Best practices for Variable declaration

What are good practices for declaring variables?

   let a, b, c;
     or 
   let a;
   let b;
   let c;

Also, how should a variable that’ll be initialized later be declared?

    let a = null;
       or 
    let a;

If a variable will be declared later, just initialise it with let var;; you don’t need to assign it to anything.