In attempting to mock a Math.floor call in Vitest, the test ends up raise an error:

• TypeError: Cannot read properties of undefined (reading ‘value’)

This error is raised on the following line: expect(mock_floor.mock.results[0].value).toBe(1);

test("test that a quote is returned from an Array of quotes", () => {
     const quotes = [{"character": "Me", "quote": "Hi"}, {"character": "You", "quote": "Bye"}];
     const mock_floor = vi.spyOn(Math, "floor");
     mock_floor.mockReturnValue(1);
     const quote = generate_quote(quotes);
     expect(mock_floor.mock.results[0].value).toBe(1);
     expect(quote).toEqual({"character": "You", "quote": "Bye"});
});

function generate_quote(quotes) {
    return quotes[Math.floor(Math.random() * quotes.length + 1)]
}

I’ve discovered that mock_floor.mock.results returns an empty array. Why would this be? When the aforementioned expect statement is commented out the test will pass with the subsequent expect statement: expect(quote).toEqual({"character": "You", "quote": "Bye"});.