WordPress' data module serves as a hub to manage application state for both plugins and WordPress itself, providing tools to manage data within and between distinct modules. It is designed as a modular pattern for organizing and sharing data: simple enough to satisfy the needs of a small plugin, while scalable to serve the requirements of a complex single-page application.

The data module is built upon and shares many of the same core principles of Redux, but shouldn't be mistaken as merely Redux for WordPress, as it includes a few of its own distinguishing characteristics. As you read through this guide, you may find it useful to reference the Redux documentation — particularly its glossary — for more detail on core concepts.

Install the module

npm install @wordpress/data --save

This package assumes that your code will run in an ES2015+ environment. If you're using an environment that has limited or no support for ES2015+ such as lower versions of IE then using core-js or @babel/polyfill will add support for these methods. Learn more about it in Babel docs.

Use the registerStore function to add your own store to the centralized data registry. This function accepts two arguments: a name to identify the module, and an object with values describing how your state is represented, modified, and accessed. At a minimum, you must provide a reducer function describing the shape of your state and how it changes in response to actions dispatched to the store.

const { data, apiFetch } = wp;
const { registerStore } = data;

const DEFAULT_STATE = {
	prices: {},
	discountPercent: 0,
};

const actions = {
	setPrice( item, price ) {
		return {
			type: 'SET_PRICE',
			item,
			price,
		};
	},

	startSale( discountPercent ) {
		return {
			type: 'START_SALE',
			discountPercent,
		};
	},

	fetchFromAPI( path ) {
		return {
			type: 'FETCH_FROM_API',
			path,
		};
	},
};

registerStore( 'my-shop', {
	reducer( state = DEFAULT_STATE, action ) {
		switch ( action.type ) {
			case 'SET_PRICE':
				return {
					...state,
					prices: {
						...state.prices,
						[ action.item ]: action.price,
					},
				};

			case 'START_SALE':
				return {
					...state,
					discountPercent: action.discountPercent,
				};
		}

		return state;
	},

	actions,

	selectors: {
		getPrice( state, item ) {
			const { prices, discountPercent } = state;
			const price = prices[ item ];

			return price * ( 1 - ( 0.01 * discountPercent ) );
		},
	},

	controls: {
		FETCH_FROM_API( action ) {
			return apiFetch( { path: action.path } );
		},
	},

	resolvers: {
		* getPrice( item ) {
			const path = '/wp/v2/prices/' + item;
			const price = yield actions.fetchFromAPI( path );
			return actions.setPrice( item, price );
		},
	},
} );

The return value of registerStore is a Redux-like store object with the following methods:

• store.getState(): Returns the state value of the registered reducer
  • Redux parallel: getState
• store.subscribe( listener: Function ): Registers a function called any time the value of state changes.
  • Redux parallel: subscribe
• store.dispatch( action: Object ): Given an action object, calls the registered reducer and updates the state value.
  • Redux parallel: dispatch

A reducer is a function accepting the previous state and action as arguments and returns an updated state value.

The actions object should describe all action creators available for your store. An action creator is a function that optionally accepts arguments and returns an action object to dispatch to the registered reducer. Dispatching actions is the primary mechanism for making changes to your state.

The selectors object includes a set of functions for accessing and deriving state values. A selector is a function which accepts state and optional arguments and returns some value from state. Calling selectors is the primary mechanism for retrieving data from your state, and serve as a useful abstraction over the raw data which is typically more susceptible to change and less readily usable as a normalized object.

A resolver is a side-effect for a selector. If your selector result may need to be fulfilled from an external source, you can define a resolver such that the first time the selector is called, the fulfillment behavior is effected.

The resolvers option should be passed as an object where each key is the name of the selector to act upon, the value a function which receives the same arguments passed to the selector, excluding the state argument. It can then dispatch as necessary to fulfill the requirements of the selector, taking advantage of the fact that most data consumers will subscribe to subsequent state changes (by subscribe or withSelect).

Note: Controls are an opt-in feature, enabled via use (the Plugins API).

A control defines the execution flow behavior associated with a specific action type. This can be particularly useful in implementing asynchronous data flows for your store. By defining your action creator or resolvers as a generator which yields specific controlled action types, the execution will proceed as defined by the control handler.

The controls option should be passed as an object where each key is the name of the action type to act upon, the value a function which receives the original action object. It should returns either a promise which is to resolve when evaluation of the action should continue, or a value. The value or resolved promise value is assigned on the return value of the yield assignment. If the control handler returns undefined, the execution is not continued.

Refer to the documentation of @wordpress/redux-routine for more information.

An optional preloaded initial state for the store. You may use this to restore some serialized state value or a state generated server-side.

It is very rare that you should access store methods directly. Instead, the following suite of functions and higher-order components is provided for the most common data access and manipulation needs.

The top-level API of @wordpress/data includes a number of functions which allow immediate access to select from and dispatch to a registered store. These are most useful in low-level code where a selector or action dispatch is called a single time or at known intervals. For displaying data in a user interface, you should use higher-order components instead.

Given the name of a registered store, returns an object of the store's selectors. The selector functions are been pre-bound to pass the current state automatically. As a consumer, you need only pass arguments of the selector, if applicable.

Example:

const { select } = wp.data;

select( 'my-shop' ).getPrice( 'hammer' );

Given the name of a registered store, returns an object of the store's action creators. Calling an action creator will cause it to be dispatched, updating the state value accordingly.

Example:

const { dispatch } = wp.data;

dispatch( 'my-shop' ).setPrice( 'hammer', 9.75 );

Given a listener function, the function will be called any time the state value of one of the registered stores has changed. This function returns a unsubscribe function used to stop the subscription.

Example:

const { subscribe } = wp.data;

const unsubscribe = subscribe( () => {
	// You could use this opportunity to test whether the derived result of a
	// selector has subsequently changed as the result of a state update.
} );

// Later, if necessary...
unsubscribe();

As your app grows more complex, you'll want to split your reducing function into separate functions, each managing independent parts of the state. The combineReducers helper function turns an object whose values are different reducing functions into a single reducing function you can pass to registerStore.

Example:

const { combineReducers, registerStore } = wp.data;

const prices = ( state = {}, action ) => {
	return action.type === 'SET_PRICE' ?
		{
			...state,
			[ action.item ]: action.price,
		} :
		state;
};

const discountPercent = ( state = 0, action ) => {
	return action.type === 'START_SALE' ?
		action.discountPercent :
		state;
};

registerStore( 'my-shop', {
	reducer: combineReducers( {
		prices,
		discountPercent,
	} ),
} );

A higher-order component is a function which accepts a component and returns a new, enhanced component. A stateful user interface should respond to changes in the underlying state and updates its displayed element accordingly. WordPress uses higher-order components both as a means to separate the purely visual aspects of an interface from its data backing, and to ensure that the data is kept in-sync with the stores.

Use withSelect to inject state-derived props into a component. Passed a function which returns an object mapping prop names to the subscribed data source, a higher-order component function is returned. The higher-order component can be used to enhance a presentational component, updating it automatically when state changes. The mapping function is passed the select function, the props passed to the original component and the registry object.

Example:

function PriceDisplay( { price, currency } ) {
	return new Intl.NumberFormat( 'en-US', {
		style: 'currency',
		currency,
	} ).format( price );
}

const { withSelect } = wp.data;

const HammerPriceDisplay = withSelect( ( select, ownProps ) => {
	const { getPrice } = select( 'my-shop' );
	const { currency } = ownProps;

	return {
		price: getPrice( 'hammer', currency ),
	};
} )( PriceDisplay );

// Rendered in the application:
//
//  <HammerPriceDisplay currency="USD" />

In the above example, when HammerPriceDisplay is rendered into an application, it will pass the price into the underlying PriceDisplay component and update automatically if the price of a hammer ever changes in the store.

Use withDispatch to inject dispatching action props into your component. Passed a function which returns an object mapping prop names to action dispatchers, a higher-order component function is returned. The higher-order component can be used to enhance a component. For example, you can define callback behaviors as props for responding to user interactions. The mapping function is passed the dispatch function, the props passed to the original component and the registry object.

function Button( { onClick, children } ) {
	return <button type="button" onClick={ onClick }>{ children }</button>;
}

const { withDispatch } = wp.data;

const SaleButton = withDispatch( ( dispatch, ownProps ) => {
	const { startSale } = dispatch( 'my-shop' );
	const { discountPercent } = ownProps;

	return {
		onClick() {
			startSale( discountPercent );
		},
	};
} )( Button );

// Rendered in the application:
//
//  <SaleButton discountPercent="20">Start Sale!</SaleButton>

In the majority of cases, it will be sufficient to use only two first params passed to mapDispatchToProps as illustrated in the previous example. However, there might be some very advanced use cases where using the registry object might be used as a tool to optimize the performance of your component. Using select function from the registry might be useful when you need to fetch some dynamic data from the store at the time when the event is fired, but at the same time, you never use it to render your component. In such scenario, you can avoid using the withSelect higher order component to compute such prop, which might lead to unnecessary re-renders of you component caused by its frequent value change. Keep in mind, that mapDispatchToProps must return an object with functions only.

function Button( { onClick, children } ) {
	return <button type="button" onClick={ onClick }>{ children }</button>;
}

const { withDispatch } = wp.data;

const SaleButton = withDispatch( ( dispatch, ownProps, { select } ) => {
	// Stock number changes frequently.
	const { getStockNumber } = select( 'my-shop' );
	const { startSale } = dispatch( 'my-shop' );
	
	return {
		onClick() {
			const dicountPercent = getStockNumber() > 50 ? 10 : 20;
			startSale( discountPercent );
		},
	};
} )( Button );

// Rendered in the application:
//
//  <SaleButton>Start Sale!</SaleButton>

Note: It is important that the mapDispatchToProps function always returns an object with the same keys. For example, it should not contain conditions under which a different value would be returned.

The @wordpress/data module offers a more advanced and generic interface for the purposes of integrating other data systems and situations where more direct control over a data system is needed. In this case, a data store will need to be implemented outside of @wordpress/data and then plugged in via three functions:

• getSelectors(): Returns an object of selector functions, pre-mapped to the store.
• getActions(): Returns an object of action functions, pre-mapped to the store.
• subscribe( listener: Function ): Registers a function called any time the value of state changes.
  • Behaves as Redux subscribe with the following differences:
    • Doesn't have to implement an unsubscribe, since the registry never uses it.
    • Only has to support one listener (the registry).

By implementing the above interface for your custom store, you gain the benefits of using the registry and the withSelect and withDispatch higher order components in your application code. This provides seamless integration with existing and alternative data systems.

Integrating an existing redux store with its own reducers, store enhancers and middleware can be accomplished as follows:

Example:

import existingSelectors from './existing-app/selectors';
import existingActions from './existing-app/actions';
import createStore from './existing-app/store';

const reduxStore = createStore();

const mappedSelectors = existingSelectors.map( ( selector ) => {
	return ( ...args ) => selector( reduxStore.getState(), ...args );
} );

const mappedActions = existingActions.map( ( action ) => {
	return actions.map( ( action ) => {
		return ( ...args ) => reduxStore.dispatch( action( ...args ) );
	} );
} );

const genericStore = {
	getSelectors() {
		return mappedSelectors;
	},
	getActions() {
		return mappedActions;
	},
	subscribe: reduxStore.subscribe;
};

registry.registerGenericStore( 'existing-app', genericStore );

It is also possible to implement a completely custom store from scratch:

Example:

function createCustomStore() {
	let storeChanged = () => {};
	const prices = { hammer: 7.50 };

	const selectors = {
		getPrice( itemName ): {
			return prices[ itemName ];
		},
	};

	const actions = {
		setPrice( itemName, price ): {
			prices[ itemName ] = price;
			storeChanged();
		},
	};

	return {
		getSelectors() {
			return selectors;
		},
		getActions() {
			return actions;
		},
		subscribe( listener ) {
			storeChanged = listener;
		}
	};
}

registry.registerGenericStore( 'custom-data', createCustomStore() );

The data module shares many of the same core principles and API method naming of Redux. In fact, it is implemented atop Redux. Where it differs is in establishing a modularization pattern for creating separate but interdependent stores, and in codifying conventions such as selector functions as the primary entry point for data access.

The higher-order components were created to complement this distinction. The intention with splitting withSelect and withDispatch — where in React Redux they are combined under connect as mapStateToProps and mapDispatchToProps arguments — is to more accurately reflect that dispatch is not dependent upon a subscription to state changes, and to allow for state-derived values to be used in withDispatch (via higher-order component composition).

Specific implementation differences from Redux and React Redux:

• In Redux, a subscribe listener is called on every dispatch, regardless of whether the value of state has changed.
  • In @wordpress/data, a subscriber is only called when state has changed.
• In React Redux, a mapStateToProps function must return an object.
  • In @wordpress/data, a withSelect mapping function can return undefined if it has no props to inject.
• In React Redux, the mapDispatchToProps argument can be defined as an object or a function.
  • In @wordpress/data, the withDispatch higher-order component creator must be passed a function.