fix(FEC-7226, FEC-7243): create comparer func to default tracks (#128)

src/player.js

@@ -1242,7 +1242,7 @@ export default class Player extends FakeEventTarget {
    */
   _setDefaultTrack(type: string, language: string, defaultTrack: Track): void {
     if (language) {
-      const track: ?Track = this._getTracksByType(type).find(track => track.language === language);
+      const track: ?Track = this._getTracksByType(type).find(track => TextTrack.langComparer(language, track.language));
       if (track) {
         this.selectTrack(track);
       }

src/track/text-track.js

@@ -6,6 +6,21 @@ import Track from './track'
  * @classdesc
  */
 export default class TextTrack extends Track {
+  /**
+   * Comparing language strings according to their length.
+   * @param {string} inputLang - The configured language.
+   * @param {string} trackLang - The default track language.
+   * @returns {boolean} - Whether the strings are equal or starts with the same substring.
+   */
+  static langComparer(inputLang: string, trackLang: string): boolean {
+    const inputLangLength = inputLang.length;
+    const trackLangLength = trackLang.length;
+    if (inputLangLength === trackLangLength) {
+      return inputLang === trackLang;
+    }
+    return (inputLangLength > trackLangLength ? inputLang.startsWith(trackLang) : trackLang.startsWith(inputLang));
+  }
+
   /**
    * The kind of the text track:
    * subtitles/captions/metadata.

test/src/track/text-track.spec.js

@@ -0,0 +1,17 @@
+import TextTrack from '../../../src/track/text-track'
+
+describe('TextTrack', () => {
+  describe('langComparer', () => {
+    it('should compare 2 languages with same length', () => {
+      TextTrack.langComparer('ita', 'ita').should.be.true;
+      TextTrack.langComparer('ita', 'eng').should.be.false;
+    });
+
+    it('should compare 2 languages with different length', () => {
+      TextTrack.langComparer('ita', 'it').should.be.true;
+      TextTrack.langComparer('it', 'ita').should.be.true;
+      TextTrack.langComparer('es', 'ita').should.be.false;
+      TextTrack.langComparer('ita', 'es').should.be.false;
+    });
+  });
+});