Introduce a conditionalPageLength feature plugin based off of conditionalPaging to hide the page length control when <= 1 page. Also add sub option for contionalOptions to hide page length options that are > the current result size.

features/conditionalPageLength/dataTables.conditionalPageLength.js

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+/**
+ * @summary     ConditionalPageLength
+ * @description Hide the page length control when the amount of pages is <= 1
+ * @version     1.0.0
+ * @file        dataTables.conditionalPageLength.js
+ * @author      Garrett Hyder (https://github.com/garretthyder)
+ * @contact     garrett.m.hyder@gmail.com
+ * @copyright   Copyright 2020 Garrett Hyder
+ *
+ * License      MIT - http://datatables.net/license/mit
+ *
+ * This feature plugin for DataTables hides the page length control when the amount
+ * of pages is <= 1. The control can either appear / disappear or fade in / out.
+ *
+ * Based off conditionalPaging by Matthew Hasbach (https://github.com/mjhasbach)
+ * Reference: https://github.com/DataTables/Plugins/blob/master/features/conditionalPaging/dataTables.conditionalPaging.js
+ *
+ * @example
+ *    $('#myTable').DataTable({
+ *        conditionalPageLength: true
+ *    });
+ *
+ * @example
+ *    $('#myTable').DataTable({
+ *        conditionalPageLength: {
+ *            style: 'fade',
+ *            speed: 500 // optional
+ *        }
+ *    });
+ *
+ * Conditionally hide the Page Length options that are less than the current result size.
+ * @example
+ *    $('#myTable').DataTable({
+ *        conditionalPageLength: {
+ *            conditionalOptions: true
+ *        }
+ *    }); 
+ */
+
+(function(window, document, $) {
+    $(document).on('init.dt', function(e, dtSettings) {
+        if ( e.namespace !== 'dt' ) {
+            return;
+        }
+
+        var options = dtSettings.oInit.conditionalPageLength || $.fn.dataTable.defaults.conditionalPageLength;
+
+        if ($.isPlainObject(options) || options === true) {
+            var config = $.isPlainObject(options) ? options : {},
+                api = new $.fn.dataTable.Api(dtSettings),
+                speed = 'slow',
+                conditionalPageLength = function(e) {
+                    var $paging = $(api.table().container()).find('div.dataTables_length'),
+                        pages = api.page.info().pages,
+                        size = api.rows({search:'applied'}).count();
+
+                    if (e instanceof $.Event) {
+                        if (pages <= 1) {
+                            if (config.style === 'fade') {
+                                $paging.stop().fadeTo(speed, 0);
+                            }
+                            else {
+                                $paging.css('visibility', 'hidden');
+                            }
+                        }
+                        else {
+                            if (config.style === 'fade') {
+                                $paging.stop().fadeTo(speed, 1);
+                            }
+                            else {
+                                $paging.css('visibility', '');
+                            }
+                        }
+                    }
+                    else if (pages <= 1) {
+                        if (config.style === 'fade') {
+                            $paging.css('opacity', 0);
+                        }
+                        else {
+                            $paging.css('visibility', 'hidden');
+                        }
+                    }
+
+                    if (config.conditionalOptions) {
+                        $paging.find('select option').each(function(index) {
+                            if ($(this).attr('value') > size) {
+                                $(this).hide();
+                            } else {
+                                $(this).show();
+                            }
+                        });
+                    }
+                };
+
+            if ( config.speed !== undefined ) {
+                speed = config.speed;
+            }
+
+            conditionalPageLength();
+
+            api.on('draw.dt', conditionalPageLength);
+        }
+    });
+})(window, document, jQuery);