[Dart] Modal (Popup)

Modal (Popup) implementation via Dart and CSS. According to w3schools, modal is dialog box/popup window that is displayed on top of the current page. The modal here is a simplified version of Bootstrap modal.

Run on DartPad


<!-- Button trigger modal -->
<button type="button" data-toggle="modal" data-target="#myModal">
  Launch demo modal

<!-- Modal -->
<div class="modal" id="myModal">
  <h4>Modal Title</h4>
     Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas turpis
     felis, tempus sit amet sollicitudin quis, aliquet ut felis.
     Nam a malesuada sem.
  <button type="button" data-dismiss="modal">Close</button>

The HTML code here is simplified version of Bootstrap modal, we use button element with data-toggle and data-target attributes to launch the modal, and button element with data-dismiss attribute in the modal to close the modal.


.modal {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translateX(-50%) translateY(-50%);
  background: azure;
  padding: 1rem;
  border-radius: 1rem;
  border: 1px solid gray;
  display: none;
  color: black;

The CSS here is to center the modal horizontally and vertically [2], and also set the modal invisible in the beginning.


import 'dart:html';

void launchModal(Event e) {
  // when user click launch button, show modal.
  ButtonElement elm = e.target;
  var selector = elm.dataset["target"];
  var modal = querySelector(selector);
  modal.style.display = "block";

  if (modal.dataset["isSetClose"] != "true") {
    // when user click button with `data-dismiss="modal"` in the modal,
    // close the modal
    var closeBtns = modal.querySelectorAll("button[data-dismiss='modal']");
    for (var closeBtn in closeBtns) {
      closeBtn.onClick.listen((e) => modal.style.display = "none");

    modal.dataset["isSetClose"] = "true";

void main() {
  var launchBtns = querySelectorAll("button[data-toggle='modal']");

  for (var launchBtn in launchBtns) {

First we search for button elements with data-toggle="modal". If users click on such buttons, show the modal referenced by data-target attribute of the button element. Also in the click event handler of modal launching buttons, we set the click event handler of modal closing button, which has the data-dismiss="modal" attribute in the modal HTML code.

Tested on:

  • DartPad.
  • Chromium Version 58.0.3029.110 Built on Ubuntu , running on Ubuntu 17.04 (64-bit)


[1][JavaScript] Modal (Popup)
[2]html - How to center an element horizontally and vertically? - Stack Overflow
[3]Synonyms - Dart, JavaScript, C#, Python | Dart