more "mental tax": interpreting code that uses `let` in many nested blocks

1.js

/*
not terribly difficult to predict which statements print what.
*/

function foo() {
   var a, b, c, d;

   if (true) {
      if (true) {
         a = 1;
         if (true) {
            b = 2;
            c = 3;
            if (true) {
               d = 4;
               console.log("d: " + d);
            }
            console.log("c: " + c);
         }
         console.log("b: " + b);
      }
      console.log("a: " + a);
   }
}

// will print:
// d: 4
// c: 3
// b: 2
// a: 1

2.js

/*
a little harder to trace visually to know what will happen. admit it: it takes some extra 
visual and mental processing to track which scope each variable is in, and whether the 
`console.log()` in each block will succeed or fail.
*/

function foo() {
   if (true) {
      if (true) {
         let a = 1;
         if (true) {
            let b = 2, c = 3;
            if (true) {
               let d = 4;
               console.log("d: " + d);
            }
            console.log("c: " + c);
         }
         console.log("b: " + b);
      }
      console.log("a: " + a);
   }
}

// will print:
// d: 4
// c: 3
// b: undefined
// a: undefined