为什么调试器会给您“未定义的不是函数”，而不仅仅是告诉您不是函数的属性/变量的名称？

Question

What is the reason js debuggers gives you "undefined is not a function" instead of just telling you the name of the property/variable that is not a function?

example:

a = {
  aFunction: function(){...}
  notAFunction: "foo"
}

a.notAFunction()

Gives:

undefined is not a function

This is more helpful:

property "notAFunction" of object "a" is not a function

What are the exact obstacles?

Answer 1

This is going to be changing very soon, it might already be in Canary too.

Improved exception messages: https://plus.google.com/+AddyOsmani/posts/DdWkiKsvbA2