Fix NaN for variables

[mikhail-vl]

Fixes #205

[codecov]

Codecov Report

Merging #206 (4eecf09) into master (0b791b6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #206   +/-   ##
=======================================
  Coverage   94.42%   94.43%           
=======================================
  Files          37       37           
  Lines        1596     1598    +2     
  Branches       74       74           
=======================================
+ Hits         1507     1509    +2     
  Misses         80       80           
  Partials        9        9           

Impacted Files	Coverage Δ
src/data-source/data-source.ts	91.66% <100.00%> (+0.36%)	⬆️

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[mikhail-vl]

Looks good!