CODE EXAMPLE FOR JAVASCRIPT

jQuery serialize form to JSON

function onSubmit( form ){
  var data = JSON.stringify( $(form).serializeArray() ); //  <-----------

  console.log( data );
  return false; //don't submit
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<form onsubmit='return onSubmit(this)'>
  <input name='user' placeholder='user'><br>
  <input name='password' type='password' placeholder='password'><br>
  <button type='submit'>Try</button>
</form>