projects are going to need to be consistent in let/const usage, some options

1. only use var (no let/const)
2. use var unless a variable needs to be block scoped, then use let.
3. use let for everything
4. use const where possible, otherwise use let
5. use const for values (strings, numbers, booleans, null, undefined) that are constant and other immutable things (frozen objects), otherwise use let.

recommendations should probably be something along the lines of pick a strategy and treat it like gospel. The difference between 4 and 5 is that for objects it's the reference to the object that is constant, not the values in the object. Since most object references are more often then not never redefined that means that in version 4 const ends up being used almost exclusively.

If we only wanted one let/const option then 5 would be the best.

The other thing about let and const is that they always have the exact scope they appear to have, meaning that declaring variables at the top of scope has less of an benefit but continues to have the downside of increasing the change a forgetting to declare a variable, e.g.

function () {
  let a,b,c;
  //stuff
  while(true){
    let d;
    //lots of stuff, not with d.
  }
  // lots of stuff
  d = 9;
  //whoops
}

meaning an argument could be made that for let and const it is a better practice to declare them when they are first assigned a value or as close to that as possible, e.g.

function () {
   // stuff
  let a; //non local variable.
  while(true){
    //stuff
    let b = something;
    a = somethingElse
  }
}