Маска ввода телефона из нескольких input

let parts = Array.from(document.querySelectorAll('[data-type="number"]')).sort((a, b) => +a.dataset.order > +b.dataset.order), 
    data = [ 
      /^\+\d$/, 
      /^\d{3}$/, 
      [/^\d{3}\-\d\d\-\d\d$/, v => /^(\d{3})\-$/.test(v) ? RegExp.$1 : v.replace(/-/g, '').replace(/((?:^\d\d\d(?!-)|\d\d(?!$)))/g, "$1-")] 
    ]; 
 
parts.forEach(e => e.addEventListener('input', check)); 
               
function check(){ 
  let id = +this.dataset.order; 
   
  let regexp = Array.isArray(data[id]) ? data[id][0] : data[id], 
      formatter = Array.isArray(data[id]) && data[id][1] ? data[id][1] : null; 
   
  if(this.value.trim() === '') 
    if(parts[id - 1]) 
      parts[--id].focus(); 
   
  if(formatter !== null){ 
    this.value = formatter(this.value); 
  } 
   
  if(regexp.test(this.value)){ 
    if(parts[id + 1]) 
      parts[++id].focus(); 
  } 
}

input[placeholder]{text-align: center;} 
input::-webkit-input-placeholder{opacity: 1; transition: opacity 0.3s ease;} 
input:focus::-webkit-input-placeholder{opacity: 0; transition: opacity 0.3s ease;} 
input:focus{outline: none;}

<input type='text' data-type='number' data-order='0' size='1' maxlength='2' placeholder='+7' autofocus /> 
<input type='text' data-type='number' data-order='1' size='1' maxlength='3' placeholder='123' /> 
<input type='text' data-type='number' data-order='2' size='6' maxlength='9' placeholder='456-78-90' />

var code = document.getElementById('code'); 
var country = document.getElementById('country'); 
var number = document.getElementById('number'); 
 
countryHandler = function() { 
  var text = document.getElementById('country').value; 
  if(text.length > 1 && text[0] =='+'){     
       code.focus(); 
    } 
}; 
 
codeHandler = function() { 
  var text = document.getElementById('code').value; 
  if(text.length > 2){     
       number.focus(); 
    }  
}; 
 
numberHandler = function() { 
  var text = document.getElementById('number').value; 
  if(text.length > 6){   
    //code.focus(); 
       number.blur(); 
    } 
}; 
 
 
country.oninput = countryHandler; 
country.onpropertychange = country.oninput; 
code.oninput = codeHandler; 
code.onpropertychange = code.oninput; 
number.oninput = numberHandler; 
number.onpropertychange = number.oninput;

<form id="form"> 
  <input type="text" name="country" id="country"> 
  <input type="text" name="code" id="code"> 
  <input type="text" name="number" id="number"> 
</form>