Document the use of operator <

[jgonggrijp]

@jashkenas I inserted a new "Notes" section between the links and the changelog. For now it only contains the note on operator <. Maybe that note is too long. Maybe the references are too short.

Implements #2846
Closes #2042
Closes #1834
Closes #1768

[jashkenas]

I love it, but it is a bit much 😉

Instead of such an excellent treatise on < and NaN behavior, perhaps try trimming it down to a version that simply briefly mentions that non-comparable JS values have undefined sort order behavior, perhaps with a link to good external docs on the matter, and then move on quickly to a more practical discussion about how to fix your sorting problems: By defining a custom comparator that allows you to specify a sub-key, or compute a value, by which to sort.

[jgonggrijp]

I do get carried away sometimes. 😅

Better now?

[jashkenas]

Looks great. The one thing I might consider adding would be what I consider the most common case of difficulties sorting in JS: sorting an array of objects or class instances. Generally, there’s a single property on the object, or a method to call, to produce the desired value for the sort iterator — but that’s not obvious to beginners!

It also makes for a good example snippet.

[jgonggrijp]

I agree there is a value in a general "sorting in Underscore for beginners" type of note. However, when writing this note I specifically set out to address issues like #1768, which I suspect people still run into after they have already figured out the property shorthand thing. After all, a property that is missing in some objects in a collection is a common way in which people may end up comparing undefined. The documentation of _.sortBy et al. already mentions the option to use an iteratee, anyway.

I think a general introduction to sorting for beginners would require a different approach. I'm not opposed to changing the note in order to suit this purpose, but I want to concentrate on other things right now. Given your approval, I'll just merge this for now. We can revise it at any time.

I have however added a link to the _.iteratee section to give a bit more of a hint of the available options.