let, const, and var

Rule 1: use let for variables, and const for constants whenever possible. Use var only when you have to maintain legacy code.

The following rule is also worth keeping.

Rule 2: Always declare and initialize all your variables at the beginning of your scope.

Using rule 2 implies that you never have to face with the temporal dead zone.

If you have a linter such as ESLint, set it up accordingly, so that it warns you when you violate the second rule.

If you stick to these two rules, you will get rid of most of the anomalies developers face.