Bootstrap: Remove form field border

In that example, to remove the border simply write:

.form-control {
    border: 0;
}

In your CSS file.

This will remove borders from all your form fields though, a more flexible approach would be to attach a class to the form fields you want to have no border, so your HTML would be something like:

<input type="email" class="form-control no-border" id="inputEmail3" placeholder="Email">

and your CSS:

.no-border {
    border: 0;
    box-shadow: none; /* You may want to include this as bootstrap applies these styles too */
}

In Bootstrap 4, you can add border-0 class to your element to remove its border.

Other classes that you can use - Bootstrap 4 Borders

<span class="border-0"></span>             /* remove all borders */
<span class="border-top-0"></span>         /* remove top border */
<span class="border-right-0"></span>       /* remove border on the right */
<span class="border-bottom-0"></span>      /* remove bottom border */
<span class="border-left-0"></span>        /* remove border on the left */

And here is a working example for what you asked:

$("#myButton").click(function() {
  $("#myInput").toggleClass('border-0')
});

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" rel="stylesheet" />
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<form>
  <input id="myInput">
  <input id="myButton" type="button" value="Toggle Border">
</form>