How to open a Bootstrap modal without jQuery or bootstrap.js javascript?

@uday's link to CSS only modal is a nice trick, but might be awkward to use if you use #tag's for other purposes (eg, Routing & param passing).

So here is an example that uses very little JS to achieve something very similar. I've tried to keep the Snippet as small as possible so that it's easy to see what's happening.

var modal = document.querySelector(".modal");
var container = modal.querySelector(".container");

document.querySelector("button").addEventListener("click", function (e) {
  modal.classList.remove("hidden")
});

document.querySelector(".modal").addEventListener("click", function (e) {
  if (e.target !== modal && e.target !== container) return;     
  modal.classList.add("hidden");
});

.modal {
  background-color: rgba(0,0,0,0.4); /* Transparent dimmed overlay */
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  display: table;
}

.modal.hidden {
  display: none;
}

.modal .container {
 display: table-cell;
 text-align: center;
 vertical-align: middle;
 width: 200px;
}

.modal .body {
  box-shadow: 5px 10px #888888;
  display: inline-block;
  background-color: white;
  border: 1px solid black; 
  padding: 10px;
}

<button>Show Modal</button>

<div class="modal hidden">
  <div class="container">
    <div class="body">
      <p>Click outside this box to close the modal.<p>
      <p>You could of course add a close button etc</p>
      <p>But this is left for the OP todo</p> 
    </div>
  </div>
</div>

You don't need any css style. You should create the bootstrap modal in your HTML, then in your js, you have to simply give it some style according to the following description:

var locModal = document.getElementById('locModal');
var btnclose = document.getElementById('w-change-close');
var btnShow= document.getElementById('w-change-location');


//show the modal
btnShow.addEventListener('click', (e) => {
    locModal.style.display = "block";
    locModal.style.paddingRight = "17px";
    locModal.className="modal fade show"; 
});
    //hide the modal
    btnclose.addEventListener('click', (e) => {
        locModal.style.display = "none";
        locModal.className="modal fade";
});

The HTML should be like this:

<!-- Button trigger modal -->
<button id="w-change-location" type="button" class="btn btn-primary mt-3" data-toggle="modal" data-target="#locModal">
                Change Location
   </button>
    <!-- Modal -->
    <div class="modal fade" id="locModal" tabindex="-1" role="dialog" aria-labelledby="locModalLabel"
        aria-hidden="true">
        <div class="modal-dialog" role="document">
        <div class="modal-content">
            <div class="modal-header">
                <h5 class="modal-title" id="locModalLabel">Choose Location</h5>
                <button type="button" class="close" data-dismiss="modal" aria-label="Close">
                    <span aria-hidden="true">&times;</span>
                </button>
            </div>
            <div class="modal-body">
                <form action="" id="w-form">
                    <div class="form-group">
                        <label for="city"> City</label>
                        <input type="text" class="form-control" id="city">
                    </div>
                    <div class="form-group">
                        <label for="state"> State </label>
                        <input type="text" class="form-control" id="state">
                    </div>
                </form>
            </div>
            <div class="modal-footer">
                <button id="w-change-close" type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
                <button id="w-change-btn" type="button" class="btn btn-primary">Save changes</button>
            </div>
        </div>
    </div>
</div>