How server callbacks are done in Night City

server_callback.lua

local on_server = IsDuplicityVersion()
local resource = GetCurrentResourceName()
local callbacks = {}
local eventName = resource..':night-callback'

if on_server then
    local callbacks = {}
    function DefineNightCallback(name, funcref)
        if callbacks[name] then
            return false
        end
        callbacks[name] = funcref
        return true
    end
    RegisterNetEvent(eventName)
    AddEventHandler (eventName, function(name, id, ...)
        local src = source
        if callbacks[name] then
            TriggerClientEvent(eventName, src, name, id, callbacks[name](src, ...))
        else
            
            print(
                ('[%s] [%s] [%s %s] requested invalid NightCallback: %s\n'):format(
                    os.date("%H:%M:%S"),
                    resource,
                    tostring(src),
                    tostring(GetPlayerName(src)),
                    tostring(name)
                )
            )
            TriggerClientEvent(eventName, src, name, id)
            
        end

    end)
else
    local callbackID = 0
    local pending = {}
    function NightCallback(name, funcref, ...)
        callbackID = callbackID + 1
        callbacks[callbackID] = funcref
        while pending[name] do
            Citizen.Wait(0)
        end
        pending[name] = true
        TriggerServerEvent(eventName, name, callbackID, ...)
    end
    exports('NightCallback', NightCallback)

    RegisterNetEvent(eventName)
    AddEventHandler (eventName, function(name, id, ...)
        if callbacks[id] then
            callbacks[id](...)
        end
        pending[name] = nil
        callbacks[id] = nil
    end)
end