(1) [var => let/const] part I:

Use eslint rule to check the diff between var and let:
    "no-use-before-define": "off"
    "@typescript-eslint/no-use-before-define": 2
    no-redeclare: 2
    block-scoped-var: 2
    no-loop-func: 2
(2) By the way add some eslint rule and unify some code style.