Arrays are Objects ( almost everything is an object ).

I suppose by

string, boolean, number or object

you mean it cannot be an array.

I would make this explicit.

In JavaScript, arrays function more or less as objects with numerical properties ([ "zero", "one" ] ~ { 0: "zero", 1: "one" }), so it would not even make it harder for the user to allow arrays as well. But you don't have to.

null is an Object. Expected results should be specified, and it should be tested with.

Also, where are the random tests?