Why Your ModuleScript Keeps Breaking at Runtime
ModuleScripts are the single most misused feature in Roblox development. I've spent years watching people use them like they're regular LocalScripts or ServerScripts, and then wondering why their game randomly crashes or why changes don't reflect when they test. Let me explain what actually happens here. A ModuleScript is fundamentally different from every other script type. It's a code container that returns a single value when required. That value can be a table, a number, a string, a function, or even another ModuleScript. When you call require() on a ModuleScript, Roblox executes it once, captures whatever it returns, and caches that result. Any subsequent require() calls to the same ModuleScript return the exact same cached object — not a new one. This caching behavior is both the feature and the trap. I learned this the hard way on a project where I had a ModuleScript that tracked active player stats. When I tested in Studio, the module would load the first player's data and then refuse to update when I swapped to a different player profile. The module had already been required by another script, so every new require() returned the stale cached table with the original player's stats locked in.
The fix was straightforward once I understood what was happening. Instead of having the ModuleScript maintain mutable state at the top level, I structured it to return functions that operate on parameters passed into them. The module itself becomes a factory, not a container for living data.
How Require Actually Works
When you write require(modulescript), Roblox follows a specific sequence. First it checks the cache for that exact ModuleScript instance. If it's there, you get the cached return value immediately — no code runs. If it's not cached, Roblox executes the ModuleScript from top to bottom, captures the return value, stores it in the cache, and gives it to you. The critical thing nobody explains clearly is that the code inside the ModuleScript runs during that first require() call, not when you define it. If your ModuleScript has side effects at the top level — connecting events, spawning parts, reading game state — those side effects fire the moment someone first requires it. This is why modules with top-level event connections cause massive problems in testing. I once debugged a ghost damage system that dealt the correct amount of damage in production but completely failed in playtest. The ModuleScript was connecting to the Touched event at the top level, but the connection was being made to a model that got cloned and parented after the module first loaded. The connection existed but was pointing at a dead reference. Moving the connection logic into a function that gets called explicitly when needed solved it.
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The Correct Pattern
Here's the standard approach that works reliably across projects: Inside the ModuleScript:
local Module = {} function Module.New(player) local self = setmetatable({}, {__index = Module}) self.Player = player self.Data = {} return self end function Module:TakeDamage(amount) if self.Player and self.Player.Health then self.Player.Health = self.Player.Health - amount end end function Module:GetData() return self.Data end return ModuleThen in your calling script: This pattern keeps your ModuleScript pure. It defines behavior without maintaining global state. Each instance is independent. You can require the same module fifty times and get fifty separate objects. The biggest issue is circular requires. If Module A requires Module B and Module B requires Module A, you get a nil return because neither module has finished executing when the other tries to require it. Roblox returns nil for the incomplete module. I've seen this kill entire systems because developers didn't trace the dependency chain properly. The workaround is restructuring so one module doesn't depend on the other, or moving the conflicting require into a function that runs later rather than at the top level.
Another problem is treating ModuleScripts like classes without using metatables properly. If you return a plain table from your module and try to share methods across instances, you end up either duplicating functions in memory or creating references that all instances share unintentionally. The metatable pattern I showed above prevents this by giving each instance its own data table while sharing the method table. ModuleScripts also don't re-execute when you change them during a running game unless you explicitly invalidate the cache. There's no built-in refresh mechanism. If you need hot-reloading during development, you have to write your own cache invalidation or use a separate development tool. This is one area where ModuleScripts feel incomplete compared to other engines.
When Not to Use Them
ModuleScripts are not a universal solution. If you're writing a script that only runs once at startup and never needs to be reused, a regular Script is simpler and more readable. If you're building something that needs frequent state changes and real-time updates across many parts of your game, consider a dedicated state management library instead. ModuleScripts add overhead through require and caching that isn't free, and they increase complexity when you don't actually need code reuse. They also don't work well for things that need per-require isolation of top-level execution. If you need the module body to run fresh every time it's required, ModuleScripts are the wrong tool. You'd need to use dofile or dynamically generate code, neither of which is particularly clean in Roblox.
Performance Notes
Require itself is fast once cached — it's essentially a dictionary lookup. The actual cost is in the first execution of the module. If your module has heavy initialization, it will stall the first script that requires it. I've seen modules with database queries or large data processing at the top level cause noticeable frame drops in busy games. Keep initialization light and move expensive setup into explicit functions. Memory-wise, cached modules persist for the lifetime of the game. In long-running servers this can accumulate. If you're loading modules dynamically based on player choices or game phases, make sure you're not holding references to modules you'll never use again. The garbage collector can't clean them up while any script holds a reference. The pattern works. It's been stable across thousands of Roblox games for years. Stop fighting the caching and design around it instead of pretending it doesn't exist.
