How to save an object to localStorage

Short answer

localStorage stores only strings. Convert an object to JSON when saving and back when reading:

const settings = { theme: 'dark', fontSize: 16 };
localStorage.setItem('settings', JSON.stringify(settings));
 
const saved = JSON.parse(localStorage.getItem('settings'));

A default value

If there is no key yet, getItem() returns null. A broken value may also be stored there. A helper handles both cases:

function load(key, fallback) {
  try {
    const value = localStorage.getItem(key);
    return value === null ? fallback : JSON.parse(value);
  } catch {
    return fallback;
  }
}
 
const cart = load('cart', []);

Removing

removeItem() deletes one key, clear() deletes everything stored by the site:

localStorage.removeItem('cart');

Common mistake

Without JSON.stringify() an object turns into the useless string [object Object]:

localStorage.setItem('user', { name: 'Anna' });
localStorage.getItem('user'); // '[object Object]'

Functions used in this recipe

JSON.stringify — Converts a value to a JSON string.
JSON.parse — Parses a JSON string and returns the value.