How to test node data chunking function

I'm working on a project which uses node and we're trying to achieve 100% coverage of our functions. This is the only function we haven't tested, and it's within another function.

 var userInput = "";
    req.on("data", function(data){
      userInput += data;
    });

How do you go about testing this function? We tried exporting the function from another file but no luck.

I should mention that we are using tape as a testing module.