Linter

[leomp12]

Is your feature request related to a problem? Please describe.
Linter is not being respected at all, we must fix current problems and maybe customize eslint config if needed.
Lint process should also be automatized with GitHub Action.

Describe the solution you'd like
Just ensure lint is respected (ESLint 😄 ).

Describe alternatives you've considered
N/A

Additional context
Using ESLint 6.x (should not be updated to v7 due to Node.js 8 support).