Testing JavaScript functions inside anonymous functions

Question

Is it possible to test myInnerFunction below?

var val = function() {
    var myInnerfunction = function(input) {
        return input + ' I ADDED THIS';
    };
    return myInnerfunction('test value');
}();

Because myInnerFunction is essentially a private member of the anonymously executed outer function, it doesn't seem like it is testable from the outside.

Answer 1

You could intentionally expose a testing hook to the outside world, like possibly this:

var val = function() {
   var myInnerfunction = function(input) {
      return input + ' I ADDED THIS';
   };
   /* START test hook */
   arguments.callee.__test_inner = myInnerFunction;
   /* END test hook */
   return myInnerfunction('test value');
}();

now, once val has been run at least once, you can reference val.__test_inner and call it with testable inputs.

The benefits to this approach: 1. you choose what is exposed and not (also a negative 'cause you have to REMEMBER to do so) 2. all you get is a copy-reference to the private method, so you can't accidentally change it, only use it and see what it produces

The drawbacks: 1. if the private member changes (or relies on) state of its host/parent function, it's harder for you to unit test around that, since you have to recreate or artificially control the host/parent state at the same time 2. As mentioned, these hooks have to be manually added

If you got really clever, you could have your build process look for blocks of comments like the above and remove the test hooks when creating your production build.

Answer 2

afaik unit testing does not concern about internal workings of things you test. The point is that you test the functionality ie: it does what it's supposed to do, not how it does it. So if it uses an internal private member, it should not be testable...