( function ( popups ) {

	popups.reducers = {};

	/**
	 * Creates the next state tree from the current state tree and some updates.
	 *
	 * N.B. OO.copy doesn't copy Element instances, whereas $.extend does.
	 * However, OO.copy does copy properties whose values are undefined or null,
	 * whereas $.extend doesn't. Since the state tree contains an Element instance
	 * - the preview.activeLink property - and we want to copy undefined/null into
	 * the state we need to manually iterate over updates and check with
	 * hasOwnProperty to copy over to the new state.
	 *
	 * In [change listeners](/doc/change_listeners.md), for example, we talk about
	 * the previous state and the current state (the `prevState` and `state`
	 * parameters, respectively). Since
	 * [reducers](http://redux.js.org/docs/basics/Reducers.html) take the current
	 * state and an action and make updates, "next state" seems appropriate.
	 *
	 * @param {Object} state
	 * @param {Object} updates
	 * @return {Object}
	 */
	popups.nextState = function ( state, updates ) {
		var result = {},
			key;

		for ( key in state ) {
			if ( state.hasOwnProperty( key ) && !updates.hasOwnProperty( key ) ) {
				result[ key ] = state[ key ];
			}
		}

		for ( key in updates ) {
			if ( updates.hasOwnProperty( key ) ) {
				result[ key ] = updates[ key ];
			}
		}

		return result;
	};

}( mediaWiki.popups ) );