ES6 - var, let, const. The temporal dead zone.

js-gist-00.js

/**
 * ES6 - var, let, const. The temporal dead zone.
 *
 * According to ECMAScript 6 specification. https://tc39.github.io/ecma262/#sec-let-and-const-declarations
 * let and const declarations define variables that are scoped to the running execution context's
 * LexicalEnvironment. The variables are created when their containing Lexical Environment is instantiated but may
 * not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a
 * LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the
 * LexicalBinding is evaluated, not when the variable is created. If a LexicalBinding in a let declaration does
 * not have an Initializer the variable is assigned the value undefined when the LexicalBinding is evaluated.
 *
 * var, let, and const are hoisted. But let and const cannot be accessed before the actual declaration is
 * evaluated at runtime.
 **/

console.log(favoriteDrink); // undefined
var favoriteDrink = 'Beer 🍺';
console.log(favoriteFood); // ReferenceError
const favoriteFood = 'Taco 🌮';
console.log(favoriteDessert); // ReferenceError
let favoriteDessert = 'Cookie 🍪';