JSFiddle - React, Tailwind, and code Playground

HTML

<form name="sample_form" method="post">
    <input class="check_field" type="text" name="first_name" id="first_name" value="" />
    <input class="check_field" type="text" name="last_name" id="last_name" value="" />
    <textarea class="check_field" name="address" id="address"></textarea>
    <input type="submit" name="submitbtn" id="submitbtn" value="Submit" disabled="disabled" />
</form>

JavaScript

$('.check_field').on('blur',function(){
    var allFilled=true;
    $('.check_field').each(function(){
    if($(this).val()===''){
        allFilled=false;
        return;
    }
    });
    if(allFilled){
        $('#submitbtn').removeAttr("disabled");
    }
});