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A ModuleScript is a type of Lua source container that runs once and must return exactly one value. This value is then returned by a call to require given the ModuleScript as the only argument. ModuleScripts run once and only once per Lua environment and return the exact same value for subsequent calls to require.

ModuleScripts are essential objects for adhering to the don’t-repeat-yourself (DRY) principle. When you write a function, write it only once and use it everywhere. Having multiple copies of a function is disastrous when you need to change that behavior. So, you should define functions or groups of functions in ModuleScripts and have your Scripts and LocalScripts call require on your ModuleScripts. Keep your code organized!

It’s important to know that return values from ModuleScripts are independent with regards to LocalScripts and Scripts, and other environments like the Command Bar. Using require on a ModuleScript in a LocalScript will run the code on the client, even if a Script did so already on the server. Similarly, in Roblox Studio, using require on a ModuleScript in the hierarchy with the Command Bar will give a similar behavior. So, be careful if you are using a ModuleScript on the client and server at the same time, or debugging it within Studio.

Note that the first call to require on a ModuleScript will not yield (halt) unless the ModuleScript yields (e.g. calls wait). The current thread that called require will yield until a ModuleScript returns a value. A run time error is generated if this doesn’t happen. If a ModuleScript is attempting to require another ModuleScript that in turn tries to requires it, the thread will hang and never halt (cyclic require calls do not generate errors). Be mindful of your module dependencies in large projects!

If a ModuleScript object is has its Name property set to ‘MainModule’ and is uploaded to Roblox as a model to your account, Scripts can use require with the uploaded model’s AssetId instead. This allows you to create private modules on your Roblox account!