test: add unit tests for player.duration() (#4459)

Unit tests for https://github.com/videojs/video.js/pull/4443

test/unit/player.test.js

@@ -1655,3 +1655,31 @@ QUnit.test('should add a class with major version', function(assert) {
 
   player.dispose();
 });
+
+QUnit.test('player.duration() returns NaN if player.cache_.duration is undefined', function(assert) {
+  const player = TestHelpers.makePlayer();
+
+  player.cache_.duration = undefined;
+  assert.ok(Number.isNaN(player.duration()), 'returned NaN for unkown duration');
+});
+
+QUnit.test('player.duration() returns player.cache_.duration if it is defined', function(assert) {
+  const player = TestHelpers.makePlayer();
+
+  player.cache_.duration = 200;
+  assert.equal(player.duration(), 200, 'returned correct integer duration');
+  player.cache_.duration = 942;
+  assert.equal(player.duration(), 942, 'returned correct integer duration');
+});
+
+QUnit.test('player.duration() sets the value of player.cache_.duration', function(assert) {
+  const player = TestHelpers.makePlayer();
+
+  // set an arbitrary initial cached duration value for testing the setter functionality
+  player.cache_.duration = 1;
+
+  player.duration(NaN);
+  assert.ok(Number.isNaN(player.duration()), 'duration() set and get NaN duration value');
+  player.duration(200);
+  assert.equal(player.duration(), 200, 'duration() set and get integer duration value');
+});