Reading through the source code of underscore.js I stumbled upon the following line:
... if (obj.length === +obj.length) { ...
That's a bit confusing for me. What is actually being compared here? I believe it has something to do about detecting native arrays, but cannot figure out what's actually going on. What does the + do? Why use === instead of ==? And what are the performance benefits of this style?
The + coerces the value to an Number (much like !! coerces it to a boolean).
if (x === +x)
...can be used to confirm that x itself contains an integer value. In this case it may be to make sure that the length property of obj is an integer and has not been overwritten by a string value, as that can screw up iteration if obj is treated as an array.