client/extensions/woocommerce/state/sites/settings/mailchimp/reducer.js
function settings( state = {}, action ) {
	switch ( action.type ) {
		case WOOCOMMERCE_MAILCHIMP_SETTINGS_REQUEST_SUCCESS:
		case WOOCOMMERCE_MAILCHIMP_SETTINGS_REQUEST_FAILURE:
		case WOOCOMMERCE_MAILCHIMP_API_KEY_SUBMIT_SUCCESS:
		case WOOCOMMERCE_MAILCHIMP_STORE_INFO_SUBMIT_SUCCESS:
		case WOOCOMMERCE_MAILCHIMP_CAMPAIGN_DEFAULTS_SUBMIT_SUCCESS:
		case WOOCOMMERCE_MAILCHIMP_NEWSLETTER_SETTINGS_SUBMIT_SUCCESS:
			return Object.assign( {}, state, action.settings );
		case WOOCOMMERCE_MAILCHIMP_LISTS_REQUEST_SUCCESS:
			const data = { mailchimp_lists: action.lists };
			const listKeys = keys( action.lists );
			if ( ! state.mailchimp_list && listKeys.length > 0 ) {
				// Just pick first that will be shown to the user in the dropdown
				// We are setting mailchimp_list just in case user likes it and clicks
				// Continue without actually sellecting something.
				data.mailchimp_list = listKeys[ 0 ];
			}
			return Object.assign( {}, state, data );
	}

	return state;
}
function settings( state = {}, action ) {
	switch ( action.type ) {
		case WOOCOMMERCE_MAILCHIMP_SETTINGS_REQUEST_SUCCESS:
		case WOOCOMMERCE_MAILCHIMP_SETTINGS_REQUEST_FAILURE:
		case WOOCOMMERCE_MAILCHIMP_API_KEY_SUBMIT_SUCCESS:
		case WOOCOMMERCE_MAILCHIMP_STORE_INFO_SUBMIT_SUCCESS:
		case WOOCOMMERCE_MAILCHIMP_CAMPAIGN_DEFAULTS_SUBMIT_SUCCESS:
		case WOOCOMMERCE_MAILCHIMP_NEWSLETTER_SETTINGS_SUBMIT_SUCCESS:
			return Object.assign( {}, state, action.settings );
		case WOOCOMMERCE_MAILCHIMP_LISTS_REQUEST_SUCCESS:
			const data = { mailchimp_lists: action.lists };
			const listKeys = keys( action.lists );
			if ( ! state.mailchimp_list && listKeys.length > 0 ) {
				// Just pick first that will be shown to the user in the dropdown
				// We are setting mailchimp_list just in case user likes it and clicks
				// Continue without actually sellecting something.
				data.mailchimp_list = listKeys[ 0 ];
			}
			return Object.assign( {}, state, data );
	}

	return state;
}
Updates state with latest MailChimp connected plugin settings

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