Evan Hahn - JavaScript Testing with Jasmine: JavaScript Behavior-Driven Development

In short, you can test software against a specification.

Lets say youre writing a simple calculator that just does addition. Before you even start, think about how it should behave. It should be able to add positive integers. It should be able to add negative integers. It should be able to add decimal numbers, not just integers. You can think of many different ways that your calculator needs to work.

Before youve written any of the code, you know how you want it to behave. You have a specification for its behavior.

You can write these specifications in code. Youd say, OK, it should work this way. Youd make tests that added 1 and 1, 2 and 2, 1 and 5, 1.2 and 6.8, 0 and 0, and so on. When you run these tests, youll either get a success (it works according to the specification) or a failure (it doesnt). If you ran all of your tests and saw success for each, then you can be pretty sure that your calculator works. If you ran these tests and saw some failures, then you know that your calculator doesnt work.

Thats software testing in a nutshell. Youre testing your code against a specification. There are many tools (Jasmine among them) that help you automate these software tests.

Its important to know that its difficult (and often impossible) to write tests for every case. In the calculator example, there are an infinite number of possible combinations. When testing, you should try to cover every reasonable case by testing a number of different groups (integers, negative numbers, mixes of the two, etc.). You should also identify boundary conditions (zeroes, for example) and edge cases, testing as many different scenarios as possible.