What is a function i can use to break out of a function?

to discontinue execution of a function you need to use the return keyword:

for instance:

function sayHello(who)
    if(not who) then
        print"you did not specify a person to say hello to."
        return --terminates further execution!
    end

    print("Hello, ",who,".  how are you?")
end

sayHello("Roblox") --//"Hello, Roblox. How are you"
sayHello()--//"you did not specify a person to say hello to."