Using Jint to unittest your Javascript in C#

I recently stumbled across Jint and found it interesting to a degree that I have spent a few hours getting to know the product. 99 times out of 100 I don’t have the time or the energy to dig deeper into new products but the timing was well so off I went.

What is Jint? It is an opensource implementation of a Javascript interpreter. The project defines itself in the following terms: “Jint is a script engine based on the Javascript language…Jint aims at providing every JavaScript functionalities to .NET applications”. Does this mean that I can take a piece of Javascript and execute it in a .NET Console application? Yes it does – and it works out to be a much more frictionless experience than you might expect. I tried integrating QUnit with CruiseControl.NET a while back to test Javascript in a managed environment and even though I made it work 95% it really didn’t feel like a comfortable way to go. Let’s see some code (example is from the project’s website)

script= @"
  function square(x) { 
    return x * x; 
  };
  
  return square(number);
  ";
  
var result = new JintEngine()
  .SetParameter("number", 3)
  .Run(script));

Assert.AreEqual(9, result);

Really?  Yes, indeed… I decided to try it out on one of our own internal Javascript API methods and came up with this

[TestMethod]
public void Basic_GetRestHost_ValueReturned()
{
    string expectedValue = "http://restservices.localhost";

    var jint = new JintEngine();
    var returnVal = jint.Run(File.ReadAllText("dgiapi.js") + "return $dgi.getRestHost();");

    Assert.AreEqual(expectedValue, returnVal);
}

Test passes, 4 lines of code, not too much ceremony along the way… It took a while to figure out that the Run-method isn’t chained – I thought I could preload our API in a base class and use a second “Run” method to invoke the call to $dgi.getRestHost() but never made it work. It might not be best practise – it probably isn’t but haven’t dug deeper there yet.

Conclusion: Jint really looks promising. One of the major showstoppers along the way of testing Javascript for me has always been the lack of integration with buildservers and Continuous Integration but Jint seems to close the gap here. I can definately see some usages in our business here where we spend more and more of our time writing Javascript instead of serverside .NET code – especially because we’re currently migrating from a self-breed internal business application to a new, shiny installation of Microsoft xRM in which we will inevitably end up with Javascript to extend the standard user interface (hide buttons, load data into dropdowns etc. etc). It is business critical that these scripts works like expected so it would be nice to be able to unittest at least parts of them in a Continuous Integration environment. I’ll look forward to a PoC of Jint under these circumstances.

The project is work in progress and I submitted a bug yesterday - it was fixed this morning in a dev branch. Thumbs up.