how to unit-test private methods in jquery plugins?

Question

Perhaps this is a bit of a novice JQuery question but:

• proper jquery plugins are written inside a closure
• thus only methods defining the plugin interface are accessible from the outside
• sometimes (or many times) one may need helper methods that it doesn't make sense to expose as part of plugin interface (for example because they alter internal state).
• how do those get unit-tested?

For example, looking at blockUI plugin, how can methods install, remove, reset get unit-tested?

To draw a parallel, in Java I would:

1. create a BlockUI interface containing public methods only (by definition)
2. create a BlockUIImpl class implementing the above interface. This class would contain install(), remove(), reset() methods that could be public, or (package) protected

So, I would unit-test the Impl but client programmers would interact with the plugin via BlockUI interface.

Answer 1

The same applies here as with any other language and testing privates: To test private methods, you should exercise them via the public interface. In other words, by calling your public methods, the private methods get tested in the process because the public methods rely on the privates.

Generally private methods are not tested separately from the public interface - the entire point is that they are implementation details, and tests should generally not know too much about the specifics of the implementation.

Answer 2

Code written inside a function in JavaScript, or closure as you called it, is not necessarily isolated from the outside of that function.

It is useful to know that functions have visibility of the scope in which they are defined. Any closure you create carries the scope, and therefore functions, of the code that contains it.

This simple example with a jQuery plugin and an artificial "namespace" might serve to prove this assumption:

// Initialise this only when running tests
my_public_test_namespace = function(){};

jQuery.fn.makeItBlue = function() {

    makeItBlue(this);

    function makeItBlue(object) {
        object.css('color','blue');
    }

    if(typeof my_public_test_namespace != "undefined") {
        my_public_test_namespace.testHarness = function() {
            return {
                _makeItBluePrivateFn: makeItBlue
            }
        };
    }
};

$("#myElement").makeItBlue(); // make something blue, initialise plugin

console.debug(my_public_test_namespace.testHarness()._makeItBluePrivateFn);

But don't forget you shouldn't really test privates. ;)