Transact
     const getTaxValue = (taxCode: string) => TransactionUtils.getTaxValue(policy, transaction, taxCode);
     const taxPercentage = getTaxValue(selectedTaxCode);
     if (taxPercentage) {
-        return TransactionUtils.calculateTaxAmount(taxPercentage, amount);
+        return TransactionUtils.calculateTaxAmount(taxPercentage, amount, getCurrency(transaction));
     }
 }
 

tests/unit/CurrencyUtilsTest.ts

@@ -107,27 +107,40 @@ describe('CurrencyUtils', () => {
 
     describe('convertToFrontendAmountAsInteger', () => {
         test.each([
-            [2500, 25],
-            [2550, 25.5],
-            [25, 0.25],
-            [2500, 25],
-            [2500.5, 25], // The backend should never send a decimal .5 value
-        ])('Correctly converts %s to amount in units handled in frontend as an integer', (amount, expectedResult) => {
-            expect(CurrencyUtils.convertToFrontendAmountAsInteger(amount)).toBe(expectedResult);
+            [2500, 25, 'USD'],
+            [2550, 25.5, 'USD'],
+            [25, 0.25, 'USD'],
+            [2500, 25, 'USD'],
+            [2500.5, 25, 'USD'], // The backend should never send a decimal .5 value
+            [2500, 25, 'VND'],
+            [2550, 26, 'VND'],
+            [25, 0, 'VND'],
+            [2586, 26, 'VND'],
+            [2500.5, 25, 'VND'], // The backend should never send a decimal .5 value
+        ])('Correctly converts %s to amount in units handled in frontend as an integer', (amount, expectedResult, currency) => {
+            expect(CurrencyUtils.convertToFrontendAmountAsInteger(amount, currency)).toBe(expectedResult);
         });
     });
 
     describe('convertToFrontendAmountAsString', () => {
         test.each([
-            [2500, '25.00'],
-            [2550, '25.50'],
-            [25, '0.25'],
-            [2500.5, '25.00'],
-            [null, ''],
-            [undefined, ''],
-            [0, '0.00'],
-        ])('Correctly converts %s to amount in units handled in frontend as a string', (input, expectedResult) => {
-            expect(CurrencyUtils.convertToFrontendAmountAsString(input)).toBe(expectedResult);
+            [2500, '25.00', 'USD'],
+            [2550, '25.50', 'USD'],
+            [25, '0.25', 'USD'],
+            [2500.5, '25.00', 'USD'],
+            [null, '', 'USD'],
+            [undefined, '', 'USD'],
+            [0, '0.00', 'USD'],
+            [2500, '25', 'VND'],
+            [2550, '26', 'VND'],
+            [25, '0', 'VND'],
+            [2500.5, '25', 'VND'],
+            [null, '', 'VND'],
+            [undefined, '', 'VND'],
+            [0, '0', 'VND'],
+            [2586, '26', 'VND'],
+        ])('Correctly converts %s to amount in units handled in frontend as a string', (input, expectedResult, currency) => {
+            expect(CurrencyUtils.convertToFrontendAmountAsString(input, currency ?? CONST.CURRENCY.USD)).toBe(expectedResult);
         });
     });