Snapshot Extension


This is the official snapshot extension for Harlem. The snapshot extension adds the ability to snapshot a store's state at a particular point in time and reapply it later.

Getting Started

Follow the steps below to get started using the snapshot extension.


Before installing this extension make sure you have installed @harlem/core.

yarn add @harlem/extension-snapshot
npm install @harlem/extension-snapshot


To get started simply register this extension with the store you wish to extend.

import snapshotExtension from '@harlem/extension-snapshot';

import {
} from '@harlem/core';

const STATE = {
    firstName: 'Jane',
    lastName: 'Smith'

const {
} = createStore('example', STATE, {
    extensions: [



The snapshot extension adds a single snapshot method to the store instance.


Taking a snapshot

To take a snapshot simply call the snapshot method returned from the store instance at the point in time you want the snapshot to be in.

const snap = snapshot();

The snapshot method also accepts an optional argument which is a function that returns a sub-branch of state. This is useful for taking more granular snapshots instead of the whole state tree.

const snap = snapshot(state => state.details);

Note: The branch function must return an object type. The sub-branch cannot be an array, date, string, number etc.

Applying a snapshot

To apply a snapshot, call the apply method on the snapshot instance.

const snap = snapshot();

// Sometime later

Once the snapshot is applied the state tree (or sub-branch) will be overwritten to what the snapshot was when it was taken. Any changes to state since the snapshot point will be lost.

Using snapshotted state

Along with the apply method, the snapshot function returns a state object. This state object is a readonly copy of the state that was snapshotted.

const snap = snapshot();