refactor : improve frontend code base

[Monirzadeh]

current frontend code base has multiple problem that i try to solve some of them in this PR

TODO:

• [x] unify variable name creation and replace all var with const or let to control variable scope
• [x] minor improvement
• [ ] change unneeded if to switch for better performance
• [ ] review all the changes

may update over time should wait until #924 merge some errors solve there

@fmartingr any specific suggestion?

[codecov[bot]]

Codecov Report

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

Project coverage is 30.76%. Comparing base (595cb45) to head (df95e81). Report is 34 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #925 +/- ## ========================================== + Coverage 25.58% 30.76% +5.18% ========================================== Files 47 60 +13 Lines 5628 4456 -1172 ========================================== - Hits 1440 1371 -69 + Misses 3989 2863 -1126 - Partials 199 222 +23 ```

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