temporal-dead-zone.js

// Example
{
  let f = function() {
    console.log( foo );
  };

  f();

  let foo = "foo!";
}


// Hoisted, with temporal dead zone highlight by HTML comments
{
  let f, foo;

<!--
  f = function() {
    console.log( foo ); // will throw
  };

  f();

  foo = "foo!";
-->
}

See https://gist.github.com/rwaldron/f0807a758aa03bcdd58a for an even better explanation.