// Variable declarations are hoisted to top of scope. 
// Variable assignments are not. 

// Outside of scope, hence ReferenceError
console.log(foo); // ReferenceError: foo is not defined

(function () {
  // Variable declaration got hoisted to top of scope, but the variable assignment did not, hence undefined
  console.log(foo); // undefined
  
  var foo = 'bar';
  
  // After variable assignment
  console.log(foo); // 'bar'
})();