JavaScript Evolution ECMAScript 6 - Using let and const

Prior executing code, JavaScript moves var declarations all the way up to the top of the scope. This is known as a hoist. Example:

function loadProfile(usernames) {
  if(usernames.lenght > 3) {
    var loadingMessage = "This will take a while";
  }
}

JavaScript runtime transform to

function loadProfile(usernames) {
  var loadingMessage;
  if(usernames.lenght > 3) {
    loadingMessage = "This will take a while";
  }
}

This explained developer can access the variable outside the block. Example:

function loadProfile(usernames){
  if(usernames.length > 3) {
    var loadingMessage = "This will take a while";
  }
  consolo.log (loadingMessage );
  // Output is unexpected - can be "This will take a while" or undefined.
}

var variable can be accessed in entitled function which might cause unexpected behaviour. One way you can avoid this by introducing a new variable declaration let.

let variables are scoped to the nested block and are Not hoisted. Block is any code section within curly braces like if, else, while, etc. Example:

function loadProfile(usernames) {
  if (usernames.length > 3) {
    let loadingMessage ="This might take a while";
  }
  consolo.log(loadingMessage);
  // Output is ReferencesError: loadingMessage is not defined.
}

Variables declared with let can be reassigned, but cannot be redeclared within the same scope.

let loadingMessage = "Hello";
loadingMessage = "Goodbye"; // Reassigning is allowed.

let loadingMessage = "Hello";
let loadingMessage = "Goodbye";
// Re declaring is not allowed.
// TypeError: Identified 'loadingMessage' has already been declared.

Second type of new variable is const. const keyword created read-only named constant. Example

const MAX_USERS = 3;

Constant cannot be reassigned. Example:

const MAX_USERS = 3;
MAX_USERS = 10; // Error

Constant must be assigned an initial value. Example:

const MAX_USERS = 3; // No error.

const MAX_USERS;
MAX_USERS = 3; // Error

Constant are scoped to the nearest block, same to let.