extend (deep) merges arrays fields instead of replacing them

[tsvetomir]

A deep extend does not overwrite array fields, but tries to merge them.

Example: [0, 1, 2] and [3] produce [3, 1, 2] instead of [3]. [

This behavior is not consistent or expected. If we apply it on strings "abc" and "d" should merge to "dbc".

Possibly related issue: [

[Rick Waldron]

Can you cite references of what you believe to be the correct behaviour?

[dmethvin]

Yeah, this looks correct to me. Object and Array are extended, but primitive string/boolean/number are not.

[Rick Waldron]

That's how I've always understood "extend" functions to behave, hopefully the OP will have references that will "enlighten" us.

[tsvetomir]

We have a defined behavior for extending objects (union), but we shouldn't assume that the user expects the same for sets of data.

Arrays should be treated as sets of data and not like objects. They can be combined in an union, intersection, a complement or in a user-defined way.

These two similar implementations yield the result I'd expect:

Object.extend (Prototype) ​http://jsfiddle.net/JV7m8/1/

Object.append (Mootools) ​http://jsfiddle.net/dThRz/1/

[Rick Waldron]

This yields the results exactly as shown above: ​http://jsfiddle.net/rwaldron/zkCZt/

[tsvetomir]

extend(a, b) vs. extend(true, a, b)

You need to supply the true (deep) parameter to reproduce the issue. Otherwise you're overwriting the "nested" object ​http://jsfiddle.net/MN5xQ/

[tsvetomir]

p.s. Turns out Prototype and Mootools don't have a true deep extend. They always overwrite nested objects and arrays. Sorry for comparing apples to oranges!

Either way, I'm not the only one confused by this behavior. See ​http://forum.jquery.com/topic/jquery-extend-modifies-but-not-replaces-array-properties-on-deep-copy

[Ariel <asjquery@…>]

I just ran into this.

See: ​http://jsfiddle.net/s5jNF/

I would expect Arrays to be overwritten or appended. But don't replace the individual elements piecemeal, which is what happens now.

Personally I need a deep extend where the array elements are appended. (Although I can see cases where I would want them overwritten.)

Can we get this as an option?

rwaldron, please reopen this. It's not worksforsome - it's wontfix or it's open.

[dmethvin]

Not sure why it can't be closed worksforme with needsdocs. The proposed changes might break existing code, which would be one reason to avoid changing it. For specific needs it is no doubt faster (performance-wise) to write a custom extend method, our poor method is already overburdened with constraints.

[addyosmani]

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