Question

Possible Duplicate:
javascript: plus symbol before variable
obj.length === +obj.length in javascript

While looking at the source of underscore.js I came across this line (#79)

//some stuff
} else if (obj.length === +obj.length) {
//do stuff

I’m not 100% certain of whats going on here, can anyone explain the
purpose of the ‘+’ before the obj.length value?? Would the comparison
be identical if it just read:

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

The same type of comparison is made multiple times in underscore.js, so
I’m fairly certain it’s not a typo.

If anyone could point me to an article, or throw some correct terminology at me, I’d appreciate it :). Thanks!

Answer 1

It’s checking if the length property is numeric. When the unary + is applied, it will return the numeric representation of an object or NaN, which will be the basis for which the comparison will pass or fail. For the first case, if obj doesn’t have a length property it will be +undefined which will return NaN. And if obj.length is numeric, the condition will pass.