refactor: enable code linting

[nfischer]

This refactors to a '.js' source file, which enables code linting. This also fixes the lint errors to use an updated style.

This also enables test coverage for index.js (which now holds all the business logic).

[codecov]

Codecov Report

Attention: Patch coverage is 77.27273% with 5 lines in your changes missing coverage. Please review.

Project coverage is 77.27%. Comparing base (b1aba2f) to head (219a95d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
index.js	77.27%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           main      #24       +/-   ##
=========================================
+ Coverage      0   77.27%   +77.27%     
=========================================
  Files         0        1        +1     
  Lines         0       22       +22     
=========================================
+ Hits          0       17       +17     
- Misses        0        5        +5     

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