chore: replace legacy `var` with `const`

[erickzhao]

Now that engines requires Node.js 16

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.06%. Comparing base (4cd9da5) to head (34a3a25). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1742 +/- ## ======================================= Coverage 89.06% 89.06% ======================================= Files 17 17 Lines 905 905 Branches 187 187 ======================================= Hits 806 806 Misses 62 62 Partials 37 37 ```

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