As you write JavaScript applications, you will soon realize that having a sound testing strategy is indispensable. In fact, not writing enough tests is almost always a bad idea. It is essential to cover all the non-trivial functionality of your code to make sure of the following points:
The existing code behaves as per the specifications
Any new code does not break the behavior defined by the specifications
Both these points are very important. Many engineers consider only the first point the sole reason to cover your code with enough tests. The most obvious advantage of test coverage is to really make sure that the code being pushed to the production system is mostly error-free. Writing test cases to smartly cover the maximum functional areas of the code generally gives you a good indication about the overall quality of the code. There should be no arguments or compromises around this point. It is unfortunate though that many production systems are still bereft...