JavaScript testing

Posted by Patrice Neff Wed, 10 Feb 2010

This post about JavaScript testing is part of the Setting up a full stack for web testing series.

Introduction

JavaScript tests are sufficiently special to deserve a post outside of unit and functional testing. You need a special set of tools to write these tests. But then again for JavaScript the usual split between unit and functional tests is still applicable. So what I wrote about those two topics is also relevant here.

In this post I cover two separate tools for JavaScript testing: JsTestDriver, which is great for JavaScript unit tests, and Selenium which can be used to functional test a web application that uses JavaScript.

Selenium

Selenium is a software suite used to remotely control browsers. It consists of various sub-projects:

  • Selenium Core: This is the original Selenium. It allows you to create a HTML page with some browser commands. Those commands can then be run in the browser when you open the page.
  • Selenium IDE: A Firefox extension that can record your actions in the browser. This is a great way to learn the Selenium syntax as you can then export the recorded actions in several programming languages.
  • Selenium Remote Control (Selenium RC): A Java server that allows controlling browsers remotely. Can be used from any programming language using a simple HTTP API.
  • Selenium Grid: Basically the same as Selenium Remote Control but for running in parallel.

Personally I only use Selenium RC, so I won’t talk about the other parts. To get started, read the excellent Selenium documentation which explains everything a lot better than I could. If you want a quick start I recommend the Selenium RC chapter.

The basic test case in Selenium works like this:

  1. Open the page to test (with by first logging in)
  2. Execute some action like a click on a link or a mouse event
  3. Wait for the action to execute (Ajax loads to finish, etc.)
  4. Assert that the result is as expected

These four steps are repeated for every test case – so a lot of pages will be opened. Opening all those pages is the reason why Selenium tests tend to be very slow.

As a test framework you can use whatever you already have – in your favorite programming language. The difference is just that in your tests you will talk to Selenium to get the current state.

Take the following Python code as an example:

def test_login():
    sel = selenium("localhost", 4444, "*firefox", "http://localhost:5000/")
    sel.start()
    sel.open("/login")
    assert sel.is_text_present("Login")
    assert sel.is_element_present("username")
    assert sel.is_element_present("password")
    assert sel.get_value("username") == 'Username'

This script launches a Firefox browser and opens a login page of an application running on the localhost. It then gets several values from Selenium and asserts the correctness of these values using the standard test framework methods.

JsTestDriver

JsTestDriver is a relatively new tool which can be used to submit tests suites to browsers. Those browsers have to register with a Java-based server and you execute tests by submitting them to that same server.

So far that sounds very similar to Selenium. The difference is that JsTestDriver works with a blank page in which it directly inserts the test suite. It does that with a lot of optimizations to make sure the test runs are as fast as possible.

After that the unit test suite is run – directly inside the browser – and the client gets the test results including stack traces if available.

I recommend the Getting started documentation on the JsTestDriver wiki to see some code.

One of the main differences to Selenium is that you write the tests directly in JavaScript. There is a built-in test framework but you can also use other frameworks depending on your taste.

To show some code that you can contrast with the Selenium example above, consider this example:

GreeterTest = TestCase("GreeterTest");

GreeterTest.prototype.testGreet = function() {
  var greeter = new myapp.Greeter();
  assertEquals("Hello World!", greeter.greet("World"));
};

Differences

Selenium is a very magic piece of software. Everybody falls in love with it when seeing their browser doing all the work. It’s something people just can’t resist. And so they end up using Selenium for all their web testing needs. I’ve been there myself.

The downside of Selenium is that it’s very brittle and slow. This is something that can’t really be avoided because all it does is control a browser. Opening pages and waiting for all scripts to load takes some time. And doing that hundreds or thousands of times, as is easily the case in a large test suite, leads to very slow test executions.

So instead of falling into that trap and only using Selenium, I recommend to clearly separate out unit tests which you can then execute in JsTestDriver. JsTestDriver does a lot less work and because of that is a lot more stable and faster. Then do integration tests with Selenium to test some basic workflows.

As an example take an autocompletion widget which is used on a search home page. Almost everything the widget does you can test by just calling its public interfaces and seeing if it does the right thing. This includes all the strange edge cases such as handling network timeouts or invalid data. So this part you do with a big JsTestDriver test suite. Then you only need one small functional test case to make sure the widget is correctly embedded in your home page. That’s your Selenium test.

As is evident I’m very happy that JsTestDriver has come along. Before that the only good solution for JavaScript testing was Selenium – and as I explained above it’s not a perfect fit for every testing need.

Conclusion

If you have followed my testing tutorial so far you now have all your testing tools set up. Some more chapters will follow but those now cover testing philosophy and tools around the principal testing framework.