function verify(username){
  return /^[a-z][a-z0-9_]{5,15}$/i.test(username);
}
console.log( verify("myjinxin2015"))  
console.log( verify("smile67"))       
console.log( verify("GiacomoSorbi"))  
console.log( verify("jhoffner"))      
console.log( verify("g964") )         
console.log( verify("matt c") )       /
console.log( verify("My_name_is_ZozoFouchtra") )

function addCommas(money){
  var re=/\d{3}/g
  return money.replace(re,x=>x+",")
}
console.log( addCommas("$123456789") )

function addCommas(money){
  var re=/\d{3}(?!$)/g
  return money.replace(re,x=>x+",")
}
console.log( addCommas("$123456789") )

const person = {
  firstName: 'John',
  lastName: 'Doe',
  age: 30,
  getFullName: function() {
    return this.firstName + ' ' + this.lastName;
  }
};

console.log(person.getFullName());

var str="javascript coffeescript vbscript javacode"
var re2=/(?=java)\b\w+script\b/g
console.log(str.match(re2))

var str="dog,cat,pig,cow"
var re2=/\b\w+\b(?=,cat|,pig)/g   
console.log(str.match(re2))

function addCommas(money){
  var re=/\d{3}(?!$)/g
  return money.replace(re,x=>x+",")
}
console.log( addCommas("$123456789") )  
console.log( addCommas("$123456") )    
console.log( addCommas("$1234567890") ) 
console.log( addCommas("$1234567") )

const a = '5';
const b = 5;
const c = 10;

const result1 = a == b;
const result2 = a === b;
const result3 = b < c;
const result4 = b >= c;

console.log(result1, result2, result3, result4);

let a = 5;
let b = a++ + ++a;
console.log(b);

console.log(Math.round(1.45)); 
console.log(Math.ceil(1.45));  
console.log(Math.floor(1.45));
console.log(Math.round(1.55)); 
console.log(Math.ceil(1.55));  
console.log(Math.floor(1.55));

var str="example";
var result=str.replace("e","@");
console.log(result);

'use strict';

function strictModeExample() {
  undeclaredVariable = 10;
  try {
    console.log(undeclaredVariable);
  } catch (e) {
    console.log('Error:', e.message);
  }
}

strictModeExample();

let obj = { a: 1 };
Object.freeze(obj);

console.log(Object.isFrozen(obj));
console.log(obj.a = 2);
console.log(obj.a);

var str="My".concat("name","is","John");
console.log(str);

function* numberGenerator() {
  yield 1;
  yield 2;
  yield 3;
}

const gen = numberGenerator();
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);

Что будет на выходе?

const arr = [];
arr.length = 3;
arr[2] = 1;
arr.forEach((el, i) => console.log(i))

const symbol1 = Symbol('symbol');
const symbol2 = Symbol('symbol');

const obj = {};
obj[symbol1] = 'value1';
obj[symbol2] = 'value2';

console.log(obj[symbol1]);

function recursivePascalTriangle(n, row = [1], triangle = []) {
  triangle.push(row);
  if (n === triangle.length) {
    return triangle;
  }
  const nextRow = [1];
  for (let i = 1; i < row.length; i++) {
    nextRow.push(row[i] + row[i - 1]);
  }
  nextRow.push(1);
  return recursivePascalTriangle(n, nextRow, triangle);
}

const result = recursivePascalTriangle(5);

console.log(result);

var arr=[1,2,3,4,5];
console.log(arr.join());      
console.log(arr.join(","));   
console.log(arr.join(" "));   
console.log(arr.join("and"));

function outerFunction() {
  let x = 10;
  function innerFunction() {
    x += 5;
    console.log(x);
  }
  return innerFunction;
}

const closureFunc = outerFunction();
closureFunc();
closureFunc();

function rndnum(from,to){
  return ~~((to-from)*Math.random()+from);
}
for (var i=0;i<5;i++) console.log(rndnum(20,30));

var arr=[[9,0],[5,3],[2,6],[7,8]];
var result=[];
for (var i=0;i<arr.length;i++) 
  result=result.concat(arr[i]);
console.log(result);