Little by little, a little becomes a lot.

A Tanzanian Proverb

What is Unit Testing?

Unit testing is testing the smallest testable unit of an application.

It is done during the coding phase by the developers.

To perform unit testing, a developer writes a piece of code (unit tests) to verify the code to be tested (unit) is correct.

An example

You have written a function to add two numbers:

int Add(int a, int b) { return a+b; }

The above function takes two numbers as input and returns their sum.

A unit test code would look something like this:

void TestAdd1() { Assert.IsEqual(Add(5, 10), 15) }

The above unit test “asserts” that 5 + 10 is equal to 15. If the Add function returns anything else Assert.IsEqual result in error and the test case will fail.

You will probably add a few more unit test cases like these:

void TestAdd2() { Assert.IsEqual(Add(500, 1000), 1500) }
void TestAdd3() { Assert.IsEqual(Add(0, 1000), 1000) }
void TestAdd4() { Assert.IsEqual(Add(-100, 100), 0) }
void TestAdd5() { Assert.IsEqual(Add(-100, -1100), -1200) }

After you write your test cases, you will run them to verify that everything is working correctly.

Later another developer, in addition to adding some of his code, accidentally modifies the Add function as:

int Add(int a, int b) { return a*b; }

As you can see, instead of adding the two numbers, the code now multiplies them.

When the developer runs the unit tests that you have designed for this function, they will fail. The new developer can trace the failed test cases back to the function and fix the code.

Automation

Automating unit testing is easy. For many teams, unit testing is integrated into their daily build process to promptly report any errors. Some of the most popular frameworks are JUnit for Java and Jest for Javascript-based framework like Node.js.

Benefits of Unit Testing

Unit tests can be a great investment if done correctly.

Unit testing helps in finding bugs early.
Developers can write unit tests as soon as they finish writing code without having to wait for others. This makes it easier for developers to identify and fix bugs as they are usually quite familiar with the code they recently worked on.
Unit testing makes the team in the long run.
Software development is an interactive process. Design and implementation changes are commonplace. If you have unit tests in place, developers can quickly run them and get feedback on the quality of their work.
Unit testing makes debugging easier.
Unit testing simplifies the debugging process. When a test fails, only the newest changes made in the code need to be checked. If you don’t have unit test cases, small changes made over the range of several weeks or months need to be inspected.
Unit testing can be automated.
Unit testing can be easily integrated into the software build process, making it easy to report errors quickly.
Unit testing decreases the total testing cost.
Since unit testing can be automated, it can be performed daily. When bugs are nipped in the bud, integration and system testing also becomes more effective.

Disadvantages of Unit Testing

Unit tests can be a lousy investment if not done correctly.

Unit testing increases the initial development time.
It takes time to write good test cases. Since a developer does this, it adds to the development time. In the long run, good test cases can save time, but many think of unit testing as an expense rather than an investment and do not allocate the required amount of time.
Unit testing can lead to an unnecessary proliferation of test cases leading to increased maintenance time.
We have often seen a hundred test cases when just twenty would have sufficed. This inflation usually happens when someone other than the developer writes the test cases, and the incentive is based on the number of test cases written rather than the quality of the test cases. When the underlying API changes, reworking the test cases takes time but it never added to the code's quality in the first place.
Unit testing can induce false confidence in the quality of the code.
It is vital to ensure that unit tests examined all the code paths. To do this, developers have to install a code coverage software and verify the unit tests' efficacy. But this is rare. Unit testing is more often than not considered a to-do item rather than a necessity. As a result, you may often see a lot of redundant test cases and a lot of necessary boundary condition testing missing. Build