Bug fixes and features should always come with tests, when applicable. Test files should live next to the file they are testing. Before submitting your changes in a pull request, always run the full test suite.
To run the test suite:
# Runs the full test suite
# Runs the full w/ the watch flag and coverage reports
Helpful Testing Process
One technique that can helpful when writing tests, is to reference the coverage report for the file/function/feature you're testing. To do this, run yarn test:dev from your terminal. This will generate a coverage report for the project. The coverage directory will be located in the root directory. There is an index.html which shows the coverage report per file.