While writing the unit tests for an app, developers try following a principle named F.I.R.S.T as much as possible because that provides accuracy in the development. The principle F.I.R.S.T is a combination of the many tenets. Below are the principles explained in detail.
The F letter in the principle of Unit Testing stands for Fast. Unit tests are smaller pieces of code that are the ones who would perform one particular task. Unit tests are short, and unlike integration tests, unit tests do not communicate over the network, nor do they perform database operations. The reason behind unit tests running faster is that they do not communicate with remote servers or databases. Developers who are practicing the test-driven development process, run unit tests very often while implementing the app features.
The I letter in the principle of Unit Testing stands for Independent. Unit tests must be independent of others. One unit test mustn't depend on the result produced by the other unit test. Mostly, it is seen that the unit tests are run randomly in order. The code that is being tested or the system under the test should also be isolated from the dependencies. This is because the dependencies carry some bugs, and to make sure it does not influence a unit test, the dependencies are mostly stubbed with predefined data. This way, a test in isolation from its dependencies produces accurate results.
The R letter in the principle of Unit Testing stands for Repeatable. A unit test should be one, and if it is run multiple numbers of times or run on a different computer should provide the same result. This is the reason why unit tests are made independent of the environment as well as from other unit tests. The input parameters are usually required predefined as well as hardcoded. If the function under the test needs to be tested with valid as well as invalid input parameters, then two or more different unit tests are produced. Each unit test and its functions are tested with its predefined parameters. This way, the unit tests become repeatable, and if tested in multiple environments, will produce the same results each time.
The S letter, in principle, stands for Self-validating. It means that if the unit test is passed, the developer should not do any additional manual checks once the test is complete. The unit test validates the result that the function under the test produced and will itself make a decision that it has passed or failed. Thus, hire application developers to make sure that the tests are not additionally checked once it's complete.
The T letter, in principle, stands for Thorough or Timely. This means that while testing the function, the developer should consider a happy path as well as the adverse scenario. And thus, multiple unit tests are created. One unit test will test a function under a valid input parameter, and another unit test will test the function under invalid input parameters. Unit tests should also be timely because it is better to create unit tests when the developer is working on the app feature. This way, confidence will be built that the feature works as expected, and there is no bug during the production. Therefore, before promoting the code, it should be covered with unit tests.