$.extend with deep copy of an array property merges the array instead of replacing it

[i23098]

If I run $.extend(true, {}, {'a': [1,2,3]}, {'a':[4]}) I would expect the result to be {"a":[4]} as property a should be overwritten instead of merged...

​http://jsfiddle.net/pm1f3utz/

[i23098]

It also happens in 2.x

[gibson042]

Merging objects and arrays rather than overwriting them is the behavior that differentiates our deep extend from our shallow extend.

​http://api.jquery.com/jQuery.extend/:

On a deep extend, Object and Array are extended, but object wrappers on primitive types such as String, Boolean, and Number are not. Deep-extending a cyclical data structure will result in an error.

For needs that fall outside of this behavior, write a custom extend method instead, or use a library like ​lodash.