Where a and b are positive or negative integers.

There are tests with zeros.

Tests could just use assert.fail.

Now wondering how to do [1..5] ( which is almost (1)[5], but you can't have a Number there ). Also wondering what people don't like. They're voting, but they're not telling you what or how to improve.

I don't think you can nest it blocks, the sample tests throw an error because you're not closing the blocks the right way, I suggest you replace the nested it blocks like this:

Test.describe("Basic Tests", function(){
  Test.describe("It should works for basic tests.", function(){

    Test.it("checking 1.55", function() {
      Test.assertEquals(f(1.55), false);
    });
    Test.it("checking -100", function() {
      Test.assertEquals(f(-100), true);
    });
  });
});

Also, be consistent about what you use for the tests.

You really need to specify isInteger is disabled, and that only Numbers will be tested.

All current solutions bug out on f(false), which is a valid input according to the description.