var a = 1;
if (true) {
  var b = 2;
}
console.log(a); // 1
console.log(b); // 2

// in ES6
let a = 1;
if (true) {
  let b = 2;
}
console.log(a); // 1
console.log(b); // ReferenceError: b is not defined

var a = [];
for (var i=0; i< 5; i++) {
  a.push(function() {
    console.log(i);
  });
}
a.forEach(function(value) {
  value();
});

var b = []; 
for ( let i=0; i< 5; i++) {
  b.push(function() {
    console.log(i);
  });
}
b.forEach(function(value) {
  value();
});

// in ES5

var MY_CONSTANT = true;

MY_CONSTANT = false;

// in ES6
const MY_CONSTANT = true;
MY_CONSTANT = false; // Uncaught TypeError: Assignment to constant variable

var collection = [];
collection.push(1, 2, 1);
console.log(collection); // [ 1, 2, 1]

function addToCollection(collection, value) {

  if (collection.indexOf(value) < 0) {

    collection.push(value)

  }

}

let collection = new Set();
collection.add(1);
collection.add(2);
collection.add(1);
console.log(collection); // Set {1, 2}

var collection = {};
collection.a = ‘abc';
collection.b = ‘xyz';

let collection = new Map();
collection.set(‘a', ‘abc');
collection.set(‘b', ‘xyz');

collection.forEach(function(value, key) {
  console.log(key + ":" + value);
});
console.log(collection.size);

function doSomething(someObject) {
  console.log(someObject);
}

function doSomething(someObject) {
  if (someObject === undefined) {
    someObject = {};
  }
  console.log(someObject);

}

function doSomething(someObject = {}) {

  console.log(someObject);

}

function doSomething(someObject) {
  var one = someObject.propOne;
  console.log(one);
  var two = someObject.propTwo;
  console.log(two);
  var three = someObject.propThree;
  console.log(three);

}

function doSomething({ propOne, propTwo, propThree }) {
  console.log(propOne);
  console.log(propTwo);
  console.log(propThree);

}