Can you fake out Array.isArray() with a user-defined object?

Question

I'm curious about whether there is any way to fake out Array.isArray() with a user-defined object.

From the book JavaScript Patterns:

Array.isArray([]); // true

// trying to fool the check
// with an array-like object
Array.isArray({
  length: 1,
  "0": 1,
  slice: function () {}
}); // false

That object clearly fails, but is there any other way to do it? This is sheer curiosity, and not because I think that you could ever screw with .isArray() in regular client code (though it would obviously be fantastic to know if you could!).

Answer 1

Only if you set the internal [[Class]] property to "Array", which is not possible afaik. From the specification:

The isArray function takes one argument arg, and returns the Boolean value true if the argument is an object whose class internal property is "Array"; otherwise it returns false.

Or you go the other way round: Create a normal array and explicitly set every array method to undefined.

Answer 2

Array.isArray = function () { return true; }

And if you want to be naughty

Array.isArray.toString = function () { 
  return 'function () { [native code] }';
};