const a = [1, 2, 3];
const b = a.flatMap(x => [x, x * 2]);
console.log(b);

function showObjectKeys(obj){
  for (var key in obj){
    console.log(key);
  }
}
function showObjectValues(obj){
  for (var key in obj){
    console.log(obj[key]);
  }
}
var ob={item1:"This",item2:"is",item3:"an",item4:"example"};
console.log("keys of ob:")
showObjectKeys(ob);
console.log("values of ob:")
showObjectValues(ob);

function showObjectKeys(obj){
  for (var key in obj){
    console.log(key);
  }
}
function showObjectValues(obj){
  for (var key in obj){
    console.log(obj[key]);
  }
}
var arr=["one","two","three"];
console.log("keys of arr:")
showObjectKeys(arr);
console.log("values of arr:")
showObjectValues(arr);

var str="this is an example.";
var result=str[0].toUpperCase()+str.slice(1);
console.log(result);

var arr1=["1","2","3","4","5"];
console.log(arr1.toString());      
console.log(JSON.stringify(arr1));  

var arr2=[[1,2],[3,4],[5]];
console.log(arr2.toString());       
console.log(JSON.stringify(arr2));  

const variable = () => {
   console.log(arguments);
}

variable([1]);

var str="Hello beautiful World!";
console.log(str.slice(6,16))
console.log(str.substring(6,16))
console.log(str.substr(6,9))

var a=[1,2,3,4,5];
a.every(x=>{
  console.log(x); 
  return x<4;
})

var a=0,b=0,arr=[3,6,9,6,9,3];
arr.some((x,i)=>{
  a=arr[i];  //or a=x
  b=arr[i+1];
  return a+b==15
})
console.log(a,b)

var arr=[1,2,3,11,21,31];
arr.sort();
console.log(arr)

var pushElement=(arr,el1,el2)=>{
  arr.push(el1);
  arr.push(el2);
  return arr;
}
console.log( pushElement([1],2,3) );

var arr1=["1","2","3","4","5"];
console.log(arr1.toString());       
console.log(JSON.stringify(arr1));  

function sum1_100(){
  var sum=0,num=1;
  do{
    sum+=num;
    num++;
  }while (num<=100)
  return sum;
}
console.log(sum1_100()) 

var arr1=[1,2,3],arr2=[4,5,6];
console.log(arr1.concat(arr2));
console.log(arr1.concat(4,5,6));
console.log(arr1.concat([4],[5],[6]));
console.log(arr1.concat(...arr2));
console.log(arr1.concat(4).concat(5).concat(6)); 

var str="My name is John";
var words1=str.split(" ",3);
console.log("words1:",words1);
var words2=str.split(" ",5);
console.log("words2:",words2);

var arr=[1,2,3,11,21,31];
arr.sort((a,b)=>a-b);
console.log(arr)
arr.sort((a,b)=>b-a);
console.log(arr)

var num=111;
var a=num.toString(2), 
    b=num.toString(8), 
    c=num.toString(16), 
console.log([a,b,c])

var arr=[[1,2],[3,4],[5,6]];
var result=[].concat(...arr);
console.log(result); 

function repeat(s,n){
  return [...Array(n+1)].join(s);
}
console.log(repeat("a",5));
console.log(repeat("abc",5));

var str="Hello World!";
console.log(str.indexOf("e"))
console.log(str.lastIndexOf("e"))
console.log(str.search("e"))

var str="Hello World!";
console.log(str.indexOf("o"))
console.log(str.lastIndexOf("o"))
console.log(str.search("o"))