Wrap any object to extend its methods and properties.

Polyfill.lua

local Polyfill = {}; do
  function Polyfill.new(base)
    local Poly; Poly = setmetatable({}, {
      __index = function(self, key)
        if (Poly[key]) then
          return Poly[key]
        end
        
        return base[key]
      end
    })
    
    return Poly
  end
  
  Polyfill.__index = Polyfill
  Polyfill.__newindex = nil
  Polyfill.__metatable = nil
  
  Polyfill = setmetatable({}, Polyfill)
end

local string = Polyfill.new(string)
function string.split(str, sep)
  local tstr, tsep = type(str), type(sep)

  assert(tstr == "string", ("Attempted to call string.split on a %s."):format(tstr))
  assert(tsep == "string" or tsep == "nil", "Attempted to call string.split with invalid separator.")
  
  sep = tsep == "nil" and "," or sep
  
  local result = {}
  for match in ("%s%s"):format(str, sep):gmatch(("(.-)%s"):format(sep)) do
    result[#result + 1] = match
  end
  
  return result
end

print(string.split("hello,world")[2])