islclosure

Checks the isC byte in the closureClosureA function value that captures its lexical environment. In Luau, closures come in two types: LClosure (Luau bytecode + Proto + upvalues) and CClosure (native C function pointer + upvalues).'s ClosureHeader:

typedef struct ClosureHeader {
  CommonHeader;     // GC header
  uint8_t isC;      // 0 = LClosure, 1 = CClosure
  uint8_t nupvalues;
  ...
} ClosureHeader;

Returns true if isC == 0 (Luau closure with bytecodeBytecodeThe compiled binary representation of Luau source code. A sequence of 32-bit instructions (opcodes + operands) stored in a Proto's code[] array. Executed by the Luau VM interpreter. Proto). newcclosure-wrapped functions return false even though their inner function may be Luau, because the outer wrapper is a C closure. This is the inverse of iscclosure().