t003.js

{ // start of the block scope

  // TODO: try uncomment
  // console.log(a);
  // hoisting works differently for blocks scope, and it will throw an error:
  // 'ReferenceError: Cannot access a before initialization'

  let a = 1; // 'a' defined and we can start to use it
             //  
  console.log(a);

} // end of the block scope, here 'a' becomes not defined

// TODO: try uncomment
// TODO: try change let to var in code above 
// console.log(a);
// 'a' is not defined here so we will see an error:
// 'ReferenceError: a is not defined