/*
* Interface representing a budget item. It has the sum — some amount spent 
* or gained, and a description that explains the purpose or source of money
*/
export class BudgetItem {
    constructor(
    public sum: number,
    public description: string
    ) { }
}
/*
* Interface representing a budget item. It has the sum — some amount spent 
* or gained, and a description that explains the purpose or source of money
*/
export interface BudgetItem {
    sum: number;
    description: string; 
}
--- 
+++ 
@@ -2,7 +2,9 @@
 * Interface representing a budget item. It has the sum — some amount spent 
 * or gained, and a description that explains the purpose or source of money
 */
-export interface BudgetItem {
-    sum: number;
-    description: string; 
+export class BudgetItem {
+    constructor(
+    public sum: number,
+    public description: string
+    ) { }
 }
Make BudgetItem a class instead of interface
