Hooking · Function

hookfunction

Redirects a function's entry point to a custom hook.

function hookfunction(targettargetfunctionThe function whose execution is to be redirected.: function, hookhookfunctionThe replacement function that will be called instead.: function): functionfunctionA trampoline to the original implementation of the target function.

Usage

Intercept print

local originalPrint = hookfunction(print, function(...)
  -- Do something before
  return originalPrint("[Intercepted]", ...)
end)

print("Hello") -- Output: "[Intercepted] Hello"

Synopsis

Overview

Replaces a function's implementation at the pointer level. Every existing reference to the target automatically runs your hook. Returns a "trampoline". A copy of the original closure. So you can call the old logic without infinite recursion.

Trampoline usage

The trampoline is critical. Always call it (not the original symbol) from inside your hook:

local old = hookfunction(target, function(...)
  return old(...)  -- calls original, not the hook
end)

Calling target(...) inside the hook would loop forever because target is now your hook.

Internals

For Luau closures the executor swaps the internal bytecode pointer and upvalueUpvalueA variable captured from an enclosing scope. When a function references a local variable from an outer function, that variable becomes an upvalue - stored in an UpVal struct that persists even after the outer function returns. array between the target and a clone. For C closures it patches the native function pointer. Because all callers still hold a reference to the same closure object, the swap is transparent.