fix(table): MatTableDataSource incorrectly sorting zero (#10561)

Fixes the `MatTableDataSource` sorting zero before negative numbers.

Fixes #10556.

src/lib/table/table-data-source.ts

@@ -123,16 +123,16 @@ export class MatTableDataSource<T> extends DataSource<T> {
       // This avoids inconsistent results when comparing values to undefined/null.
       // If neither value exists, return 0 (equal).
       let comparatorResult = 0;
-      if (valueA && valueB) {
+      if (valueA != null && valueB != null) {
         // Check if one value is greater than the other; if equal, comparatorResult should remain 0.
         if (valueA > valueB) {
           comparatorResult = 1;
         } else if (valueA < valueB) {
           comparatorResult = -1;
         }
-      } else if (valueA) {
+      } else if (valueA != null) {
         comparatorResult = 1;
-      } else if (valueB) {
+      } else if (valueB != null) {
         comparatorResult = -1;
       }
 

src/lib/table/table.spec.ts

@@ -267,6 +267,35 @@ describe('MatTable', () => {
       ]);
     });
 
+    it('should sort zero correctly', () => {
+      // Activate column A sort
+      dataSource.data[0].a = 1;
+      dataSource.data[1].a = 0;
+      dataSource.data[2].a = -1;
+
+      // Expect that zero comes after the negative numbers and before the positive ones.
+      component.sort.sort(component.sortHeader);
+      fixture.detectChanges();
+      expectTableToMatchContent(tableElement, [
+        ['Column A\xa0Sorted by a ascending', 'Column B', 'Column C'],
+        ['-1', 'b_3', 'c_3'],
+        ['0', 'b_2', 'c_2'],
+        ['1', 'b_1', 'c_1'],
+      ]);
+
+
+      // Expect that zero comes after the negative numbers and before
+      // the positive ones when switching the sorting direction.
+      component.sort.sort(component.sortHeader);
+      fixture.detectChanges();
+      expectTableToMatchContent(tableElement, [
+        ['Column A\xa0Sorted by a descending', 'Column B', 'Column C'],
+        ['1', 'b_1', 'c_1'],
+        ['0', 'b_2', 'c_2'],
+        ['-1', 'b_3', 'c_3'],
+      ]);
+    });
+
     it('should be able to page the table contents', fakeAsync(() => {
       // Add 100 rows, should only display first 5 since page length is 5
       for (let i = 0; i < 100; i++) {
@@ -299,9 +328,9 @@ describe('MatTable', () => {
 });
 
 interface TestData {
-  a: string|undefined;
-  b: string|undefined;
-  c: string|undefined;
+  a: string|number|undefined;
+  b: string|number|undefined;
+  c: string|number|undefined;
 }
 
 class FakeDataSource extends DataSource<TestData> {