UI: rearrange logic for adding timezone in date-format helper

changelog/26693.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+ui: Remove possibility of returning an undefined timezone from date-format helper
+```

ui/lib/core/addon/helpers/date-format.js

@@ -66,15 +66,21 @@ export function dateFormat([value, style], { withTimeZone = false }) {
     return value || '';
   }
 
+  const zone = withTimeZone ? formatTimeZone(date) : '';
+  return format(date, style) + zone;
+}
+
+// separate function for testing
+export function formatTimeZone(date) {
   let zone; // local timezone ex: 'PST'
   try {
     // passing undefined means default to the browser's locale
-    zone = ' ' + date.toLocaleTimeString(undefined, { timeZoneName: 'short' }).split(' ')[2];
+    zone = date.toLocaleTimeString(undefined, { timeZoneName: 'short' }).split(' ')[2];
   } catch (e) {
     zone = '';
   }
-  zone = withTimeZone ? zone : '';
-  return format(date, style) + zone;
+
+  return zone ? ` ${zone}` : '';
 }
 
 export default helper(dateFormat);

ui/tests/integration/helpers/date-format-test.js

@@ -7,6 +7,7 @@ import { module, test } from 'qunit';
 import { setupRenderingTest } from 'ember-qunit';
 import { find, render, settled } from '@ember/test-helpers';
 import hbs from 'htmlbars-inline-precompile';
+import { formatTimeZone } from 'core/helpers/date-format';
 
 const TEST_DATE = new Date('2018-04-03T14:15:30');
 
@@ -68,4 +69,21 @@ module('Integration | Helper | date-format', function (hooks) {
     await settled();
     assert.dom(this.element).hasText('', 'renders empty string when falsey');
   });
+
+  test('it formats timezone', async function (assert) {
+    assert.strictEqual(formatTimeZone('not a date'), '', 'returns an empty string for a non-date value');
+
+    // compute expected because we can't reliably assume timezone of the test suite
+    const expected = new Intl.DateTimeFormat('en', { timeZoneName: 'shortOffset' })
+      .format(TEST_DATE)
+      .split(',')[1];
+
+    const formatted = formatTimeZone(TEST_DATE);
+
+    assert.strictEqual(
+      formatted,
+      expected,
+      `formatted timezone: "${formatted}" equals expected: "${expected}"`
+    );
+  });
 });