A simple rule that works well in modern JavaScript is to start with const and only switch to let if you know the variable needs to be reassigned later.

The biggest difference is scope. let and const are block-scoped, so they're only available within the block where they're declared. var is function-scoped, which can make variables accessible in places you didn't intend and lead to harder-to-find bugs.

Another important point is that const doesn't make an object or array immutable. It only prevents the variable from pointing to a different value. You can still modify the contents unless you intentionally make the object immutable.

For new projects, let and const are generally the preferred choice.