The test cases are not specifically enough, I wonder if you can add the following cases:

// for negative integers
Test.expect(((-2).twos(4) == "1110"),"-2 in 4 bits == 1110");

// for results over 32bits
Test.expect(((-10).twos(32) == "11111111111111111111111111110110"),"-10 in 32 bits == 11111111111111111111111111110110");

This doesn't work! (-2).twos(4) returns '1010' which should be '1110'

This is awesome but will go wrong while I wanna get a string of length >= 32. In JavaScript, numbers are storaged with 64bits but presented with 32bits, so 1 << 32(or higher) is '0...000' not '1...000', which causes the error.

I suggest adding test case for variable length arguments like this :

function add() {
    return [].slice.call(arguments).reduce(function(p, c) {
        return p + c;
    }, 0);
}

var add_ = defaultArguments(add);
Test.assertEquals(add_(1,2,3), 6);

add_ = defaultArguments(add_);
Test.assertEquals(add_(2,3), 5);