Преобразовать номер телефона к виду +7(000)000-00-00

const numberValueReduced = phone => { 
    return phone.replace(/^(\d{3})(\d{3})(\d{2})(\d{2})$/, '+7 ($1) $2-$3-$4'); 
}; 
 
let btnTest = document.getElementById('test'),  
    result  = document.getElementById('result');  
btnTest.addEventListener('click', () => { 
  let nmbr  = rnd(10), 
      frmtd = numberValueReduced(nmbr);  
  result.innerHTML = `${nmbr}<br>${frmtd}`;  
}); 
 
function rnd(n) { 
  let str = '';  
  while (n-- > 0)  
    str += Math.floor(Math.random() * 10);  
  return str;  
}

body, 
button { 
  font: 16px sans-serif;  
  text-align: center;  
} 
 
#result { 
  min-height: 70px;  
  margin-top: 30px;  
  font-size: 20px;  
} 
 
#test { 
  width: 250px; height: 35px; 
  border: 0; 
  text-transform: uppercase;  
  background-color: #44d; color: #fff;  
  box-shadow: 0 2px 8px -2px #0007;   
}

<div id="result"></div> 
<button id="test">Случайный номер</button>