The rule

const by default. let when you must reassign. var never in new code. That is the whole guidance, and it is near-universal in modern codebases.

Naming rules

A name may start with a letter, an underscore or a dollar sign, and may contain digits after that. It may not be a reserved word — class and function are out. Names are case-sensitive, so myVar, MyVar and myvar are three different variables, which is a good reason not to rely on capitalisation alone to tell things apart. The convention is camelCase for variables and functions, PascalCase for classes, and UPPER_SNAKE for true constants.

const does not mean immutable

This trips up everyone. const stops the binding being reassigned; it says nothing about the value. A const array can still be pushed to, and a const object can still gain properties. What you cannot do is point the name at something else.

Block scope versus function scope

let and const are scoped to the nearest block — anything between { and }. var is scoped to the whole enclosing function, so a var declared inside an if leaks out of it.

The temporal dead zone

var declarations are hoisted and initialised to undefined, so reading one before its line gives undefined. let and const are hoisted too but left uninitialised, so reading one early throws ReferenceError: Cannot access 'x' before initialization. That window is the temporal dead zone, and throwing is the better behaviour — it turns a silent undefined into a loud error.