[3.1.x] Refs #31493 -- Replaced var with const/let in documentation JS.

Backport of 2afa61e7d9 from master

docs/internals/contributing/writing-code/javascript.txt

@@ -82,13 +82,13 @@ Django's JavaScript tests use `QUnit`_. Here is an example test module:
 
     QUnit.module('magicTricks', {
         beforeEach: function() {
-            var $ = django.jQuery;
+            const $ = django.jQuery;
             $('#qunit-fixture').append('<button class="button"></button>');
         }
     });
 
     QUnit.test('removeOnClick removes button on click', function(assert) {
-        var $ = django.jQuery;
+        const $ = django.jQuery;
         removeOnClick('.button');
         assert.equal($('.button').length, 1);
         $('.button').click();
@@ -96,7 +96,7 @@ Django's JavaScript tests use `QUnit`_. Here is an example test module:
     });
 
     QUnit.test('copyOnClick adds button on click', function(assert) {
-        var $ = django.jQuery;
+        const $ = django.jQuery;
         copyOnClick('.button');
         assert.equal($('.button').length, 1);
         $('.button').click();

docs/ref/csrf.txt

@@ -85,11 +85,11 @@ You can acquire the token like this:
 .. code-block:: javascript
 
     function getCookie(name) {
-        var cookieValue = null;
+        let cookieValue = null;
         if (document.cookie && document.cookie !== '') {
-            var cookies = document.cookie.split(';');
-            for (var i = 0; i < cookies.length; i++) {
-                var cookie = cookies[i].trim();
+            const cookies = document.cookie.split(';');
+            for (let i = 0; i < cookies.length; i++) {
+                const cookie = cookies[i].trim();
                 // Does this cookie string begin with the name we want?
                 if (cookie.substring(0, name.length + 1) === (name + '=')) {
                     cookieValue = decodeURIComponent(cookie.substring(name.length + 1));
@@ -99,14 +99,14 @@ You can acquire the token like this:
         }
         return cookieValue;
     }
-    var csrftoken = getCookie('csrftoken');
+    const csrftoken = getCookie('csrftoken');
 
 The above code could be simplified by using the `JavaScript Cookie library
 <https://github.com/js-cookie/js-cookie/>`_ to replace ``getCookie``:
 
 .. code-block:: javascript
 
-    var csrftoken = Cookies.get('csrftoken');
+    const csrftoken = Cookies.get('csrftoken');
 
 .. note::
 
@@ -137,7 +137,7 @@ and read the token from the DOM with JavaScript:
 
     {% csrf_token %}
     <script>
-    var csrftoken = document.querySelector('[name=csrfmiddlewaretoken]').value;
+    const csrftoken = document.querySelector('[name=csrfmiddlewaretoken]').value;
     </script>
 
 Setting the token on the AJAX request
@@ -148,7 +148,7 @@ Finally, you'll need to set the header on your AJAX request. Using the
 
 .. code-block:: javascript
 
-    var request = new Request(
+    const request = new Request(
         /* URL */,
         {headers: {'X-CSRFToken': csrftoken}}
     );

docs/ref/templates/builtins.txt

@@ -1830,7 +1830,7 @@ The resulting data can be accessed in JavaScript like this:
 
 .. code-block:: javascript
 
-    var value = JSON.parse(document.getElementById('hello-data').textContent);
+    const value = JSON.parse(document.getElementById('hello-data').textContent);
 
 XSS attacks are mitigated by escaping the characters "<", ">" and "&". For
 example if ``value`` is ``{'hello': 'world</script>&amp;'}``, the output is:

docs/topics/i18n/translation.txt

@@ -1080,9 +1080,12 @@ interface within your Python code::
 The ``ngettext`` function provides an interface to pluralize words and
 phrases::
 
-    var object_count = 1 // or 0, or 2, or 3, ...
-    s = ngettext('literal for the singular case',
-            'literal for the plural case', object_count);
+    const objectCount = 1 // or 0, or 2, or 3, ...
+    const string = ngettext(
+        'literal for the singular case',
+        'literal for the plural case',
+        objectCount
+    );
 
 ``interpolate``
 ~~~~~~~~~~~~~~~
@@ -1096,23 +1099,29 @@ function supports both positional and named interpolation:
   corresponding ``fmt`` placeholders in the same order they appear.
   For example::
 
-    fmts = ngettext('There is %s object. Remaining: %s',
-            'There are %s objects. Remaining: %s', 11);
-    s = interpolate(fmts, [11, 20]);
-    // s is 'There are 11 objects. Remaining: 20'
+    const formats = ngettext(
+      'There is %s object. Remaining: %s',
+      'There are %s objects. Remaining: %s',
+      11
+    );
+    const string = interpolate(formats, [11, 20]);
+    // string is 'There are 11 objects. Remaining: 20'
 
 * Named interpolation: This mode is selected by passing the optional
   boolean ``named`` parameter as ``true``. ``obj`` contains a JavaScript
   object or associative array. For example::
 
-    d = {
-        count: 10,
-        total: 50
+    const data = {
+      count: 10,
+      total: 50
     };
 
-    fmts = ngettext('Total: %(total)s, there is %(count)s object',
-    'there are %(count)s of a total of %(total)s objects', d.count);
-    s = interpolate(fmts, d, true);
+    const formats = ngettext(
+        'Total: %(total)s, there is %(count)s object',
+        'there are %(count)s of a total of %(total)s objects',
+        data.count
+    );
+    const string = interpolate(formats, data, true);
 
 You shouldn't go over the top with string interpolation, though: this is still
 JavaScript, so the code has to make repeated regular-expression substitutions.