Why do I get -0 when dividing any number by -Infinity in JavaScript?

Question

I was writing some JavaScript tests in Jest just to practice. I wrote the following function:

const divide = (a, b) => {
  if (isNaN(a) || isNaN(b)) {
    throw new Error("This function expect 2 numbers");
  } 
  return a / b;
};

Then I wrote the following test:

describe("Test divide operator", () => {
  it("Check the divide function", () => {
    expect(divide(10, 0)).toBe(Infinity);
    expect(divide(0, 0)).toBe(NaN);
    expect(divide(Infinity, Infinity)).toBe(NaN);
    expect(divide(10, Infinity)).toBe(0);
    expect(divide(10, -Infinity)).toBe(0);  // THIS TEST FAILS
  });
});

I received the following error:

    expect(received).toBe(expected) // Object.is equality

    Expected: 0
    Received: -0

Is it a JavaScript bug? Anyone knows why 0 can be negative in JavaScript? In my opinion, it is a little awkward to have to write "-0" just to pass the test.

Thanks.

Answer 1

This is not a bug it is a "feature" from IEEE-754 about floating point arithmetic. -0 is a signed 0 and javascript implements it correctly.

It's extremely rare to actually encounter and controversial as well. Other languages handle it differently.

Also -0 === 0 AND -0 === +0 so it normaly does not matter.