Unlike var, const/let are bound to lexical scope rather than function scope.

{
  var x = 1 // will escape the scope
  let y = 2 // bound to lexical scope
  const z = 3 // bound to lexical scope, constant
}

console.log(x) // 1
console.log(y) // ReferenceError: y is not defined
console.log(z) // ReferenceError: z is not defined

Run in RunKit

Arrow functions automatically bind to the 'this' lexical scope of the surrounding code.

performSomething(result => {
  this.someVariable = result
})

vs

performSomething(function(result) {
  this.someVariable = result
}.bind(this))

Let's consider this example, that outputs the squares of the numbers 3, 5, and 7:

let nums = [3, 5, 7]
let squares = nums.map(function (n) {
  return n * n
})
console.log(squares)

Run in RunKit

The function passed to .map can also be written as arrow function by removing the function keyword and instead adding the arrow =>:

let nums = [3, 5, 7]
let squares = nums.map((n) => {
  return n * n
})
console.log(squares)

Run in RunKit

However, this can be written even more concise. If the function body consists of only one statement and that statement computes the return value, the curly braces of wrapping the function body can be removed, as well as the return keyword.

let nums = [3, 5, 7]
let squares = nums.map(n => n * n)
console.log(squares)

Run in RunKit

    let [x,y, ...nums] = [0, 1, 2, 3, 4, 5, 6];
console.log(x, y, nums);

let {a, b, ...props} = {a:1, b:2, c:3, d:{e:4}}
console.log(a, b, props);

let dog = {name: 'fido', age: 3};
let {name:n, age} = dog;
console.log(n, age);

/* @flow */

function product(a: number, b: number){
  return a * b;
}

const b = 3;
let c = [1,2,3,,{}];
let d = 3;

import request from 'request';

request('http://dev.markitondemand.com/MODApis/Api/v2/Quote/json?symbol=AAPL', (err, res, payload)=>{
  payload = JSON.parse(payload);
  let {LastPrice} = payload;
  console.log(LastPrice);
});

class Mammel {
  constructor(legs){
    this.legs = legs;
  }
  eat(){
    console.log('eating...');
  }
  static count(){
    console.log('static count...');
  }
}

class Dog extends Mammel{
  constructor(name, legs){
    super(legs);
    this.name = name;
  }
  sleep(){
    super.eat();
    console.log('sleeping');
  }
}

let d = new Dog('fido', 4);
d.sleep();
d.eat();
console.log('d', d);