Function builtin_is_callable 

pub fn builtin_is_callable(n: &str) -> bool

Property read that walks the prototype chain (used by iteration helpers). Whether the builtin named n is CALLABLE. Most are (Array, parseInt, Math.floor); the exceptions are the namespace objects a script can only read properties off (Math, JSON, every require()d core module), which report typeof === "object" and carry no name/length.