Fix/support old browsers

[alextremp]

Description

Support old browsers (ie11, safari 9), due to detected bug in built JS with let usage:

Expected behavior

Memetized description

[codecov-commenter]

Codecov Report

Merging #9 (44a5466) into master (a87269c) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master        #9   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          119       119           
  Branches        17        17           
=========================================
  Hits           119       119           

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