fix(orderBy): sort by identity if no predicate is given

Closes #5847
Closes #4579
Closes #9403

src/ng/filter/orderBy.js

@@ -11,7 +11,7 @@
  * correctly, make sure they are actually being saved as numbers and not strings.
  *
  * @param {Array} array The array to sort.
- * @param {function(*)|string|Array.<(function(*)|string)>} expression A predicate to be
+ * @param {function(*)|string|Array.<(function(*)|string)>=} expression A predicate to be
  *    used by the comparator to determine the order of elements.
  *
  *    Can be one of:
@@ -24,10 +24,13 @@
  *      is interpreted as a property name to be used in comparisons (for example `"special name"`
  *      to sort object by the value of their `special name` property). An expression can be
  *      optionally prefixed with `+` or `-` to control ascending or descending sort order
- *      (for example, `+name` or `-name`).
+ *      (for example, `+name` or `-name`). If no property is provided, (e.g. `'+'`) then the array
+ *      element itself is used to compare where sorting.
  *    - `Array`: An array of function or string predicates. The first predicate in the array
  *      is used for sorting, but when two items are equivalent, the next predicate is used.
  *
+ *    If the predicate is missing or empty then it defaults to `'+'`.
+ *
  * @param {boolean=} reverse Reverse the order of the array.
  * @returns {Array} Sorted copy of the source array.
  *
@@ -116,15 +119,21 @@ orderByFilter.$inject = ['$parse'];
 function orderByFilter($parse){
   return function(array, sortPredicate, reverseOrder) {
     if (!(isArrayLike(array))) return array;
-    if (!sortPredicate) return array;
     sortPredicate = isArray(sortPredicate) ? sortPredicate: [sortPredicate];
+    if (sortPredicate.length === 0) { sortPredicate = ['+']; }
     sortPredicate = map(sortPredicate, function(predicate){
       var descending = false, get = predicate || identity;
       if (isString(predicate)) {
         if ((predicate.charAt(0) == '+' || predicate.charAt(0) == '-')) {
           descending = predicate.charAt(0) == '-';
           predicate = predicate.substring(1);
         }
+        if ( predicate === '' ) {
+          // Effectively no predicate was passed so we compare identity
+          return reverseComparator(function(a,b) {
+            return compare(a, b);
+          }, descending);
+        }
         get = $parse(predicate);
         if (get.constant) {
           var key = get();

test/ng/filter/orderBySpec.js

@@ -6,9 +6,18 @@ describe('Filter: orderBy', function() {
     orderBy = $filter('orderBy');
   }));
 
-  it('should return same array if predicate is falsy', function() {
-    var array = [1, 2, 3];
-    expect(orderBy(array)).toBe(array);
+  it('should return sorted array if predicate is not provided', function() {
+    expect(orderBy([2, 1, 3])).toEqual([1, 2, 3]);
+
+    expect(orderBy([2, 1, 3], '')).toEqual([1, 2, 3]);
+    expect(orderBy([2, 1, 3], [])).toEqual([1, 2, 3]);
+    expect(orderBy([2, 1, 3], [''])).toEqual([1, 2, 3]);
+
+    expect(orderBy([2, 1, 3], '+')).toEqual([1, 2, 3]);
+    expect(orderBy([2, 1, 3], ['+'])).toEqual([1, 2, 3]);
+
+    expect(orderBy([2, 1, 3], '-')).toEqual([3, 2, 1]);
+    expect(orderBy([2, 1, 3], ['-'])).toEqual([3, 2, 1]);
   });
 
   it('shouldSortArrayInReverse', function() {