When is obj.length not equal to +obj.length?

Question

I was perusing the underscore.js annotated source when I encountered this:

if (obj.length === +obj.length) {...}

I now know from this stackoverflow question that the plus sign (+) operator returns the numeric representation of the object.

That said, obj.length returns a number. When would obj.length not be equal to +obj.length?

Answer 1

The === operator does not make any typecast when it checks, so different types of data will immediately return false even if '5' == 5. The + as you said typecasts the object into number. If you typecast a number into a number, it is still a number, so you basically check if your object.length exists and is a number. Values like undefined, NaN, null, string and others will return false. You are not sure what happens with obj, so you have to check...