First, let's learn about the different types of tests and how they all fit into our project's workflow. The first thing to note is that some tests are more technically-focused, while others are more business-focused; some tests are only concerned with a very small part of the whole system, while others test the system as a whole. Here's a brief overview of the most common types of tests you'll encounter:

• Unit tests: These test the smallest testable parts of an application, called units. For example, if we have a function called createUser, we can write a unit test that tests that the function always returns a promise.
  With unit tests, we are only concerned with the function of the unit, independent of external dependencies. If the unit has external dependencies, such as a database, we must substitute...