Source: misc/announce.js

/**
 * @file
 * Adds an HTML element and method to trigger audio UAs to read system messages.
 *
 * Use {@link Drupal.announce} to indicate to screen reader users that an
 * element on the page has changed state. For instance, if clicking a link
 * loads 10 more items into a list, one might announce the change like this.
 *
 * @example
 * $('#search-list')
 *   .on('itemInsert', function (event, data) {
 *     // Insert the new items.
 *     $(data.container.el).append(data.items.el);
 *     // Announce the change to the page contents.
 *     Drupal.announce(Drupal.t('@count items added to @container',
 *       {'@count': data.items.length, '@container': data.container.title}
 *     ));
 *   });
 */

(function (Drupal, debounce) {

  'use strict';

  var liveElement;
  var announcements = [];

  /**
   * Builds a div element with the aria-live attribute and add it to the DOM.
   *
   * @type {Drupal~behavior}
   *
   * @prop {Drupal~behaviorAttach} attach
   *   Attaches the behavior for drupalAnnouce.
   */
  Drupal.behaviors.drupalAnnounce = {
    attach: function (context) {
      // Create only one aria-live element.
      if (!liveElement) {
        liveElement = document.createElement('div');
        liveElement.id = 'drupal-live-announce';
        liveElement.className = 'visually-hidden';
        liveElement.setAttribute('aria-live', 'polite');
        liveElement.setAttribute('aria-busy', 'false');
        document.body.appendChild(liveElement);
      }
    }
  };

  /**
   * Concatenates announcements to a single string; appends to the live region.
   */
  function announce() {
    var text = [];
    var priority = 'polite';
    var announcement;

    // Create an array of announcement strings to be joined and appended to the
    // aria live region.
    var il = announcements.length;
    for (var i = 0; i < il; i++) {
      announcement = announcements.pop();
      text.unshift(announcement.text);
      // If any of the announcements has a priority of assertive then the group
      // of joined announcements will have this priority.
      if (announcement.priority === 'assertive') {
        priority = 'assertive';
      }
    }

    if (text.length) {
      // Clear the liveElement so that repeated strings will be read.
      liveElement.innerHTML = '';
      // Set the busy state to true until the node changes are complete.
      liveElement.setAttribute('aria-busy', 'true');
      // Set the priority to assertive, or default to polite.
      liveElement.setAttribute('aria-live', priority);
      // Print the text to the live region. Text should be run through
      // Drupal.t() before being passed to Drupal.announce().
      liveElement.innerHTML = text.join('\n');
      // The live text area is updated. Allow the AT to announce the text.
      liveElement.setAttribute('aria-busy', 'false');
    }
  }

  /**
   * Triggers audio UAs to read the supplied text.
   *
   * The aria-live region will only read the text that currently populates its
   * text node. Replacing text quickly in rapid calls to announce results in
   * only the text from the most recent call to {@link Drupal.announce} being
   * read. By wrapping the call to announce in a debounce function, we allow for
   * time for multiple calls to {@link Drupal.announce} to queue up their
   * messages. These messages are then joined and append to the aria-live region
   * as one text node.
   *
   * @param {string} text
   *   A string to be read by the UA.
   * @param {string} [priority='polite']
   *   A string to indicate the priority of the message. Can be either
   *   'polite' or 'assertive'.
   *
   * @return {function}
   *   The return of the call to debounce.
   *
   * @see http://www.w3.org/WAI/PF/aria-practices/#liveprops
   */
  Drupal.announce = function (text, priority) {
    // Save the text and priority into a closure variable. Multiple simultaneous
    // announcements will be concatenated and read in sequence.
    announcements.push({
      text: text,
      priority: priority
    });
    // Immediately invoke the function that debounce returns. 200 ms is right at
    // the cusp where humans notice a pause, so we will wait
    // at most this much time before the set of queued announcements is read.
    return (debounce(announce, 200)());
  };
}(Drupal, Drupal.debounce));