why is typeof (/./) !== 'function' used in underscore

Question

I was reading through the source for the _.isFunction() function and saw this line:

if (typeof (/./) !== 'function') {

and I don't understand why it's there. /./ is a regex that always seem to have the type object. Why wouldn't _.isFunction be redefined if /./ type was a function?

Answer 1

Some versions of various JavaScript engines have allowed for calling RegExp as another way of using .exec():

var pattern = /./;

pattern('abc');
pattern.exec('abc');

And, since they were Callable, typeof considered them functions:

Type of val: Object (native or host and does implement [[Call]])
Result: "function"

To my knowledge, though, current versions don't exhibit this behavior and will throw a TypeError. But, if you're concerned with backwards compatibility, as Underscore is, you may need to check for it.