Metatables

A table can be used as a metatable of another table. Multiple tables can have the same metatable. A table's metatable may have a metatable itself. Even a table can be its own metatable!

• You can use setmetatable(x, m) to set m to be x's metatable.

• You can use getmetatable(x) to get x's metatable. This will return nil if x has no metatable.

Think of this as attaching one table to another. You're attaching m, the table you wish to be the metatable of x, to x. You'd store metamethods in the metatable.

Metamethods

Think of metamethods as ROBLOX events. For example, with a ClickDetector and its MouseClick event

clickDetector.MouseClick:Connect(function(client)
    -- # ...
end)

Something happens that will invoke a metamethod/fire an event.

Now, I am not saying ROBLOX events and metamethods are the exact same, I am merely using that as an abstraction to show you how it would work. I will be covering __index metamethod in this answer. There are a lot more metamethods.

Note: The keys must start with two underscores and named exactly how they are documented

__index

__index is pretty simple. It gets invoked when you index x for a nil value; something that does not exist in x.

__index can be set to a function or a table.

As a table

When __index is set to a table, Lua will look up the index in the table.

local x = {}
local m = {
    __index = {inTheMeta = "hello"}
}

setmetatable(x, m)
-- # setmetatable returns x back, so you can do `local x = setmetatable({...}, m))` if you please.
print(x.inTheMeta) --> hello

Lua first checks if inTheMeta is in x. If it is, produce the value. If not, check if x has a metatable. If it does, look for __index. If __index is there, look up the key in the table. If the key exists, produce the value of the key. If no metatable or no __index metamethod, produce nil.

As a function

If __index is set to a function, produce the return value of said function

local x = {}
local m = {
    __index = function(t, k)
        return "__index invoked; x was indexed for " .. k .. " but the key didn't exist"
    end
}

setmetatable(x, m)

print(x.inTheMeta) --> __index invoked; x was indexed for inTheMeta but the key didn't exist

Lua first checks if inTheMeta is in x. If it is, produce the value. If not, check if x has a metatable. If it does, look for __index. If it's there, produce the value that the __index function returns.

• t is the table that m is the metatable of; x.
• k is the key you indexed for.

This does not cover everything about metatables and metamethods. You can learn more on them here and here.

Hopefully this answered your question, and if it did, then don't forget to hit that "Accept Answer" button. If you have any other questions, then feel free to leave them down in the comments.