Unit testing self-invoking functions

Question

I have a self-invoking function in a JavaScript file. Something like this:

com.renderer = (function(){

    render(data){

    }

    .....other functions 

    return{

        init : function(){
            $(document).ready(function(){
              jsonData = fetchFromServer();
              render(jsonData);
            });

         }
    }

})().init();

I am thinking of how to unit test this. I am using JSUnitTestDriver. If I could somehow inject jsonData to the render function from outside, that would be good, but that seems impossible to me.

Any suggestions or alternatives?

I really do not want to remove the self-invoking nature of the function. And does it really make sense to change what I consider good design for the sake of unit tests? (In this particular case, not in general.)

Note: I can't talk to a server while running tests.

Answer 1

The problem is that you are trying to unit test a singleton, and singletons are generally not considered good design.

Instead, I would consider something like the following:

function createRenderer(dataFetcher) {
    function render(data) {
    }

    // other functions

    return {
        init: function () {
            $(document).ready(function () {
                jsonData = dataFetcher();
                render(jsonData);
            });
        }
    };
}

// in your production code
com.renderer = createRenderer(fetchFromServer);
com.renderer.init();

// in your unit test
var renderer = createRenderer(function () {
    return { test: "data" };
});

renderer.init();
// run your tests against renderer.

This uses a technique called dependency injection, where the dependency on fetching JSON data is injected into your renderer, separating out that responsibility.