For starters, we want to make sure we can hold the return value of Math.random() in place so that we can test against it.

beforeEach(() => {
	vi.spyOn(Math, 'random').mockReturnValue(0.5);
});

We also want to make sure we put everything back after every test. This is mostly precautionary. But, it’s good practice. So, let’s do it.

afterEach(() => {
	vi.restoreAllMocks();
});

Now, I probably wouldn’t keep this test. But, for our purposes, it does illustrate that our function is being mocked correctly.

it('should have a secret number', () => {
	const game = new Game();
	expect(game.secretNumber).toBeTypeOf('number');
	expect(game.secretNumber).toBe(50); // This is the new part.
});

And now that we can consistently rely on the secret number holding steady, we can test the rest of our implementation.

it('should return the correct response if the number is too low', () => {
	const game = new Game();
	expect(game.guess(49)).toBe('Too low!');
});

it('should return the correct response if the number is too low', () => {
	const game = new Game();
	expect(game.guess(51)).toBe('Too high!');
});

it('should return the correct response if the number is correct', () => {
	const game = new Game();
	expect(game.guess(50)).toBe('Correct! You guessed the number in 1 attempts.');
});

it('should return the correct response if the number is already guessed', () => {
	const game = new Game();
	game.guess(49);
	expect(game.guess(49)).toBe('You already guessed that number!');
});

it('should return the correct number of guesses made', () => {
	const game = new Game();

	game.guess(49);
	game.guess(51);
	game.guess(50);

	expect(game.guesses.size).toBe(3);
});

Completed Example

This is the entire file if you want to see everything in one place.

import { it, expect, describe, vi, beforeEach, afterEach } from 'vitest';
import { Game } from './game';

describe('Game', () => {
	beforeEach(() => {
		vi.spyOn(Math, 'random').mockReturnValue(0.5);
	});

	afterEach(() => {
		vi.restoreAllMocks();
	});

	it('should return an instance of a game', () => {
		// This is mostly a dummy test.
		const game = new Game();
		expect(game).toBeInstanceOf(Game);
	});

	it('should have a secret number', () => {
		// Thisn't really a useful test.
		// Do I *really* care about the type of the secret number?
		// Do I *really* care about the name of a "private" property?
		const game = new Game();
		expect(game.secretNumber).toBeTypeOf('number');
		expect(game.secretNumber).toBe(50);
	});

	it('should return the correct response if the number is too low', () => {
		const game = new Game();
		expect(game.guess(49)).toBe('Too low!');
	});

	it('should return the correct response if the number is too low', () => {
		const game = new Game();
		expect(game.guess(51)).toBe('Too high!');
	});

	it('should return the correct response if the number is correct', () => {
		const game = new Game();
		expect(game.guess(50)).toBe('Correct! You guessed the number in 1 attempts.');
	});

	it('should return the correct response if the number is already guessed', () => {
		const game = new Game();
		game.guess(49);
		expect(game.guess(49)).toBe('You already guessed that number!');
	});

	it('should return the correct number of guesses made', () => {
		const game = new Game();

		game.guess(49);
		game.guess(51);
		game.guess(50);

		expect(game.guesses.size).toBe(3);
	});
});