Fullstack Javascript
Course Notes
04 - NPM Packages; testing with Jest
2. Unit Testing

2. Unit Testing

What is Unit Testing?

  • A unit test is a test that validates a unit of code produced the expected output when provided with some known input
  • A unit is usually the smallest piece of code you can isolate, frequently (but not always) a function is used to represent a single unit
  • An example of a unit test could be:
function addNumbers(first, second) {
    return first + second;
}

test(`Verify numbers are added correctly`, () => {
    expect(addNumbers(1, 1)).toReturnValue(2);
});
  • That example is really simple, but the idea is clear:
    • The function addNumbers should add the two numbers passed as arguments together
    • Our test attempts to add two numbers and verify the output is what is expected

Why do we bother with unit tests?

  • Mainly because writing perfect code is hard (average is 1-25 bugs per 1000 lines of code delivered to customers)
  • Unit tests help us to:
    • Catch bugs early as we can test the outputs of our code before we release
    • Prevent regression bugs, aka prevents a bug you fixed last week from coming back after you changed something this week. If you have a test to verify that you fixed the bug, if you reintroduce the bug the test will fail
    • Improves code quality: Code has to be written in a certain way to be testable, and testable code is usually higher quality code
      • We won’t be focusing on that much
  • Unit tests are really just a way to make sure that when you release code to the public at least most of the bugs have already been squashed because your unit tests caught them

How does a unit test work?

  • Unit tests, structurally, are pretty simple:
    • They provide a piece of code with some inputs, and verify the ouputs match what is expected
  • Frequently, when writing unit tests you’ll often want to try and cover the following scenarios:
    • You should be testing against the typical use cases for your code
    • Test that any edge cases are covered
    • Test that any invalid input or errors are handled in an expected way
      • When it comes to testing errors, a succesful test doesn’t mean that your code handles every error
      • Sometimes just throwing the error is acceptable
      • However, the test should validate that the expected behvaiour (which can be throwing the error) happens

Characteristics of a good unit test

  • Isolated
    • A unit test should only test one piece of functionality at a time
    • It should not depend on other, outside systems in order to work
  • Deterministic
    • Rerunning the same unit test multiple times should always produce the same results
  • Fast
    • Unit tests should be small, and only test one thing at a time
    • As such, they should be able to run very quickly