Javascript ES6 :When to use "let" and "const"

I have been learning javascript for less than a year now. I have a question in regards to ES6 syntax namely “let” and “const”. I know that they are supposed to replace “var”, but they are both block variable meaning that they cant be used outside the a block of code or function. But if you are developing a large program, when should you use"let" and “const”, and when should you revert to “var”, because it just seems easy to revert to using “var” whenever you are re-using a variable.

The large application that I work on doesn’t use var at all. We always use const if the value is never reassigned and let if it is.

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There are vanishingly few valid uses for var: if you need the ability to reassign you get with let, you almost always want the better scoping rules + inability to redeclare variables in the same scope that you get with that over var