feat(orderBy): consider null and undefined greater than other values

Previously, `null` values where sorted using type `string` resulting in a string
comparison. `undefined` values where compared to other values by type and were
usually considered greater than other values (since their type happens to start
with a `u`), but this was coincidental.

This commit ensures that `null` and `undefined ` values are explicitly
considered greater than other values (with `undefined` > `null`) and will
effectively be put at the end of the sorted list (for ascending order sorting).

Closes #15294

Closes #16376

BREAKING CHANGE:

When using `orderBy` to sort arrays containing `null` values, the `null` values
will be considered "greater than" all other values, except for `undefined`.
Previously, they were sorted as strings. This will result in different (but more
intuitive) sorting order.

Before:
```js
orderByFilter(['a', undefined, 'o', null, 'z']);
//--> 'a', null, 'o', 'z', undefined
```

After:
```js
orderByFilter(['a', undefined, 'o', null, 'z']);
//--> 'a', 'o', 'z', null, undefined
```

src/ng/filter/orderBy.js

@@ -40,6 +40,7 @@
  *      index: ...
  *    }
  *    ```
+ *    **Note:** `null` values use `'null'` as their type.
  * 2. The comparator function is used to sort the items, based on the derived values, types and
  *    indices.
  *
@@ -74,11 +75,15 @@
  *
  * The default, built-in comparator should be sufficient for most usecases. In short, it compares
  * numbers numerically, strings alphabetically (and case-insensitively), for objects falls back to
- * using their index in the original collection, and sorts values of different types by type.
+ * using their index in the original collection, sorts values of different types by type and puts
+ * `undefined` and `null` values at the end of the sorted list.
  *
  * More specifically, it follows these steps to determine the relative order of items:
  *
- * 1. If the compared values are of different types, compare the types themselves alphabetically.
+ * 1. If the compared values are of different types:
+ *    - If one of the values is undefined, consider it "greater than" the other.
+ *    - Else if one of the values is null, consider it "greater than" the other.
+ *    - Else compare the types themselves alphabetically.
  * 2. If both values are of type `string`, compare them alphabetically in a case- and
  *    locale-insensitive way.
  * 3. If both values are objects, compare their indices instead.
@@ -89,9 +94,10 @@
  *
  * **Note:** If you notice numbers not being sorted as expected, make sure they are actually being
  *           saved as numbers and not strings.
- * **Note:** For the purpose of sorting, `null` values are treated as the string `'null'` (i.e.
- *           `type: 'string'`, `value: 'null'`). This may cause unexpected sort order relative to
- *           other values.
+ * **Note:** For the purpose of sorting, `null` and `undefined` are considered "greater than"
+ *           any other value (with undefined "greater than" null). This effectively means that `null`
+ *           and `undefined` values end up at the end of a list sorted in ascending order.
+ * **Note:** `null` values use `'null'` as their type to be able to distinguish them from objects.
  *
  * @param {Array|ArrayLike} collection - The collection (array or array-like object) to sort.
  * @param {(Function|string|Array.<Function|string>)=} expression - A predicate (or list of
@@ -658,8 +664,7 @@ function orderByFilter($parse) {
   function getPredicateValue(value, index) {
     var type = typeof value;
     if (value === null) {
-      type = 'string';
-      value = 'null';
+      type = 'null';
     } else if (type === 'object') {
       value = objectValue(value);
     }
@@ -690,7 +695,11 @@ function orderByFilter($parse) {
         result = value1 < value2 ? -1 : 1;
       }
     } else {
-      result = type1 < type2 ? -1 : 1;
+      result = (type1 === 'undefined') ? 1 :
+        (type2 === 'undefined') ? -1 :
+        (type1 === 'null') ? 1 :
+        (type2 === 'null') ? -1 :
+        (type1 < type2) ? -1 : 1;
     }
 
     return result;

test/ng/filter/orderBySpec.js

@@ -309,6 +309,16 @@ describe('Filter: orderBy', function() {
 
         expect(orderBy(items, expr)).toEqual(sorted);
       });
+
+      it('should consider null and undefined greater than any other value', function() {
+        var items = [undefined, null, 'z', {}, 999, false];
+        var expr = null;
+        var sorted = [false, 999, {}, 'z', null, undefined];
+        var reversed = [undefined, null, 'z', {}, 999, false];
+
+        expect(orderBy(items, expr)).toEqual(sorted);
+        expect(orderBy(items, expr, true)).toEqual(reversed);
+      });
     });
 
     describe('(custom comparator)', function() {
@@ -376,7 +386,7 @@ describe('Filter: orderBy', function() {
       });
 
 
-      it('should treat a value of `null` as `"null"`', function() {
+      it('should treat a value of `null` as type `"null"`', function() {
         var items = [null, null];
         var expr = null;
         var reverse = null;
@@ -386,8 +396,8 @@ describe('Filter: orderBy', function() {
         var arg = comparator.calls.argsFor(0)[0];
 
         expect(arg).toEqual(jasmine.objectContaining({
-          type: 'string',
-          value: 'null'
+          type: 'null',
+          value: null
         }));
       });