client/lib/plugins/store.js
function( sites, pluginSlug ) {
		let plugin,
			plugins = this.getPlugins( sites ),
			pluginSites;
		if ( ! plugins ) {
			return;
		}

		plugin = find( plugins, _filters.isEqual.bind( this, pluginSlug ) );
		if ( ! plugin ) {
			return null;
		}

		pluginSites = plugin.sites
			.filter( site => site.visible )
			.map( site => {
				// clone the site object before adding a new property. Don't modify the return value of getSite
				const pluginSite = clone( getSite( reduxGetState(), site.ID ) );
				pluginSite.plugin = site.plugin;
				return pluginSite;
			} );

		return pluginSites.sort( function( first, second ) {
			return first.title.toLowerCase() > second.title.toLowerCase() ? 1 : -1;
		} );
	}
function( sites, pluginSlug ) {
		let plugin,
			plugins = this.getPlugins( sites ),
			pluginSites;
		if ( ! plugins ) {
			return;
		}

		plugin = find( plugins, _filters.isEqual.bind( this, pluginSlug ) );
		if ( ! plugin ) {
			return null;
		}

		pluginSites = plugin.sites
			.filter( site => site.visible )
			.map( site => {
				// clone the site object before adding a new property. Don't modify the return value of getSite
				const pluginSite = clone( getSite( reduxGetState(), site.ID ) );
				pluginSite.plugin = site.plugin;
				return pluginSite;
			} );

		return pluginSites.sort( function( first, second ) {
			return first.title.toLowerCase() > second.title.toLowerCase() ? 1 : -1;
		} );
	}
Array of sites with a particular plugin.