Temporal Dead Zone

tdz-1.js

{
  // The block has begun, we're in a new block scope. The TDZ for the "a" binding has begun
 
  var f = function() {
    // 2. Because f() is evaluated before `a` is actually declared, 
    // an exception will be thrown indicating to the author that
    // `a` is not yet defined.
    console.log(a); 
  };
 
  f(); // 1. f is called and results evaluated before the next line... 
  let a = 1; 
  // The `a` binding is now safe, but we never got here because f() killed us :(
  // that's ok, because we can fix this bug and celebrate the win!
}

tdz-2.js

{
  // The block has begun, we're in a new block scope. The TDZ for the "a" binding has begun
  // Declare `a` before use.
  let a = 1; 
  var f = function() {
    // 2. Because f() is evaluated after `a` is declared, 
    // the value of `a` will be logged.
    console.log(a); 
  };
 
  f(); // 1. f is called and results evaluated
}