Question

I came across this line of code in Underscore.js’s _.each implementation and I am curious what is going on here. What does the ‘+’ in front of the obj do?

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

Answer 1

The if tests that obj.length is numeric and not NaN. The right-hand side is always a number (or NaN if obj.length cannot be interpreted as a number). It will only be === to the left-hand side if obj.length is also a number.

Note that using isNaN won’t work if obj.length is a numeric-looking string; that is, isNan("3") returns false. Note also that NaN === NaN is false—NaN is never === to anything.