The Basics

let and const

These two keywords provide Block Scope variables (and constants) in JavaScript. Before ES6, JavaScript had only two types of scope: Global Scope and Function Scope.

Variables declared with the let keyword can have Block Scope. However variables declared inside a block can not be accessed from outside the block.

{
  var x = 2;
}
// x CAN be used here
{
  let y = 2;
}
// y can NOT be used here

Declaring a variable with const is similar to let when it comes to Block Scope. JavaScript const variables must be assigned a value when they are declared. However, The keyword const is a little misleading. You can't change constant primitive values, but you can change the properties of a constant objects.

// You can create a const object:
const car = {type:"Fiat", model:"500", color:"white"};
// You can change a property:
car.color = "red";
// You can add a property:
car.owner = "Johnson";
// You CAN'T reassign a constant object:
car = {type:"Volvo", model:"EX60", color:"red"};    // ERROR

Arrow functions

Arrow functions are a different way of creating functions in JavaScript. Besides a shorter syntax, they offer advantages when it comes to keeping the scope of the this keyword.

function callMe(name) {
  console.log(name);
}
// Or
const callMe = function(name) {
  console.log(name);
}
// Becomes
const callMe = (name) => {
  console.log(name);
}