Using eval method to get class from string in Firefox

Question

What I tried (which works in chrome)

var class_str = "class Test {};";
var a = eval(class_str);
console.log(new a());

Raises following error in Firefox 46:

TypeError: a is not a constructor

a is undefined and using new A() returns ReferenceError: A is not defined.

What is different on Firefox?

Answer 1

Putting the whole class string in parentheses works.

Fixed code:

var class_str = "(class Test {})";
var a = eval(class_str);
console.log(new a());

Answer 2

I tried another method that works just as using parentheses and seems much simpler as it doesn't pollute global names.

result = eval(`class a{} window.a=a`)

console.log(result)