Apply column formats to slider labels (rstudio#1119)

NEWS.md

@@ -10,6 +10,8 @@
 
 - Fixed a bug that caused the date picker widget from the DateTime extension to render incorrectly (thanks, @mikmart, #1116).
 
+- Column formatting now also applies to range labels shown on filter sliders (thanks, @GitChub, @mikmart, #247).
+
 # CHANGES IN DT VERSION 0.31
 
 - Upgraded DataTables version to 1.13.6 (thanks, @stla, #1091).

inst/htmlwidgets/datatables.js

@@ -651,7 +651,25 @@ HTMLWidgets.widget({
             }
             r1  = t1; r2 = t2;
           })();
-          $span1.text(formatDate(r1)); $span2.text(formatDate(r2));
+          // format with active column renderer, if defined
+          var colDef = data.options.columnDefs.find(function(def) {
+            return (def.targets === i || inArray(i, def.targets)) && 'render' in def;
+          });
+          var updateSliderText = function(v1, v2) {
+            // we only know how to use function renderers
+            if (colDef && typeof colDef.render === 'function') {
+              var restore = function(v) {
+                v = scaleBack(v, scale);
+                return type !== 'date' ? v : new Date(+v);
+              }
+              $span1.text(colDef.render(restore(v1), 'display'));
+              $span2.text(colDef.render(restore(v2), 'display'));
+            } else {
+              $span1.text(formatDate(v1, false));
+              $span2.text(formatDate(v2, false));
+            }
+          };
+          updateSliderText(r1, r2);
           var updateSlider = function(e) {
             var val = filter.val();
             // turn off filter if in full range