What's diffirence between __index and __newindex in metatables?

[GamEditoPro]

Hello, can someone answer on this short question: what’s diffirence between __index and __newindex in metatables?

[Blockzez]

x[y] (OP_GETTABLE) fires __index as a fallback when there’s no primitive get for the type or the primitive get of the table is nil.
x[y] = z (OP_SETTABLE) fires __newindex as a fallback when there’s no primitive set for the type or the primitive get of the table is nil.

So basically, __index is get when the table is nil and __newindex is new set when the key of the table doesn’t exist (is nil).

[Forummer]

https://www.lua.org/pil/13.4.1.html
https://www.lua.org/pil/13.4.2.html

[tomsterBG]

That explanation is pure madness! Anyway, in Roblox metatables there’s a much simpler explanation:

In other words __index(table, index) is a function that you manually write in a metatable. For example:

myMetaTable = {
    __index = function(table, index)
        print("Sorry, but the element ".. tostring(index).. " does not exist.")
        -- runs when you try to index a nil element of the table this metatable is connected to, table[index]
    end,
    __newindex = function(table, index, value)
        print("Sorry, but the element ".. tostring(index).. " can't be set to ".. tostring(value).. " because this element does not exist.")
        -- runs when you try to index and set a nil element of the table this metatable is connected to, table[index] = value
    end
}