_.extend copies undefined values

[jzaefferer]

Using _.extend as part of Backbone.js I stumbled upon the problem that _.extend copies all properties, even those were the value is undefined. This makes defining and extending defaults a lot more complicated then necessary as I have to filter out manually and properties that are specified as an option, but have no value - which happens a lot when the option is passed based on some other optional argument.

For now my workaround is to replace _.extend with jQuery's $.extend. So far it looks like both are binary compatible, but I'd rather not rely on that.

[polotek]

+1. Also, I'm sure this has been answered before. But why no deep copy function?

[jashkenas]

Thanks, _.extend no longer copies undefined values.

As for deep copies -- there are no good semantics for deep copying in JS ... even jQuery's version has nasty bugs.

[jashkenas]

I'm afraid I'm going to have to revert this change -- Underscore's extend is a primitive enough utility that it should be copying all properties, even undefined ones.

[polotek]

I think I see your argument. But extend isn't used like a primitive utility. It's used like an object extension method with particular semantics. I would argue that we need both: a dumb copy utility and a utility that "does the right thing" when you want to extend an object. It seems the difference is whether you care about the values or whether you care about extending properties that may or may not have a value. I don't want to turn this into a pedantic language debate thread. But it helps to differentiate undefined and null. undefined is "no value", meaning it shouldn't override over values. If you want a value that explicitly says "nothing" that's what null is for, and null should override previous values.

That said, I don't feel terribly strongly about this. I agree that everybody wants their extend, mixin, objectCopy, clone to work slightly differently depending on the situation. People should just build their own.

[jashkenas]

Totally, and that's why the version of it that ships with Underscore.js should be the "unfanciest" version possible.

[claudioc]

I still think that the current behaviour is really a bug, no matter what. Underscore could override actual values with "undefined" (it happened to me, and it was quite a pain to find this).
I think that we should at least: 1. state this behaviour in doc, 2. try to add a parameter to not copy undefined values (this sucks on so many levels, giving the nature itself of _.extend(), but still...)
On a side note, jQuery's extend method does not copy undefined values.

[STRd6]

For those concerned about overwriting existing values with undefined values perhaps you are more interested in a reverseMerge method?