Call string constructor function

Question

I have a constructor function, for example webapp.core.cf1 or Backbone.Router. But I get this constructor function not as a reference, but as a string! I cannot change this requirement, the constr. must be in a string. How can I make a new object from this, for instance classfun("Backbone.Router")?

function classfun (cfstr) 
{ 
  ...
  cf = new window[cfstr]();

If I try it this way, I get the error: ... is not a constructor.

Why this does not work? Is there an alternative way without using eval()?

Thanks a lot in advance

EDIT

Thank you all for your answers!

Thank you, Tronix117, this was the problem!!

Thank you, Benjamin Schulte, for the function!

Answer 1

If you try in your console:

var A=function(a){this.test=a};
b = new window["A"](3);
console.log(b.test); // return 3

So that means, you are trying to access something which is not in the scope of window, it should rather be something like that: window.something["A"], if you don't know what is something then you should use the function of Benjamin, otherwise use this one, because it's way faster.