code: function( elems, callback, arg ) {
  		var value, key, ret = [],
  			i = 0,
  			length = elems.length,
  			// jquery objects are treated as arrays
  			isArray = elems instanceof jQuery || length !== undefined && typeof length === "number" && ( ( length > 0 && elems[ 0 ] && elems[ length -1 ] ) || length === 0 || jQuery.isArray( elems ) ) ;
  
  		// Go through the array, translating each of the items to their
  		if ( isArray ) {
  			for ( ; i < length; i++ ) {
  				value = callback( elems[ i ], i, arg );
  
  				if ( value != null ) {
  					ret[ ret.length ] = value;
  				}
  			}
  
  		// Go through every key on the object,
  		} else {
  			for ( key in elems ) {
  				value = callback( elems[ key ], key, arg );
  
  				if ( value != null ) {
  					ret[ ret.length ] = value;
  				}
  			}
  		}
  
  		// Flatten any nested arrays
  		return ret.concat.apply( [], ret );
  	}
docs: arg is for internal usage only