Function not_a_function_message 

pub fn not_a_function_message(v: &Value) -> String

V8’s “not a function” wording for a value that was expected to be callable. A number/string/boolean is named WITH its value (number 1 is not a function, string "s" is not a function); every other type is named by type alone (object is not a function, symbol is not a function).