label of input

<div class="container">
  <form>
    <h1>Sign-Up Now</h1>
    <div class="field">
      <label for="firstName">First Name</label>
      <input type="text" id="firstName" name="firstNameTextField" placeholder="First Name">
    </div>
    <div class="field">
      <label for="lastName">Last Name</label>
      <input type="text" id="lastName" name="lastNameTextField" placeholder="Last Name">
    </div>
    <div class="field">
      <label for="email">Email</label>
      <input type="text" id="email" name="emailTextField" placeholder="Email">
    </div>
    <button>Start</button>
  </form>
</div>





html, body {
  height: 100%;
  margin: 0;
}
body {
  -ms-flex-align: center;
  align-items: center;
  display: -ms-flexbox;
  display: flex;
  font-family: "Open Sans", sans-serif;
  line-height: 1.75;
}
.container {
  margin: 0 auto;
}
form {
  text-align: center;
}
h1 {
  font-size: 2.5em;
  margin: 0 0 .75em;
}
.field {
  margin-bottom: 1.5em;
  position: relative;
}
label {
  background: #fff;
  color: #4481c4;
  font-size: 1.25em;
  font-weight: 600;
  left: .75em;
  opacity: 0;
  padding: 0 .25em;
  position: absolute;
  top: 2em;
  transition: all 0.1s linear;
  z-index: -1;
}
label.show {
  opacity: 1;
  top: -1em;
  z-index: 1;
}
input {
  border: 1px solid #ddd;
  border-radius: 4px;
  font-size: 1.75em;
  padding: .5em;
}
button {
  background: #03a9c4;
  border: none;
  border-radius: 5px;
  color: #fff;
  font-size: 1.5em;
  margin-top: .75em;
  padding: .75em;
  text-transform: uppercase;
}









$(function () {
  var showClass = 'show';
  
  $('input').on('checkval', function () {
    var label = $(this).prev('label');
    if(this.value !== '') {
      label.addClass(showClass);
    } else {
      label.removeClass(showClass);
    }
  }).on('keyup', function () {
    $(this).trigger('checkval');
  }); 
});