Class PersistentStore<State, Persisted>

A store that is automatically saved to and restored from local storage. This is suitable for small stores that can very quickly be (de)serialized to/from JSON without any noticeable delay.

class MyState implements VersionedObject {
  _version = 1;
  // eslint-disable-next-line camelcase -- will fix in next version
  my_state_key = 'my default value';
}

class MyStore extends PersistentStore<MyState> {
  constructor() {
    super('myPersistenceKey', new MyState());
  }
}

let store = new MyStore();
store('my_state_key').state ='my persisted value';

// the user leaves the page and comes back later ...
TestBed.tick();

store = new MyStore();
expect(store('my_state_key').state).toBe('my persisted value');
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At this point, if you change _version the store will be reset to the default state. This is a convenience during initial development of your app. Once it is released to real users, you will want to use a MigrationManager to avoid wiping out your users' data:

class MyState implements VersionedObject {
  _version = 2; // bump version to 2
  myStateKey = 'my default value'; // schema change: my_state_key => myStateKey
}

class MyMigrationManager extends MigrationManager<MyState> {
  constructor() {
    super();
    this.registerMigration(1, this.#migrateFromV1);
  }

  #migrateFromV1(oldState: any): any {
    return { _version: 2, myStateKey: oldState.my_state_key };
  }
}

class MyStore extends PersistentStore<MyState> {
  constructor() {
    // pass in our new `MyMigrationManager`
    super('myPersistenceKey', new MyState(), {
      migrator: new MyMigrationManager(),
    });
  }
}

// the store gets the value persisted from version 1 in our previous example
const store = new MyStore();
expect(store('myStateKey').state).toBe('my persisted value');
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If you want to persist something a little different from what is in the store, for example to omit some properties, use a PersistenceCodec:

class MyState implements VersionedObject {
  _version = 1;
  sessionStart = Date.now();
}
type Persisted = Omit<MyState, 'sessionStart'>;

class MyCodec implements PersistenceCodec<MyState, Persisted> {
  encode(left: MyState): Persisted {
    return omit(left, 'sessionStart');
  }

  decode(right: Persisted): MyState {
    return { ...right, sessionStart: Date.now() };
  }
}

class MyStore extends PersistentStore<MyState, Persisted> {
  constructor() {
    super('myPersistenceKey', new MyState(), { codec: new MyCodec() });
  }
}

const session1Start = Date.now();
let store = new MyStore();
expect(store('sessionStart').state).toBe(session1Start);
expect(localStorage.getItem('myPersistenceKey')).toBe('{"_version":1}');

// the user leaves the page and comes back later...

tick(300_000); // 5 minutes pass
store = new MyStore();
expect(store('sessionStart').state).toBe(session1Start + 300_000);
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