how to button property disable change in javascript or jqury

myform.php 

<script src="asset/myscript.js"></script>

<form action="sending.php" method="POST" enctype="multipart/form-data">

     <h2> Click This! </h2>
     <input type="file" accept="jpg">

     <input id="submit-btn" type="submit" value="submit">


</form>

myscript.js

// myjs.js File
$(document).ready(function(){


/* Here this ! */
   document.getElementById('submit-btn').disabled= true;

/* you need change false */

document.getElementById('submit-btn').disabled= false;


});