fix: fix usd amount in swap notifications (#25444)

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## **Description**

This PR fixes the fiat (USD) conversion for the values displayed in the
Swap notifications.

[![Open in GitHub
Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/MetaMask/metamask-extension/pull/25444?quickstart=1)

## **Related issues**

Fixes:

## **Manual testing steps**

1. Go to this page...
2.
3.

## **Screenshots/Recordings**

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### **Before**


![image](https://github.com/MetaMask/metamask-extension/assets/1284304/8a7c817a-6f5f-4ee9-9e33-b02c44cd01f8)

<!-- [screenshots/recordings] -->

### **After**

![Screenshot 2024-06-20 at 18 00
31](https://github.com/MetaMask/metamask-extension/assets/1284304/17a5c7bf-a797-4e1f-a618-9408723f6fed)

## **Pre-merge author checklist**

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Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
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Standards](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [x] I’ve included tests if applicable
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Not required for external contributors.

## **Pre-merge reviewer checklist**

- [x] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [x] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

ui/helpers/utils/notification.util.ts

@@ -260,8 +260,11 @@ export const getUsdAmount = (amount: string, decimals: string, usd: string) => {
     return '';
   }
 
-  const amountInEther = getAmount(amount, decimals);
-  const numericAmount = parseFloat(amountInEther) * parseFloat(usd);
+  const amountInEther = calcTokenAmount(
+    amount,
+    parseFloat(decimals),
+  ).toNumber();
+  const numericAmount = parseFloat(`${amountInEther}`) * parseFloat(usd);
 
   return formatAmount(numericAmount);
 };

ui/helpers/utils/notification.utils.test.ts

@@ -3,6 +3,7 @@ import {
   formatMenuItemDate,
   getLeadingZeroCount,
   getRandomKey,
+  getUsdAmount,
 } from './notification.util';
 
 describe('formatMenuItemDate', () => {
@@ -182,3 +183,38 @@ describe('getRandomKey', () => {
     expect(result).toContain('testtext');
   });
 });
+
+describe('getUsdAmount', () => {
+  it('should return formatted USD amount based on token amount, decimals, and USD rate', () => {
+    const amount = '1000000000000000000'; // 1 Ether (1e18 wei)
+    const decimals = '18';
+    const usdRate = '2000'; // 1 Ether = $2000
+
+    const result = getUsdAmount(amount, decimals, usdRate);
+    expect(result).toBe('2K'); // Since 1 Ether * $2000 = $2000, formatted as '2K'
+  });
+
+  it('should return an empty string if any of the parameters are missing', () => {
+    expect(getUsdAmount('', '18', '2000')).toBe('');
+    expect(getUsdAmount('1000000000000000000', '', '2000')).toBe('');
+    expect(getUsdAmount('1000000000000000000', '18', '')).toBe('');
+  });
+
+  it('should handle small amounts correctly', () => {
+    const amount = '1000000000000000'; // 0.001 Ether (1e15 wei)
+    const decimals = '18';
+    const usdRate = '1500'; // 1 Ether = $1500
+
+    const result = getUsdAmount(amount, decimals, usdRate);
+    expect(result).toBe('1.5'); // Since 0.001 Ether * $1500 = $1.5
+  });
+
+  it('should handle large amounts correctly', () => {
+    const amount = '5000000000000000000000'; // 5000 Ether
+    const decimals = '18';
+    const usdRate = '1000'; // 1 Ether = $1000
+
+    const result = getUsdAmount(amount, decimals, usdRate);
+    expect(result).toBe('5M'); // Since 5000 Ether * $1000 = $5,000,000, formatted as '5M'
+  });
+});