How to do basic OOP stuff in Lua

spritesheet.lua

local Spritesheet = {}
Spritesheet.__index = Spritesheet

function Spritesheet.new(sheetPath, spriteWidth, spriteHeight)
  assert(spriteWidth, 'Missing sprite width!')
  assert(spriteHeight, 'Missing sprite height!')

  local spritesheet = {}
  setmetatable(spritesheet, Spritesheet)

  spritesheet.sheet = love.graphics.newImage(sheetPath)
  spritesheet.spriteWidth = spriteWidth
  spritesheet.spriteHeight = spriteHeight

  return spritesheet
end

function Spritesheet:getImage()
  return self.sheet
end

function Spritesheet:getSprite(x, y)
  if not y then
    return self:getSpriteQuadAtIndex(x)
  else
    return self:getSpriteQuadAtCoord(x, y)
  end
end