Question

a = /\d+/.exec; 
a("hello,123")

I getting the following error:

Error: can't convert undefined to object

Why is a returned as undefined?

Answer 1

a in this case is a reference to the RegExp.prototype.exec function itself, and is not actually bound to any regular expression. Either of these would work:

var a = /\d+/.exec('hello,123');

---

var rx = /\d+/,
     a = rx.exec;

a.call(rx, 'hello,123');

Your undefined is coming from an internal reference to this from within the exec function.