can someone explain why exactly the vitest below is failing given the implementation provided.

import { createCacheKeyFromArgs, memoize } from './memoize';

const mocks = {
  add: vi.fn((x, y) => x + y),
};

const addMemoize = memoize(mocks.add);

afterEach(() => {
  mocks.add.mockClear();
});

const asyncAddMemoize = memoize(function asyncAdd(x, y) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(mocks.add(x, y));
    }, 500);
  });
});

describe('memoize', () => {
  it('should memoize the result of add fn and only call it once', () => {
    expect.assertions(4);

    // Arrange
    const addSpy = vi.spyOn(mocks, 'add');

    // Act
    const resultOne = addMemoize(2, 2);
    const resultTwo = addMemoize(2, 2);
    const resultThree = addMemoize(2, 2);

    // Assert
    expect(resultOne).toEqual(4);
    expect(resultTwo).toEqual(4);
    expect(resultThree).toEqual(4);
    expect(addSpy).toHaveBeenCalledTimes(1);
  });
});

where memoize implementation is

export const createCacheKeyFromArgs = <ARGS extends unknown[]>(args: ARGS[]) =>
  args.reduce((cacheKey, arg) => (cacheKey += `_${typeof arg === 'object' ? JSON.stringify(args) : `${arg}`}_`), '');

export const memoize = <ARGS extends unknown[], RET>(fn: (...args: ARGS) => RET) => {
  let cache: Record<string, RET> = {};

  return (...args: ARGS) => {
    const cacheKey = createCacheKeyFromArgs(args);

    if (cache[cacheKey]) {
      return cache[cacheKey];
    }

    const asyncFn = fn.call(undefined, ...args);
    cache[cacheKey] = asyncFn;
    return asyncFn;
  };
};

export default memoize;

It complains with error ‘AssertionError: expected “spy” to be called 1 times, but got 0 times’. I must say I just started to work with vitest, migrating from jest so I’m fairly new to it.