The old way to define a component (as part of ECS). This script normally saved as individual file.

component-area-legacy.lua

---@alias entity {} # also known as character/object/thing (unique)
---@aliss component.area { width: number, height: number } # a 2d object component, also known as size

---The component constructor/giver/assembler, I don't know, I don't really know.
---@type table<entity, component.area>|fun(width?: number, height?: number): c: component
return setmetatable({}, {
  -- stored as weak reference
  __mode = 'kv',

  ---@param list table<entity, component.size>
  ---@param e entity
  ---@param w number
  ---@parsm h number
  __call = function(list, e, w, h)
    -- notice: argument variables' name are shorter to avoid confusion

    -- two dimensions object with width and height. pure data.
    local area = {
      width = w,
      height = h
    }

    -- replaces the existing table regardless it is existed or not
    list[e] = area

    -- return is optional, assuming the components can be easily retrieved
    return list[e]
  end
})