#define IS_NULL(x)      x === null
#define IS_DEFINED(x)   x !== void 0
#define IS_UNDEFINED(x) x === void 0

#define IS_NUMBER(x)    typeof x === "number"
#define IS_STRING(x)    typeof x === "string"
#define IS_BOOLEAN(x)   typeof x === "boolean"
#define IS_FUNCTION(x)  typeof x === "function"