extending-core-data-types.md

Extending

You can extend the entity record definitions using TypeScript's declaration merging.

For example, if you're building a plugin that displays a number of views of each comment, you can add a new numberOfViews field to the Comment type like this:

// In core-data
export namespace WPBaseTypes {
	// This is the parent interface – it can be extended
	export interface Comment< C extends Context > {
		id: number;
		// ...
	}
}

// This the child type used in function signatures – it cannot be extended
export type Comment< C extends Context > = OmitNevers<
	WPBaseTypes.Comment< C >
>;

// In the plugin
import type { Context } from '@wordpress/core-data';
// Target the core-data module
declare module '@wordpress/core-data' {
	// Extend the WPBaseTypes namespace through declaration merging
	export namespace WPBaseTypes {
		// Extend the parent Commet interface through declaration merging
		export interface Comment< C extends Context > {
			numberOfViews: number;
		}
	}
}

import type { Comment } from '@wordpress/core-data';
// Create an instance of the child type
const c : Comment< 'view' > = ...;

// Extending the parent had the desired effect on the child type:
// c.numberOfViews is a number
// c.id is still present

Of course, you will also need to extend the REST API to expose the numberOfViews property.