JSFiddle - React, Tailwind, and code Playground

HTML

<input class="placeholder_required" value="hi_1" placeholder="hi_1">
<br>
<input class="placeholder_required" value="hi_2" placeholder="hi_2">
<br>
<input class="placeholder_required" value="hi_3" placeholder="hi_3">
<br>
<input id="add_input" class="" value="後來加的" placeholder="後來加的">
<br>
<input class="placeholder_optional" value="hi_4" placeholder="hi_4">
<br>

<textarea name="" id="" cols="30" rows="10" class="placeholder_optional" placeholder="text">text</textarea>
<button id="add">add</button>
<button id="submit">
  submit
</button>

JavaScript

$(function() {
	$("#add").click(function(){
  	$("#add_input").addClass('placeholder_required');
  })
  $("#submit").click(function() {
    var chk = chk_palceholder();
    //如果 chk 是true就往下送 
    alert(chk);
  });
})

function chk_palceholder() {
  //檢查必填
  var chk = true;
  $(".placeholder_required").each(function(k, v) {
      if ($(this).val() == $(this).attr("placeholder")) {
        chk = false;
      }
    })
    //檢查非必填
  $(".placeholder_optional").each(function(k, v) {
    if ($(this).val() == $(this).attr("placeholder")) {
      $(this).val('');
    }
  })

  return chk;
}