loadFromLocalStorage

1/**
2 * Loads and parses a value from localStorage by key.
3 *
4 * @param key - The localStorage key to load.
5 * @param fallback - A fallback value if parsing fails or key is missing.
6 * @returns The parsed value or the fallback.
7 * @throws Error if JSON parsing fails and no fallback is provided.
8 */
9export function loadFromLocalStorage<T>(key: string, fallback?: T): T | undefined {
10  try {
11    const item = localStorage.getItem(key);
12    if (item === null) return fallback;
13    return JSON.parse(item) as T;
14  } catch (err) {
15    console.warn(`[loadFromLocalStorage] Failed to parse key "${key}":`, err);
16    if (fallback !== undefined) return fallback;
17    throw err;
18  }
19}

1const key = 'testKey';
2
3beforeEach(() => {
4  localStorage.clear();
5  jest.spyOn(console, 'error').mockImplementation(() => {});
6});
7
8afterEach(() => {
9  jest.restoreAllMocks();
10});
11
12test('returns parsed value from localStorage', () => {
13  localStorage.setItem(key, JSON.stringify({ a: 1 }));
14  expect(loadFromLocalStorage(key)).toEqual({ a: 1 });
15});
16
17test('returns fallback when key not found', () => {
18  expect(loadFromLocalStorage('missingKey', 'fallback')).toBe('fallback');
19});
20
21test('returns fallback on parse error if fallback provided', () => {
22  localStorage.setItem(key, 'not json');
23  expect(loadFromLocalStorage(key, 'fallback')).toBe('fallback');
24});
25
26test('throws on parse error when fallback not provided', () => {
27  localStorage.setItem(key, 'not json');
28  expect(() => loadFromLocalStorage(key)).toThrow();
29  expect(console.error).toHaveBeenCalledWith(
30    expect.stringMatching(/Failed to load/),
31    expect.anything()
32  );
33});