javascript - jQuery $.extend() - how to not add new members to the first object if they don't exist? -

i stumbled on small problem when extending javascript objects using jquery. when executing

var item = $.extend(options.itemdefaults, item); 

options.itemdefaults extended properties in item , result passed item. far, good.

but next time line executed, options.itemdefaults has property values item had, instead of original defaults. defaults lost!

i realize store defaults object in temporary variable, , extend temporary variable instead, seems bit lengthy. there way i'm after (overriding default values supplied ones, taking defaults when no values supplied, not changing default object) without detour?

update: seems wasn't easy around hoped. when

var defaults = options.itemdefaults; var $item = $.extend(defaults, { attributemodel: options.attributemodel }, item); defaults = undefined  

in each iteration, still add properties item on options.itemdefaults! how around this?

try,

var item = $.extend(true, {}, options.itemdefaults, item); 

the true flag indicates deep copy must made.

we use empty object {} target defaults not tampered with.

properties of options.itemdefaults, , item copied empty object.