Why does debuggers give you “undefined is not a function” instead of just telling you the name of the property/variable that is not a function?

问题

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?

回答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

来源：https://stackoverflow.com/questions/28612341/why-does-debuggers-give-you-undefined-is-not-a-function-instead-of-just-tellin