JS var keyword

js_var.lua

local _G = _G
return function(implicit_declaration, stacklevel)
    assert(implicit_declaration == "var" or implicit_declaration == "global" or not implicit_declaration)
    stacklevel = (stacklevel or 1) + 1
    local _P = getfenv(stacklevel + 1)
    local _V = {}
    local declarations = {}
    local environment = {
        _P = _P,
        _V = _V,
        var = function(name)
            declarations[name] = "var"
        end,
        global = function(name)
            declarations[name] = "global"
        end
    }
    setmetatable(environment, {
        __index = function(_, name)
            local declaration = declarations[name]
            if declaration == "var" then
                return _V[name]
            end
            if declaration == "global" then
                return _G[name]
            end
            -- vars of outer functions are accessed just fine throug this
            return _P[name]
        end,
        __newindex = function(self, name, value)
            local declaration = declarations[name]
            if declaration == "var" then
                _V[name] = value
                return
            end
            if declaration == "global" then
                _G[name] = value
                return
            end
            if implicit_declaration then
                declarations[name] = implicit_declaration
                -- recursive call
                self[name] = value
                return
            end
            -- Set in parent environment
            -- TODO check for declaration, perhaps recursively?
            _P[name] = value
        end
    })
    -- Set the environment of the calling function
    setfenv(stacklevel, environment)
end