refactor: small syntax changes

[erisu]

Motivation, Context & Description

• Modernizing Syntax
  • var to let & const
  • arrow functions

Testing

• npm t

Checklist

• I've run the tests to see all new and existing tests pass

[codecov-io]

Codecov Report

Merging #64 into master will decrease coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
- Coverage   89.70%   89.55%   -0.16%     
==========================================
  Files           1        1              
  Lines          68       67       -1     
==========================================
- Hits           61       60       -1     
  Misses          7        7              

Impacted Files	Coverage Δ
index.js	89.55% <100.00%> (-0.16%)	⬇️

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.55%. Comparing base (a399529) to head (3bc4b2b).
⚠️ Report is 37 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
- Coverage   89.70%   89.55%   -0.16%     
==========================================
  Files           1        1              
  Lines          68       67       -1     
==========================================
- Hits           61       60       -1     
  Misses          7        7              

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