_.superMerge: Like merge, but concatenates plucked arrays (rather than overwriting with the rightmost).

_.superMerge.js

_.mixin( {
	// Like uniq, but tests for equivalence rather than identity.
	// Does this by running comparison on stringified representations.
	// DOM nodes and other objects with circular references can't be stringified and will break.
	looseUniq  : function uniqBasedOnStringify( source ){
		var output = _( source )
			.map( function stringify( x ){ return JSON.stringify( x ); } )
			.uniq()
			.map( function parse( x ){     return JSON.parse( x );     } )
			.valueOf();

		return output;
	},
	// Like pluck, but takes an array of keys. eg:
	// _.deepPluck( DOMelements, [ 'parentNode', 'parentNode', 'id' ] );
	// Useful recurssion wrapper when array is generated programmatically. 
	deepPluck  : function deepPluck( sources, sourceKeys ){
		var keys = _.clone( sourceKeys );

		return ( function recursivePluck( sources, keys ){
			var key        = keys.shift();
			var candidates = _.filter( sources, function hasKey( source ){ return _.has( source, key ); } );
			var pluck      = _.pluck( candidates, key );

			if( keys.length ){
				return recursivePluck( pluck, keys );
			}

			return pluck;
		}( sources, keys ) );
	},
	// Like merge, but concatenates plucked arrays (rather than overwriting with the rightmost). eg:
	// _.superMerge( { a: 1, b: [ 1 , 2 ] }, { c: 3, b: [ 3, 4 ] } ); // => { a: 1, c: 3, b: [ 1, 2, 3, 4 ] };
	superMerge : function mergeAndConcatArrays(){
		var sources = _.toArray( arguments );
		var merge   = _.merge.apply( _, [ {} ].concat( sources ) );

		void function findArrays( object, path ){
			_.each( object, function ( value, key ){
				var currentPath = _( path ).clone();

				currentPath.push( key );

				if( _.isArray( value ) ){
					object[ key ] = _( sources )
						.deepPluck( currentPath )
						.looseUniq()
						.flatten( true )
						.valueOf();
				}
				else if( _.isObject( value ) ){
					findArrays( value, currentPath );
				}
			} );
		}( merge, [] );

		return ( sources[ 0 ] = merge );
	}
} );