I have a few doubts that popped up in this project. Here's the code.

class Expenses: ObservableObject {
    @Published var items = [ExpenseItem]() {
          didSet {
            if let data = try? JSONEncoder().encode(items) {
                UserDefaults.standard.set(data, forKey: "Items")
            }
        }
    }

    init() {
        if let savedItems = UserDefaults.standard.data(forKey: "Items") {
            if let decodedItems = try? JSONDecoder().decode([ExpenseItem].self, from: savedItems) { //i dont know whats [EspenseItem].self

                items = decodedItems
                return
            }
        }
        items = [] //Huh? whats this?
    }
}

The commented parts explain what i dont understand. Thank you for reading. Funny thing is that i actually had 4-5 questions but while writing the comments the solution came to me. So. only 2 things that i dont understand.

Thanks @Bnerd! 1 more thing left if you have time & know it.whats [ExpenseItem].self?

ExpenseItem is the struct you made to store your expenses. .self = (if I am not mistaken) means that you refer to the ExpenseItem Struct itself and not an new instance of it. Found the below which is a good read to understand the "self" have a quick look to get a better idea. https://www.codingem.com/self-in-swift/

Thanks!