I was looking at the source code for the [Underscore.js](https://web.archive.org/web/20130403134123/http://documentcloud.github.com/underscore/) library, specifically for the [map](https://github.com/jashkenas/underscore/blob/master/underscore.js) method (around line 85 on that page, and copied here):

```
  _.map = function(obj, iterator, context) {
    var results = [];
    if (obj == null) return results;
    if (nativeMap && obj.map === nativeMap) return obj.map(iterator, context);
    each(obj, function(value, index, list) {
      results[results.length] = iterator.call(context, value, index, list);
    });
    if (obj.length === +obj.length) results.length = obj.length;
    return results;
  };

```

completely straightforward EXCEPT for the following line

```
    if (obj.length === +obj.length) results.length = obj.length;

```

Now, I read this to mean "if the object length is not a negative number..."  

which, if my interpretation is right, implies that it might be!

So, dear experts, for what kinds of objects might

```
    obj.length === +obj.length 

```

be `false`? My understanding of `===` means it could return `false` if the type of `obj.length` didn't match the type of `+obj.length`, but here my knowledge falls short. What kinds of things could `+` do to the kinds of things that `obj.length` might be? Is `x === +x` some sort of generic idiomatic test that I just don't know? Is it response to some sort of special case that arises deeper in underscore.js, e.g., does underscore.js assign and track negative object.lengths for some conventional purpose? Guidance and advice will be much appreciated.