It is possible, albeit counterproductive, to write a unit test which executes some code, and asserts truth. This is a deliberately extreme, and simplified example - I'm sure most people have come across tests which execute code without actually making use of it.
Are there any code coverage tools which assess whether code covered is actually used as part of the assertions of the test?
What you want in essence is to compute the intersection between the covered code, and a backward code slice (minus the unit test itself) on the assertion in the covered test.
If that intersection is empty, the assertion doesn't test any part of the application.
Most code coverage tools don't compute slices, so I'd guess the answer to your question is "no".
You would need some kind of dependency analysis, i.e. to find out which statements the assertion depends on. Then this would have to be cross-checked with the coverage. CodeSurfer is one commercial tool that does dependency analysis for C.
Once again, I'd just investigate why this is happening ? Tests added just for the sake of increasing coverage numbers often are a symptom of a more serious cause. Educating everyone instead of picking out specific offenders usually works out better.
For identifying mistakes that have already been made: You can use a static code analysis tool that checks that
each test has atleast ONE assert. Of course this can also be defeated by rogue asserts.
there are no unused return values.
I saw the first one in something called TestLint from RoyOsherove and the second one I can think of Parasoft or similar.
Still this isn't a foolproof method and will sink significant time poring through the SCA reported issues. But that's the best I can think of.
I finished an app and after that I'm trying to write unit test to cover all methods.
The thing is that Im seeing that I'm testing my code with the knowledge of how it works.
For me is a bit stupid because I know how the code works and I'm testing what is my code actually doing.
My question is:
Is this useless? Im testing what it does, not what it is supposing to do. My code works but I can improve it. Then:
Should I complete all my tests and then try to refactor my code changing my test to "How the code should work" and making the changes to the app to pass the test?
Thank you.
You need to test "How the code should work". That is, you have a clear idea of what a particular method should behave like, then create the set of tests that covers that behaviour. If your code fails the test then you can fix it.
Even though your code is working now you still need the tests for regression testing. Later when you modify your code or add new extensions to it you need to ensure that you did not break the existing functionality. The set of tests that you derive today will be able to tell you how well you did.
While I don't always do Test-Driven Development (write tests before implementing the class), I do always write tests after I implement the code. Even some days after. At least I try to follow a path coverage strategy (that is, every route flow from the beginning to the method up to when it returns). Also for unexpected parameter values. This is useful for enhancing your confidence of correct behaviour of your functions, also when you change the code.
Quite always I find unexpected behaviors or little bugs :) So it works
It will be extremely useful if you ever change the code.
Pretty useless, I'd say. It is called "test driven" for a reason. Every line of code is motivated by a test.
That means the code line is also protected against changes.
Test driven development requires a minimalistic approach and tons of discipline. Add a test to extend the functionality a little. Implement the functionality so it just makes the light green but nothing more. Make foolish implementations until the tests force you to make serious ones.
I have tried to add tests to existing code and found it difficult. The tendency is that only some of the functionality becomes tested. This is dangerous since the test suite will make people think the code is protected. One method is to go through the code line by line and ensure there is a test for it. Make foolish changes to the code until a test forces you to back to the original version. In the end, any change to the code should break the tests.
However, there is no way you can derive the requirements from the code. No test suite that is the result of reverse-engineering will be complete.
You should at least try to build the tests on how its supposed to work. Therefor it is better to build the tests in advance, but it is still not useless to build the tests now.
The reason: you don't build the tests to test your current code, but you're building them to test future modifications. Unit tests are especially useful to test if a modification didn't break earlier code.
Better late than never!
Of course, the best option is to do the unit tests before you start implementing so that you can profit from them even more, but having a good set of unit tests will prevent you from introducing many bugs when you refactor or implement new features, regardless of whether you implemented the unit tests before or after the implementation.
I know that TDD style is writing the test first, see it fails then go and make it green, which is good stuff. Sometimes it really works for me.
However especially when I was experimenting with some stuff (i.e. not sure about the design, not sure if it's going to work) or frantically writing code, I don't want to write unit tests, it breaks my flow.
I tend to write unit tests later on and especially just before stuff getting too complicated. Also there is another problem writing them later is generally more boring.
I'm not quite sure if this is a good approach (definitely not the best).
What do you think? Do you code write your unit tests later? Or how do you deal this flow problem or experimental design / code stage.
What I've learned is that there is no experimental code, at least not working in production environments and/or tight deadlines. Experiments are generally carried out until something "works" at which point that becomes the production code.
The other side of this is that TDD from the start will result in better design of your code. You'll be thinking more about it, reworking it, refactoring it more frequently than if you write the tests after the fact.
I've written tests after the fact. Better late then never. They are always worth having.
However, I have to say, the first time I wrote them before writing the tested code, it was extremely satisfying. No more fiddling around with manual testing. I was surprised just how good it felt.
Also, I tend to write unit tests before refactoring legacy code - which, almost by definition, means that I'm writing tests to test code that's already written. Provides a security blanket that makes me more comfortable with getting into big blocks of code written by others.
"I'm not quite sure if this is a good approach (definitely not the best)."
Not good? Why not?
Are you designing for testability? In that case, your design is test-driven. What more can anyone ask for?
Whether the tests come first, in the middle or last doesn't matter as much as designing for testability. In the end, changes to the design will make tests fail, and you can fix things. Changes to the tests in anticipation of design changes will make the tests fail, also. Both are fine.
If you get to the end of your design work, and there's something hard to test in the middle, then you failed to do TDD. You'll have to refactor your design to make it testable.
I often take the same approach you're talking about. What seems to work well is to treat the exerimental code exactly as such, and then start a proper design based on what you've learned. From here you can write your tests first. Otherwise, you're left with lots of code that was written as temporary or experimental, and probably won't get around to writing tests for all of it.
I would say that for normal development, TDD works extremely well. There are cases where you may not need to write the tests first (or even at all), but those are infrequent. Sometimes, however, you need to do some exploration to see what will work. I would consider this to be a "spike", and I don't necessarily think that TDD is absolutely necessary in this case. I would probably not use the actual "spike" code in my project. After all, it was just an exploration and now that I have a better idea of how it ought to work, I can probably write better code (and tests) than my "spike" code. If I did decide to use my "spike" code, I'd probably go back and write tests for it.
Now, if you find that you've violated TDD and written some production code before your tests - it happens - then, too, I'd go back and write the tests. In fact, on the occasions where this has happened to me I've often found things that I've neglected once I start writing the tests because more tests come to mind that aren't handled by the code. Eventually, you get back in the TDD rythym (and vow never to do that again).
Consider the psychological tendencies associated with sunk cost. That is, when you get to the second part of the equation, that laziness gene we all have makes us want to protect the work we have already done. The consequences?
If you write the tests first...
You tend to write the code to fit the tests. This encourages the "simplest thing that solves the problem" type development and keeps you focused on solving the problem not working on meta-problems.
If you write the code first...
You will be tempted to write the tests to fit the code. In effect this is the equivalent of writing the problem to fit your answer, which is kind of backwards and will quite often lead to tests that are of lesser value.
Although I'd be surprised if 1 programmer out of 50 ALWAYS writes tests first, I'd still argue that it is something to strive for if you want to write good software.
I usually write my tests first but sometime while experimenting I write the code after. Once I get an idea of what my code is supposed to do, I stop the code and start the tests.
Writing the code first is natural when you're trying to figure out how your code is going to work. Writing the test first helps you determine what your code show do (not how it should do it). If you're writing the code first, you're trying to solve the problem without completely defining the problem. This isn't necessarily "bad", but you are using unit tests as a regression tool rather than a development tool (again, not "bad" - just not TDD).
VS 2008 has a nice feature that will generate test classes for an object, the tests needs to me tweaked but it dose a lot of the grunt work for you. Its really nice for crating tests for your less then diligent co-workers.
Another good point for this is it help to prevent you from missing something, expectantly when your working on code that isn't yours.
if your using a different testing framework then MSUnitTest, it's fairly simple to convert then tests from MSUnit to Nunit, etc. just do some copy and past.
I would like to say that I always write Unit tests first but of course I don't (for numerous reasons well known to any real programmer :-)). What I (ok, also not always...) do is to convert every bug which takes me more than five minutes to find into a unit test. Even before I fix it. This has the following advantages:
It documents the bug and alerts me if it shows up again at a later point of time.
It helps in finding the bug, since I have a well-defined place to put debugging code into (setting up my data structures, call the right methods, set breakpoints on etc.) Before I discovered unit testing I modified the main() function for this testing code resulting in strange results when I forgot to remove it afterwards ...
Usually it gives me good ideas what else could go wrong, so it quite often evolves in a whole bunch of unit tests and resulting in more than one bug getting discovered resp. fixed.
How do you effectively test code that you wrote? I find that it is very hard to tiger test code that I have written for my site because part of me feels that I don't want to find bugs in my code because that shows I'm not perfect. (Even writing that I might not be perfect bother me a little bit.)
I believe in unit testing, but lately I've become a bit of a gunslinger and have been deploying lots of code to production from the hip (which isn't always bad for a new web startup, but often is.)
I've been bitten in the last few weeks by serious bugs that have gotten by my own testing. I have a partner who I push to black box test my own code, but due to my knowledge of the implementation details I really should be the best person to test the weak points of the code using white box testing.
So what methods and tools are useful to help you test code you have written yourself?
Writing the test before the code helps since I can think (without the bias set by the implementation in the mind) about what the behavior of the code should be in various conditions.
OK, here goes.
You are not perfect. I am not perfect. Nobody is perfect. So don't worry about exposing errors. The more you find yourself, rather than letting someone else find them for you, the closer to perfect you become.
Get into the habit of writing the tests before the code where you can. Define what the code should do with a meaningful set of tests, then code to get the tests passing. Then refactor - classic TDD.
If there are bugs, then immediately write a test that exposes the bug (i.e. that should pass, but fails because of the bug). Then get the test to pass, and you've fixed the bug.
To sum up:
TDD:
Before writing code, write a unit test to take in inputs and check for outputs.
Write your code only to make the unit test pass -- nothing more, nothing less.
Repeat for every method you're working on.
REGRESSION:
Every time you commit a change/feature/fix, run all your unit tests.
For each unit test which fails, go to BUG FIX
BUG FIX:
Every time you find a bug, write a unit test to reveal the bug.
Implement a fix which causes the unit test to pass.
FUNCTIONAL TESTING:
Find a decent functional testing framework (e.g. selenium for web tests)
Create a number of tests which help exercise the end-to-end functionality of your app
Find and fix tests using the REGRESSION and BUG FIX processes.
I've started to write unit tests before the code for all my personal projects bigger than 20 lines of code. It helps a lot.
Now I'm always surprised when people say that they hate unittests and TDD.
First you should note all the possibilities of inputs, Then you have to test with all the possibilities. You can find our bugs and it will clear out lot of bugs. I am using this way only, so far i have no idea about the Tools
If you do TDD, you may write the test code, forget it for one week and then write the productive code. So you will rethink if your approach is the right one.
Avoid embarrassment. If you find bugs in your code and you are able to fix it, no one can accuse you to write bad code.
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I saw many questions asking 'how' to unit test in a specific language, but no question asking 'what', 'why', and 'when'.
What is it?
What does it do for me?
Why should I use it?
When should I use it (also when not)?
What are some common pitfalls and misconceptions
Unit testing is, roughly speaking, testing bits of your code in isolation with test code. The immediate advantages that come to mind are:
Running the tests becomes automate-able and repeatable
You can test at a much more granular level than point-and-click testing via a GUI
Note that if your test code writes to a file, opens a database connection or does something over the network, it's more appropriately categorized as an integration test. Integration tests are a good thing, but should not be confused with unit tests. Unit test code should be short, sweet and quick to execute.
Another way to look at unit testing is that you write the tests first. This is known as Test-Driven Development (TDD for short). TDD brings additional advantages:
You don't write speculative "I might need this in the future" code -- just enough to make the tests pass
The code you've written is always covered by tests
By writing the test first, you're forced into thinking about how you want to call the code, which usually improves the design of the code in the long run.
If you're not doing unit testing now, I recommend you get started on it. Get a good book, practically any xUnit-book will do because the concepts are very much transferable between them.
Sometimes writing unit tests can be painful. When it gets that way, try to find someone to help you, and resist the temptation to "just write the damn code". Unit testing is a lot like washing the dishes. It's not always pleasant, but it keeps your metaphorical kitchen clean, and you really want it to be clean. :)
Edit: One misconception comes to mind, although I'm not sure if it's so common. I've heard a project manager say that unit tests made the team write all the code twice. If it looks and feels that way, well, you're doing it wrong. Not only does writing the tests usually speed up development, but it also gives you a convenient "now I'm done" indicator that you wouldn't have otherwise.
I don't disagree with Dan (although a better choice may just be not to answer)...but...
Unit testing is the process of writing code to test the behavior and functionality of your system.
Obviously tests improve the quality of your code, but that's just a superficial benefit of unit testing. The real benefits are to:
Make it easier to change the technical implementation while making sure you don't change the behavior (refactoring). Properly unit tested code can be aggressively refactored/cleaned up with little chance of breaking anything without noticing it.
Give developers confidence when adding behavior or making fixes.
Document your code
Indicate areas of your code that are tightly coupled. It's hard to unit test code that's tightly coupled
Provide a means to use your API and look for difficulties early on
Indicates methods and classes that aren't very cohesive
You should unit test because its in your interest to deliver a maintainable and quality product to your client.
I'd suggest you use it for any system, or part of a system, which models real-world behavior. In other words, it's particularly well suited for enterprise development. I would not use it for throw-away/utility programs. I would not use it for parts of a system that are problematic to test (UI is a common example, but that isn't always the case)
The greatest pitfall is that developers test too large a unit, or they consider a method a unit. This is particularly true if you don't understand Inversion of Control - in which case your unit tests will always turn into end-to-end integration testing. Unit test should test individual behaviors - and most methods have many behaviors.
The greatest misconception is that programmers shouldn't test. Only bad or lazy programmers believe that. Should the guy building your roof not test it? Should the doctor replacing a heart valve not test the new valve? Only a programmer can test that his code does what he intended it to do (QA can test edge cases - how code behaves when it's told to do things the programmer didn't intend, and the client can do acceptance test - does the code do what what the client paid for it to do)
The main difference of unit testing, as opposed to "just opening a new project and test this specific code" is that it's automated, thus repeatable.
If you test your code manually, it may convince you that the code is working perfectly - in its current state. But what about a week later, when you made a slight modification in it? Are you willing to retest it again by hand whenever anything changes in your code? Most probably not :-(
But if you can run your tests anytime, with a single click, exactly the same way, within a few seconds, then they will show you immediately whenever something is broken. And if you also integrate the unit tests into your automated build process, they will alert you to bugs even in cases where a seemingly completely unrelated change broke something in a distant part of the codebase - when it would not even occur to you that there is a need to retest that particular functionality.
This is the main advantage of unit tests over hand testing. But wait, there is more:
unit tests shorten the development feedback loop dramatically: with a separate testing department it may take weeks for you to know that there is a bug in your code, by which time you have already forgotten much of the context, thus it may take you hours to find and fix the bug; OTOH with unit tests, the feedback cycle is measured in seconds, and the bug fix process is typically along the lines of an "oh sh*t, I forgot to check for that condition here" :-)
unit tests effectively document (your understanding of) the behaviour of your code
unit testing forces you to reevaluate your design choices, which results in simpler, cleaner design
Unit testing frameworks, in turn, make it easy for you to write and run your tests.
I was never taught unit testing at university, and it took me a while to "get" it. I read about it, went "ah, right, automated testing, that could be cool I guess", and then I forgot about it.
It took quite a bit longer before I really figured out the point: Let's say you're working on a large system and you write a small module. It compiles, you put it through its paces, it works great, you move on to the next task. Nine months down the line and two versions later someone else makes a change to some seemingly unrelated part of the program, and it breaks the module. Worse, they test their changes, and their code works, but they don't test your module; hell, they may not even know your module exists.
And now you've got a problem: broken code is in the trunk and nobody even knows. The best case is an internal tester finds it before you ship, but fixing code that late in the game is expensive. And if no internal tester finds it...well, that can get very expensive indeed.
The solution is unit tests. They'll catch problems when you write code - which is fine - but you could have done that by hand. The real payoff is that they'll catch problems nine months down the line when you're now working on a completely different project, but a summer intern thinks it'll look tidier if those parameters were in alphabetical order - and then the unit test you wrote way back fails, and someone throws things at the intern until he changes the parameter order back. That's the "why" of unit tests. :-)
Chipping in on the philosophical pros of unit testing and TDD here are a few of they key "lightbulb" observations which struck me on my tentative first steps on the road to TDD enlightenment (none original or necessarily news)...
TDD does NOT mean writing twice the amount of code. Test code is typically fairly quick and painless to write and is a key part of your design process and critically.
TDD helps you to realize when to stop coding! Your tests give you confidence that you've done enough for now and can stop tweaking and move on to the next thing.
The tests and the code work together to achieve better code. Your code could be bad / buggy. Your TEST could be bad / buggy. In TDD you are banking on the chances of BOTH being bad / buggy being fairly low. Often its the test that needs fixing but that's still a good outcome.
TDD helps with coding constipation. You know that feeling that you have so much to do you barely know where to start? It's Friday afternoon, if you just procrastinate for a couple more hours... TDD allows you to flesh out very quickly what you think you need to do, and gets your coding moving quickly. Also, like lab rats, I think we all respond to that big green light and work harder to see it again!
In a similar vein, these designer types can SEE what they're working on. They can wander off for a juice / cigarette / iphone break and return to a monitor that immediately gives them a visual cue as to where they got to. TDD gives us something similar. It's easier to see where we got to when life intervenes...
I think it was Fowler who said: "Imperfect tests, run frequently, are much better than perfect tests that are never written at all". I interprete this as giving me permission to write tests where I think they'll be most useful even if the rest of my code coverage is woefully incomplete.
TDD helps in all kinds of surprising ways down the line. Good unit tests can help document what something is supposed to do, they can help you migrate code from one project to another and give you an unwarranted feeling of superiority over your non-testing colleagues :)
This presentation is an excellent introduction to all the yummy goodness testing entails.
I would like to recommend the xUnit Testing Patterns book by Gerard Meszaros. It's large but is a great resource on unit testing. Here is a link to his web site where he discusses the basics of unit testing. http://xunitpatterns.com/XUnitBasics.html
I use unit tests to save time.
When building business logic (or data access) testing functionality can often involve typing stuff into a lot of screens that may or may not be finished yet. Automating these tests saves time.
For me unit tests are a kind of modularised test harness. There is usually at least one test per public function. I write additional tests to cover various behaviours.
All the special cases that you thought of when developing the code can be recorded in the code in the unit tests. The unit tests also become a source of examples on how to use the code.
It is a lot faster for me to discover that my new code breaks something in my unit tests then to check in the code and have some front-end developer find a problem.
For data access testing I try to write tests that either have no change or clean up after themselves.
Unit tests aren’t going to be able to solve all the testing requirements. They will be able to save development time and test core parts of the application.
This is my take on it. I would say unit testing is the practice of writing software tests to verify that your real software does what it is meant to. This started with jUnit in the Java world and has become a best practice in PHP as well with SimpleTest and phpUnit. It's a core practice of Extreme Programming and helps you to be sure that your software still works as intended after editing. If you have sufficient test coverage, you can do major refactoring, bug fixing or add features rapidly with much less fear of introducing other problems.
It's most effective when all unit tests can be run automatically.
Unit testing is generally associated with OO development. The basic idea is to create a script which sets up the environment for your code and then exercises it; you write assertions, specify the intended output that you should receive and then execute your test script using a framework such as those mentioned above.
The framework will run all the tests against your code and then report back success or failure of each test. phpUnit is run from the Linux command line by default, though there are HTTP interfaces available for it. SimpleTest is web-based by nature and is much easier to get up and running, IMO. In combination with xDebug, phpUnit can give you automated statistics for code coverage which some people find very useful.
Some teams write hooks from their subversion repository so that unit tests are run automatically whenever you commit changes.
It's good practice to keep your unit tests in the same repository as your application.
LibrarIES like NUnit, xUnit or JUnit are just mandatory if you want to develop your projects using the TDD approach popularized by Kent Beck:
You can read Introduction to Test Driven Development (TDD) or Kent Beck's book Test Driven Development: By Example.
Then, if you want to be sure your tests cover a "good" part of your code, you can use software like NCover, JCover, PartCover or whatever. They'll tell you the coverage percentage of your code. Depending on how much you're adept at TDD, you'll know if you've practiced it well enough :)
Unit-testing is the testing of a unit of code (e.g. a single function) without the need for the infrastructure that that unit of code relies on. i.e. test it in isolation.
If, for example, the function that you're testing connects to a database and does an update, in a unit test you might not want to do that update. You would if it were an integration test but in this case it's not.
So a unit test would exercise the functionality enclosed in the "function" you're testing without side effects of the database update.
Say your function retrieved some numbers from a database and then performed a standard deviation calculation. What are you trying to test here? That the standard deviation is calculated correctly or that the data is returned from the database?
In a unit test you just want to test that the standard deviation is calculated correctly. In an integration test you want to test the standard deviation calculation and the database retrieval.
Unit testing is about writing code that tests your application code.
The Unit part of the name is about the intention to test small units of code (one method for example) at a time.
xUnit is there to help with this testing - they are frameworks that assist with this. Part of that is automated test runners that tell you what test fail and which ones pass.
They also have facilities to setup common code that you need in each test before hand and tear it down when all tests have finished.
You can have a test to check that an expected exception has been thrown, without having to write the whole try catch block yourself.
I think the point that you don't understand is that unit testing frameworks like NUnit (and the like) will help you in automating small to medium-sized tests. Usually you can run the tests in a GUI (that's the case with NUnit, for instance) by simply clicking a button and then - hopefully - see the progress bar stay green. If it turns red, the framework shows you which test failed and what exactly went wrong. In a normal unit test, you often use assertions, e.g. Assert.AreEqual(expectedValue, actualValue, "some description") - so if the two values are unequal you will see an error saying "some description: expected <expectedValue> but was <actualValue>".
So as a conclusion unit testing will make testing faster and a lot more comfortable for developers. You can run all the unit tests before committing new code so that you don't break the build process of other developers on the same project.
Use Testivus. All you need to know is right there :)
Unit testing is a practice to make sure that the function or module which you are going to implement is going to behave as expected (requirements) and also to make sure how it behaves in scenarios like boundary conditions, and invalid input.
xUnit, NUnit, mbUnit, etc. are tools which help you in writing the tests.
Test Driven Development has sort of taken over the term Unit Test. As an old timer I will mention the more generic definition of it.
Unit Test also means testing a single component in a larger system. This single component could be a dll, exe, class library, etc. It could even be a single system in a multi-system application. So ultimately Unit Test ends up being the testing of whatever you want to call a single piece of a larger system.
You would then move up to integrated or system testing by testing how all the components work together.
First of all, whether speaking about Unit testing or any other kinds of automated testing (Integration, Load, UI testing etc.), the key difference from what you suggest is that it is automated, repeatable and it doesn't require any human resources to be consumed (= nobody has to perform the tests, they usually run at a press of a button).
I went to a presentation on unit testing at FoxForward 2007 and was told never to unit test anything that works with data. After all, if you test on live data, the results are unpredictable, and if you don't test on live data, you're not actually testing the code you wrote. Unfortunately, that's most of the coding I do these days. :-)
I did take a shot at TDD recently when I was writing a routine to save and restore settings. First, I verified that I could create the storage object. Then, that it had the method I needed to call. Then, that I could call it. Then, that I could pass it parameters. Then, that I could pass it specific parameters. And so on, until I was finally verifying that it would save the specified setting, allow me to change it, and then restore it, for several different syntaxes.
I didn't get to the end, because I needed-the-routine-now-dammit, but it was a good exercise.
What do you do if you are given a pile of crap and seem like you are stuck in a perpetual state of cleanup that you know with the addition of any new feature or code can break the current set because the current software is like a house of cards?
How can we do unit testing then?
You start small. The project I just got into had no unit testing until a few months ago. When coverage was that low, we would simply pick a file that had no coverage and click "add tests".
Right now we're up to over 40%, and we've managed to pick off most of the low-hanging fruit.
(The best part is that even at this low level of coverage, we've already run into many instances of the code doing the wrong thing, and the testing caught it. That's a huge motivator to push people to add more testing.)
This answers why you should be doing unit testing.
The 3 videos below cover unit testing in javascript but the general principles apply across most languages.
Unit Testing: Minutes Now Will Save Hours Later - Eric Mann - https://www.youtube.com/watch?v=_UmmaPe8Bzc
JS Unit Testing (very good) - https://www.youtube.com/watch?v=-IYqgx8JxlU
Writing Testable JavaScript - https://www.youtube.com/watch?v=OzjogCFO4Zo
Now I'm just learning about the subject so I may not be 100% correct and there's more to it than what I'm describing here but my basic understanding of unit testing is that you write some test code (which is kept separate from your main code) that calls a function in your main code with input (arguments) that the function requires and the code then checks if it gets back a valid return value. If it does get back a valid value the unit testing framework that you're using to run the tests shows a green light (all good) if the value is invalid you get a red light and you then can fix the problem straight away before you release the new code to production, without testing you may actually not have caught the error.
So you write tests for you current code and create the code so that it passes the test. Months later you or someone else need to modify the function in your main code, because earlier you had already written test code for that function you now run again and the test may fail because the coder introduced a logic error in the function or return something completely different than what that function is supposed to return. Again without the test in place that error might be hard to track down as it can possibly affect other code as well and will go unnoticed.
Also the fact that you have a computer program that runs through your code and tests it instead of you manually doing it in the browser page by page saves time (unit testing for javascript). Let's say that you modify a function that is used by some script on a web page and it works all well and good for its new intended purpose. But, let's also say for arguments sake that there is another function you have somewhere else in your code that depends on that newly modified function for it to operate properly. This dependent function may now stop working because of the changes that you've made to the first function, however without tests in place that are run automatically by your computer you will not notice that there's a problem with that function until it is actually executed and you'll have to manually navigate to a web page that includes the script which executes the dependent function, only then you notice that there's a bug because of the change that you made to the first function.
To reiterate, having tests that are run while developing your application will catch these kinds of problems as you're coding. Not having the tests in place you'd have to manually go through your whole application and even then it can be hard to spot the bug, naively you send it out into production and after a while a kind user sends you a bug report (which won't be as good as your error messages in a testing framework).
It's quite confusing when you first hear of the subject and you think to yourself, am I not already testing my code? And the code that you've written is working like it is supposed to already, "why do I need another framework?"... Yes you are already testing your code but a computer is better at doing it. You just have to write good enough tests for a function/unit of code once and the rest is taken care of for you by the mighty cpu instead of you having to manually check that all of your code is still working when you make a change to your code.
Also, you don't have to unit test your code if you don't want to but it pays off as your project/code base starts to grow larger as the chances of introducing bugs increases.
Unit-testing and TDD in general enables you to have shorter feedback cycles about the software you are writing. Instead of having a large test phase at the very end of the implementation, you incrementally test everything you write. This increases code quality very much, as you immediately see, where you might have bugs.