Use var instead of let and const, to make sure features always work in older browsers as well.

Author: Clorith

js/deprecation-notice.js

@@ -2,18 +2,18 @@
  * Show all jQuery Migrate warnings in the UI.
  */
 jQuery( document ).ready( function( $ ) {
-const notice = $( '.notice.jquery-migrate-deprecation-notice' );
-const warnings = jQuery.migrateWarnings || [];
-const adminbar = $( '#wp-admin-bar-enable-jquery-migrate-helper' );
-const countWrapper = $( '.count-wrapper', adminbar );
+var notice = $( '.notice.jquery-migrate-deprecation-notice' );
+var warnings = jQuery.migrateWarnings || [];
+var adminbar = $( '#wp-admin-bar-enable-jquery-migrate-helper' );
+var countWrapper = $( '.count-wrapper', adminbar );
 
 var previousDeprecations = [];
 
 /**
  * Filter the trace, return the first URI that is to a plugin or theme script.
  */
 function getSlugFromTrace( trace ) {
-let traceLines = trace.split( '\n' ),
+var traceLines = trace.split( '\n' ),
 match = null;
 
 // Loop over each line in the stack trace
@@ -85,7 +85,7 @@ jQuery( document ).ready( function( $ ) {
  * @param message
  */
 function appendNoticeDisplay( message ) {
-const list = notice.find( '.jquery-migrate-deprecation-list' );
+var list = notice.find( '.jquery-migrate-deprecation-list' );
 
 if ( ! notice.length ) {
 return;
@@ -122,7 +122,7 @@ jQuery( document ).ready( function( $ ) {
 return;
 }
 
-let data = {
+var data = {
 action: 'jquery-migrate-log-notice',
 notice: message,
 nonce: JQMH.report_nonce,
@@ -158,8 +158,8 @@ jQuery( document ).ready( function( $ ) {
 
 // Add handler for dismissing of the dashboard notice.
 $( document ).on( 'click', '.jquery-migrate-dashboard-notice .notice-dismiss', function() {
-const $notice = $( this ).closest( '.notice' );
-const notice_id = $notice.data( 'notice-id' );
+var $notice = $( this ).closest( '.notice' );
+var notice_id = $notice.data( 'notice-id' );
 
 $.post( {
 url: window.ajaxurl,