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History Extension preview

The history extension adds undo and redo capabilities to your store for easily integrating history features into your application.

Getting Started

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


Before installing this extension make sure you have installed harlem.

yarn add @harlem/extension-history
# or
npm install @harlem/extension-history


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

import historyExtension from '@harlem/extension-history';

import {
} from 'harlem';

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

const {
} = createStore('example', STATE, {
    extensions: [
            max: 50,
            mutations: [
                    name: 'set-name',
                    description: 'Set the current user\'s name'

The history extension adds several new methods to the store instance (highlighted above).



The history extension method accepts an options object with the following properties:

  • max: number - The maximum number of history steps to store. The default value is 50.
  • mutations: object[] - The mutations you wish to track. leaving this undefined will cause all mutations to be tracked. The default is undefined.
    • name: string - The name of the mutation to track. This must match the name of a registered mutation.
    • description: string? - An optional description of the mutations intentions. This is useful for displaying a list of commands in the UI.

Undoing/Redoing mutations

To undo or redo a mutation simply call the undo or redo methods returned from the store instance.


Please keep the following points in mind when using this extension:

  • This extension has a slight performance cost. Each tracked mutation is proxied and values are cloned pre/post mutation. If you are assigning/moving/deleting large object structures around state this cause a performance hit and increase in memory usage. It is recommended to keep mutations granular and precise when tracking them with the history (or trace) extension.
  • This extension cannot handle untracked mutation side-effects. For example, say you have 2 mutations, mutation1 and mutation2. Both mutations modify the same branch of state but mutation1 is tracked by the history extension and mutation2 is not. Undoing the changes from mutation1 any time after running mutation2 will cause changes from mutation2 to be lost and unrecoverable.

MIT Licensed