if(true){
  let a=1;
}
console.log(a);//undifined

for(let i=0;i<lists.length;i++){
  //do somthing with lists[i]
}
console.log(i)l//undifined

const MY_CONSTENT=1;
MY_CONSTENT=2;//error
const MY_CONSTENT;//error

const MY_OBJECT={some:1};
MY_OBJECT.some='body';

//箭头函数
let books=[{"title":"X","price":20},{"title":"Y","price":40}];
let titles=books.map(item=>item.title);
//ES5函数
var titles=books.map(function(item){
  return item.title;
});

//no arguments
books.map(()=>1);//[1,1]
//Mutiple arguments
[1,2].map((n,index)=>n*index);//[0,2]

let result=[1,2,3,4,5].map(n=>{
  n=n%3;
  retutn n;
})

//ES6
let books={
  title'X',
  sellers:['A','B'],
  printSellers(){
    this.sellers.forEach(seller=>console.log(seller+'seller'+this.title))
  }
};
//ES5
var books={
  title='X',
  sellers:['A','B'],
  printSellers:function(){
   var that=this;
   this.sellers.forEach(function(seller){
     console.log(seller+'seller'+that.title)
   })
  }
};

'my string'.startsWith('my');//true
'my string'.endsWith('my');//false
'my string'.includes('str');//true

'my'.repeat(3);//'my my my'

//ES6
let name='json',
  apples=5,
  pears=7,
  bananas=function(){return 3};
console.log('this is${name}');
console.log('he carries ${apples} apples,${pears} pears,and ${bananas()} bananas');
//ES5
console.log('he carries'+ ${apples}+' apples,'+${pears}+' pears,and '+${bananas()}+' bananas');

let x='1...
  2...
  3 lines long!';
//ES5
var x='1...'+
  '2...'+
  '3 lines long!';
var x="1...2...3 lines long!';