ProxyStorageAPI.js

const handlerStorage = {
  get(target, key) {
    const value = target.getItem(key);

    try {
      return JSON.parse(value);
    } catch (e) {
      return value;
    }
  },
  set(target, key, value) {
    try {
      target.setItem(key, JSON.stringify(value));
    } catch (e) {
      target.setItem(key, new String(value));
    }

    return true;
  },
  ownKeys(target) {
    return Array.from(target, (value, index) => target.key(index))
  },
  has(target, key) {
    return target.getItem(key) != null
  },
  delete(target, key) {
    target.removeItem(key);

    return true;
  }
};

const store = {
  local: new Proxy(localStorage, handlerStorage),
  session: new Proxy(sessionStorage, handlerStorage)
};

// set trap
store.local.test = {foo: 'bar'};
// get trap
console.log(store.local.test); // {foo: 'bar'}
// ownKeys trap
console.log(Object.keys(store.local)); // ['test']
// has trap
console.log('test' in store.local); // true
console.log('clé inexistante' in store.local); // false
// delete trap
delete store.local.test;
// has trap
console.log('test' in store.local); // false