Self and Colon Methods

Writing rect.area(rect) works, but you learned a better way back in Lua OOP: the colon. rect:area() is exactly the same call: Lua passes the table before the colon as the first argument, self, automatically.

The colon helps at the definition site too. function account:deposit(amount) declares a hidden first parameter named self for you:

type Account = {
    owner: string,
    balance: number,
    deposit: (self: Account, amount: number) -> (),
    getBalance: (self: Account) -> number,
}

local account = {owner = "Maya", balance = 1500}

function account:deposit(amount: number)
    self.balance += amount -- self is implicit
end

function account:getBalance(): number
    return self.balance
end

Notice how the shape declares self explicitly, deposit: (self: Account, amount: number) -> (), even though the colon hides it in the implementation. The colon is sugar; the parameter is still there, and the type spells it out. A method that returns nothing is typed with the empty return -> ().

Calling follows the same rule you already know: account:deposit(250) is identical to account.deposit(account, 250). Same runtime behavior as your OOP chapter: Luau just adds types on top.

Notice that account itself is left unannotated. The Account type describes the finished table, and the table only matches it once both methods are attached; local account: Account = {owner = "Maya", balance = 1500} would make strict mode report the missing methods.