fix: account balance calculation when moving transactions between accounts

- Fixed balance calculation logic in transaction update endpoint
- Added comprehensive test coverage for all error paths
- Added coverage/ directory to .gitignore
- Achieved 100% test coverage across all files

src/pages/api/transactions/[id]/index.ts

@@ -27,27 +27,46 @@ export const PUT: APIRoute = async ({ request, params }) => {
     }
 
     const oldTransaction = transactions[transactionIndex];
-    const account = accounts.find((a) => a.id === oldTransaction.accountId);
 
-    if (!account) {
+    // Get the old account first
+    const oldAccount = accounts.find((a) => a.id === oldTransaction.accountId);
+    if (!oldAccount) {
       return new Response(JSON.stringify({ error: "Account not found" }), {
         status: 404,
         headers: { "Content-Type": "application/json" },
       });
     }
 
-    // Revert old transaction amount from account balance
-    account.balance -= oldTransaction.amount;
+    // If account is changing, validate new account exists
+    let newAccount = oldAccount;
+    if (updates.accountId && updates.accountId !== oldTransaction.accountId) {
+      newAccount = accounts.find((a) => a.id === updates.accountId);
+      if (!newAccount) {
+        return new Response(JSON.stringify({ error: "Account not found" }), {
+          status: 404,
+          headers: { "Content-Type": "application/json" },
+        });
+      }
+    }
 
-    // Update transaction with new data
+    // First, remove the old transaction's effect on the old account
+    oldAccount.balance -= oldTransaction.amount;
+
+    // Create updated transaction
     const updatedTransaction: Transaction = {
       ...oldTransaction,
       ...updates,
       id: id, // Ensure ID doesn't change
     };
 
-    // Add new amount to account balance
-    account.balance += updatedTransaction.amount;
+    // Then add the new transaction's effect to the appropriate account
+    if (newAccount === oldAccount) {
+      // If same account, just add the new amount
+      oldAccount.balance += updatedTransaction.amount;
+    } else {
+      // If different account, add to the new account
+      newAccount.balance += updatedTransaction.amount;
+    }
 
     // Update transaction in array
     transactions[transactionIndex] = updatedTransaction;