client/extensions/woocommerce/state/sites/settings/mailchimp/reducer.js
function syncStatus( state = {}, action ) {
	switch ( action.type ) {
		case WOOCOMMERCE_MAILCHIMP_SYNC_STATUS_REQUEST_SUCCESS:
		case WOOCOMMERCE_MAILCHIMP_RESYNC_REQUEST_SUCCESS:
			return Object.assign( {}, action.syncStatus );
	}

	return state;
}
function syncStatus( state = {}, action ) {
	switch ( action.type ) {
		case WOOCOMMERCE_MAILCHIMP_SYNC_STATUS_REQUEST_SUCCESS:
		case WOOCOMMERCE_MAILCHIMP_RESYNC_REQUEST_SUCCESS:
			return Object.assign( {}, action.syncStatus );
	}

	return state;
}
Updates state with latest MailChimp connected plugin sync status.
This is sync between plugin and mailchimp server

@param  {Object} state  Current state
@param  {Object} action Action
@return {Object}        Updated state