Default values

You can attach a metatable to any table: a second table that tells Lua what to do in special situations. Its best-known field is __index. When you read a key that isn't in the table, Lua looks for it in __index before it returns nil.

That's how you build default values. You keep a single defaults table, and each enemy stores only what's different about it, so boss.hp comes from the boss itself and boss.speed comes from defaults. A thousand goblins share one table instead of a thousand copies, and any change to it reaches all of them. Writing doesn't go through __index, though: boss.speed = 8 writes to the boss alone.

The classic mistake here is writing setmetatable(boss, defaults). It looks right but doesn't work, because Lua doesn't search the metatable itself but its __index field. The correct version is setmetatable(boss, { __index = defaults }).