Как взять значение из placeholder?

let inp = document.querySelector("input"); 
let sbm = document.querySelector("form button"); 
let form = document.querySelector("form"); 
 
//alert(inp.getAttribute("placeholder")); 
 
sbm.onclick = function(){ 
  if ((inp.value !== undefined || inp.value !== null) && inp.value === ""){ 
    inp.value = inp.getAttribute("placeholder"); 
  } 
};

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> 
<form method="post"  action="" onsubmit="console.log($(this).serialize());return false"> 
<input type="text" id="search-box" name="sel"  placeholder='123' value=""/> 
<button type="submit">Otpravit</button>