Increase test coverage to 80% ⚡️

[JamieSlome]

The codebase currently has low test coverage. To ensure the stability of the library and speed up pull request reviews, the test coverage should be increased to a common standard of 80%.

Code coverage reports are currently being developed in #203.

### Tasks
- [ ] Create test files in `test` folder
- [ ] Run `nyc` to understand the current test score and code areas requiring more test coverage
- [ ] Ensure all files reported by `nyc` have at least 80% coverage on all code aspects (functions, lines, etc.)