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Ask HN: What are Unit Tests and Why are they important?

• keytweetlouie 16y ago
  A test that isolates a unit of work from the main program. It can be many functions or one. The idea is to "exercise" your code. That way when it's used in a full integrated flow there are no surprises. Check out TestDrivenDevelopment methodology.
• ashleyreddy 16y ago
  There are other people telling you what a unit test is so I won't bother. What I will tell you is that you will have a eureka moment when your unit test unexpectly fails. Then you will say "Boy I'm sure glad I have unit tests."
• c00p3r 16y ago
  The idea behind unit tests is quite simple - you're considering your code as several black-box abstractions divided by abstraction layers with clean interfaces. Unit tests are used to be sure that after any code change each black box is still working as it should (gives the same output for the same input data) and so do each of your interfaces. Think of self-test of an aircraft - thats it.
• weaksauce 16y ago
  to expand on c00p3r's comment, you can test whatever you want to test in a unit test. basically you are testing the response of a "unit." The unit can be whatever you feel like but it it typically a class of some sort depending on the language. Typically you test the extremes of inputs, invalid inputs and some valid inputs inside the extremes. It's hard to write comprehensive tests but once you write them you can keep adding to them when you find a new bug. Just remember that nothing is infalible.
• alaithea 16y ago
  A simple example: if I wrote a function, add(x, y), I would want to test that it behaved as expected. So, I'd write a test (among others) to assert that add(4,5) returned 9. I would also test things like add(null, 5) to make sure that my code gracefully handled error conditions.
• basicxman 16y ago
  In dummy terms, it's a checklist for your programs.
  • PKeeble 16y ago
    A checklist that gets checked automatically. Once you have good coverage of your code base you can make large changes, run the tests and be confident the system still works without actually starting it up and checking manually.
• tjpick 16y ago
  They serve a couple of purposes. 1. That the code you have written or are using behaves in a way you expect, for the inputs you expect. 2. As the code changes over time, a regression test suite helps ensure that assumptions from an earlier point in time still hold true.
  • briandoll 16y ago
    3. That the design of the API makes sense to the user of that new code. If you practice TDD (Test Driven Development), you first write the test(s), then code to make those tests pass. You're much less likely to end up with SomeClass.some_reference.a_really_bad_name(with,lots,of,required,arguments) when you first think of the caller, rather than of the method itself.
• locopati 16y ago
  If you have an application that has a lifespan of years, developers will come and go. The unit tests provide some provability that is abstracted from the developers. A developer leaves, they've left behind tests that demonstrate that the code fulfills the requirements and perhaps shows how the code interacts with other areas of code. A developer arrives, they can see the requirements and how an area of the codebase works by examining unit tests. Of course, this presumes well-written tests that are actually thorough. Unit tests also give you a measure of confidence that allows a freer hand when refactoring code. You can change code all you want and if the unit tests still pass, you haven't broken things (again - if the tests do cover all the expectations). Some level of sanity is required around test-first, test once the code is written, how many tests you need, what level of depth, etc. A one-off app to fulfill a quick-and-dirty need doesn't need as much testing as a core platform that will be running business critical code for years.
• kevinpet 16y ago
  People can quibble about the definition of "unit" tests, but essentially, automated testing is an executable specification of your code's behavior. Let's say I write a piece of code to calculate a dollar value for an investment portfolio. Then, I want to validate it. I could poke at it and see how it behaves on some examples, but this would be a one-time thing. Better for me to write some tests which set up some sample data and assert that the code calculates the right results. Later on down the line, maybe I find a bug -- maybe it doesn't work right when you purchased a stock, then purchased some more of it. I could fix it, manually test it, and call it a day, but it would be better if I first expressed the bug as a unit test (i.e. I set up some data which causes the code to generate a result different from what I expected, and so the test fails), and then fix the code. Since I also have my original test cases, I can be sure that I didn't break some other behavior when fixing this bug. Similar reasoning applies to adding functionality -- leave the old tests intact and add new tests. Tests are not about configuration. It may use the word "test" but "test whether XYZ is installed" is a different question from what unit tests are about. Tools which look for problems like XSS are not tests either. I don't know what to call them. They are a good idea. Selenium is a testing tool for validating web apps at the application level. Purists might call it a tool for functional or integration testing rather than unit testing.
• wlievens 16y ago
  I really don't want to troll with this comment, but this kind of information is widely available on sources such as Wikipedia. If you'd read that generic information first, and then came here to ask about specifics or anecdotes, the discussion would be more valuable.
• samstokes 16y ago
  Pat Maddox recently defined unit testing in a tweet (and being so concise is surprisingly enlightening): "a unit test is any test that you'd be 99% happy to have if it ran continuously" (http://twitter.com/patmaddox/status/23988815286 http://twitter.com/patmaddox/status/23988815286) Followed up with: "99% happy implies: fast; no false positives; flexible under code changes; catch errors as they occur" (http://twitter.com/patmaddox/status/23988868590 http://twitter.com/patmaddox/status/23988868590)
• radu_floricica 16y ago
  It's interesting that so far everybody seems to ignore the last part of the question, which seems to me interesting enough: is unit testing useful for web apps? In my experience, I can tell that I'd really like to be able to use them, but changing my apps to fit them would make them a lot "thicker". Let me explain. When I make a page in a web apps which does something, the code is very thing. It has roughly 3 layers: sql database (static, no stored procedures), business logic including sql statements (java), and html template (in my case html+apache velocity). There is no POO dividing my apps horizontally. In order to make this testable I'd have to basically put another layer on top of everything. I still have no idea how to do it without making everything horrible.
• known 16y ago
  All Unit Tests can be combined to create http://en.wikipedia.org/wiki/Automated_testing http://en.wikipedia.org/wiki/Automated_testing