function union() {
	var cache;
	var arr;
	var i;
	var j;
	cache = {};
	for ( i = 0; i < arguments.length; i++ ) {
		arr = arguments[ i ];
		for ( j = 0; j < arr.length; j++ ) {
			if ( !hasOwnProp( cache, arr[ j ] ) ) {
				cache[ arr[ j ] ] = true;
			}
		}
	}
	return objectKeys( cache );
}
Returns the union.

@private
@param {...Array} arr - input arrays
@returns {Array} union