Unit tests verify that individual and isolated parts of an app work as expected. These tests generally execute very fast, thus giving us a very tight feedback loop so that we know the part of the app that we are developing is working correctly.

These tests can be quick to implement but this is not necessarily the case if we need to mock out the dependencies of the unit we are testing. This is often the case when unit-testing a React frontend because a true unit test on a component needs to mock out any child components that are referenced in its JSX.

Perhaps the biggest downside of these tests is that they give us the least amount of confidence that the app as a whole is working correctly. We can have a large unit test suite that covers all the different parts of our app, but this is no guarantee that all the parts work together as expected.