fixed 296f3059 from https://gitee.com/zengyawen/docs/pulls/8421

update zh-cn/application-dev/reference/apis/js-apis-media.md.
Signed-off-by: Nzengyawen <zengyawen1@huawei.com>

zh-cn/application-dev/reference/apis/js-apis-media.md

@@ -60,7 +60,7 @@ createVideoPlayer(callback: AsyncCallback\<[VideoPlayer](#videoplayer8)>): void
 let videoPlayer
 
 media.createVideoPlayer((error, video) => {
-   if (typeof(video) != 'undefined') {
+   if (video != null) {
        videoPlayer = video;
        console.info('video createVideoPlayer success');
    } else {
@@ -89,7 +89,7 @@ createVideoPlayer(): Promise<[VideoPlayer](#videoplayer8)>
 let videoPlayer
 
 media.createVideoPlayer().then((video) => {
-   if (typeof(video) != 'undefined') {
+   if (video != null) {
        videoPlayer = video;
        console.info('video createVideoPlayer success');
    } else {
@@ -142,7 +142,7 @@ createVideoRecorder(callback: AsyncCallback\<[VideoRecorder](#videorecorder9)>):
 let videoRecorder
 
 media.createVideoRecorder((error, video) => {
-   if (typeof(video) != 'undefined') {
+   if (video != null) {
        videoRecorder = video;
        console.info('video createVideoRecorder success');
    } else {
@@ -172,7 +172,7 @@ createVideoRecorder(): Promise<[VideoRecorder](#videorecorder9)>
 let videoRecorder
 
 media.createVideoRecorder().then((video) => {
-    if (typeof(video) != 'undefined') {
+    if (video != null) {
        videoRecorder = video;
        console.info('video createVideoRecorder success');
    } else {
@@ -369,7 +369,7 @@ seek(timeMs: number): void
 
 ```js
 audioPlayer.on('timeUpdate', (seekDoneTime) => {    //设置'timeUpdate'事件回调
-    if (typeof (seekDoneTime) == 'undefined') {
+    if (seekDoneTime == null) {
         console.info('audio seek fail');
         return;
     }
@@ -442,7 +442,7 @@ function printfDescription(obj) {
 }
 
 audioPlayer.getTrackDescription((error, arrlist) => {
-    if (typeof (arrlist) != 'undefined') {
+    if (arrlist != null) {
         for (let i = 0; i < arrlist.length; i++) {
             printfDescription(arrlist[i]);
         }
@@ -478,7 +478,7 @@ function printfDescription(obj) {
 }
 
 audioPlayer.getTrackDescription().then((arrlist) => {
-    if (typeof (arrlist) != 'undefined') {
+    if (arrlist != null) {
         arrayDescription = arrlist;
     } else {
         console.log('audio getTrackDescription fail');
@@ -553,7 +553,7 @@ audioPlayer.on('reset', () => {               //设置'reset'事件回调
     audioPlayer = undefined;
 });
 audioPlayer.on('timeUpdate', (seekDoneTime) => {  //设置'timeUpdate'事件回调
-    if (typeof(seekDoneTime) == "undefined") {
+    if (seekDoneTime == null) {
         console.info('audio seek fail');
         return;
     }
@@ -608,7 +608,7 @@ on(type: 'timeUpdate', callback: Callback\<number>): void
 
 ```js
 audioPlayer.on('timeUpdate', (seekDoneTime) => {    //设置'timeUpdate'事件回调
-    if (typeof (seekDoneTime) == 'undefined') {
+    if (seekDoneTime == null) {
         console.info('audio seek fail');
         return;
     }
@@ -726,7 +726,7 @@ setDisplaySurface(surfaceId: string, callback: AsyncCallback\<void>): void
 
 ```js
 videoPlayer.setDisplaySurface(surfaceId, (err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('setDisplaySurface success!');
     } else {
         console.info('setDisplaySurface fail!');
@@ -784,7 +784,7 @@ prepare(callback: AsyncCallback\<void>): void
 
 ```js
 videoPlayer.prepare((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('prepare success!');
     } else {
         console.info('prepare fail!');
@@ -834,7 +834,7 @@ play(callback: AsyncCallback\<void>): void;
 
 ```js
 videoPlayer.play((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('play success!');
     } else {
         console.info('play fail!');
@@ -884,7 +884,7 @@ pause(callback: AsyncCallback\<void>): void
 
 ```js
 videoPlayer.pause((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('pause success!');
     } else {
         console.info('pause fail!');
@@ -934,7 +934,7 @@ stop(callback: AsyncCallback\<void>): void
 
 ```js
 videoPlayer.stop((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('stop success!');
     } else {
         console.info('stop fail!');
@@ -984,7 +984,7 @@ reset(callback: AsyncCallback\<void>): void
 
 ```js
 videoPlayer.reset((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('reset success!');
     } else {
         console.info('reset fail!');
@@ -1036,7 +1036,7 @@ seek(timeMs: number, callback: AsyncCallback\<number>): void
 ```js
 let seekTime = 5000;
 videoPlayer.seek(seekTime, (err, result) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('seek success!');
     } else {
         console.info('seek fail!');
@@ -1067,7 +1067,7 @@ import media from '@ohos.multimedia.media'
 let seekTime = 5000;
 let seekMode = media.SeekMode.SEEK_NEXT_SYNC;
 videoPlayer.seek(seekTime, seekMode, (err, result) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('seek success!');
     } else {
         console.info('seek fail!');
@@ -1133,7 +1133,7 @@ setVolume(vol: number, callback: AsyncCallback\<void>): void
 ```js
 let vol = 0.5;
 videoPlayer.setVolume(vol, (err, result) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('setVolume success!');
     } else {
         console.info('setVolume fail!');
@@ -1190,7 +1190,7 @@ release(callback: AsyncCallback\<void>): void
 
 ```js
 videoPlayer.release((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('release success!');
     } else {
         console.info('release fail!');
@@ -1248,7 +1248,7 @@ function printfDescription(obj) {
 }
 
 videoPlayer.getTrackDescription((error, arrlist) => {
-    if (typeof (arrlist) != 'undefined') {
+    if (arrlist) != null) {
         for (let i = 0; i < arrlist.length; i++) {
             printfDescription(arrlist[i]);
         }
@@ -1285,7 +1285,7 @@ function printfDescription(obj) {
 
 let arrayDescription;
 videoPlayer.getTrackDescription().then((arrlist) => {
-    if (typeof (arrlist) != 'undefined') {
+    if (arrlist != null) {
         arrayDescription = arrlist;
     } else {
         console.log('video getTrackDescription fail');
@@ -1320,7 +1320,7 @@ import media from '@ohos.multimedia.media'
 let speed = media.PlaybackSpeed.SPEED_FORWARD_2_00_X;
 
 videoPlayer.setSpeed(speed, (err, result) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('setSpeed success!');
     } else {
         console.info('setSpeed fail!');
@@ -1381,7 +1381,7 @@ selectBitrate(bitrate:number, callback: AsyncCallback\<number>): void
 ```js
 let bitrate = 1024000;
 videoPlayer.selectBitrate(bitrate, (err, result) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('selectBitrate success!');
     } else {
         console.info('selectBitrate fail!');
@@ -1639,7 +1639,7 @@ function printfItemDescription(obj, key) {
 }
 
 audioPlayer.getTrackDescription((error, arrlist) => {
-    if (typeof (arrlist) != 'undefined') {
+    if (arrlist != null) {
         for (let i = 0; i < arrlist.length; i++) {
             printfItemDescription(arrlist[i], MD_KEY_TRACK_TYPE);  //打印出每条轨道MD_KEY_TRACK_TYPE的值
         }
@@ -1995,7 +1995,7 @@ let eventEmitter = new events.EventEmitter();
 
 eventEmitter.on('prepare', () => {
     videoRecorder.prepare(videoConfig, (err) => {
-        if (typeof (err) == 'undefined') {
+        if (err == null) {
             console.info('prepare success');
         } else {
             console.info('prepare failed and error is ' + err.message);
@@ -2004,7 +2004,7 @@ eventEmitter.on('prepare', () => {
 });
 
 media.createVideoRecorder((err, recorder) => {
-    if (typeof (err) == 'undefined' && typeof (recorder) != 'undefined') {
+    if (err == null && recorder) != null) {
         videoRecorder = recorder;
         console.info('createVideoRecorder success');
         eventEmitter.emit('prepare');                                        // prepare事件触发
@@ -2064,7 +2064,7 @@ let videoConfig = {
 // promise
 let videoRecorder = null;
 media.createVideoRecorder().then((recorder) => {
-    if (typeof (recorder) != 'undefined') {
+    if (recorder != null) {
         videoRecorder = recorder;
         console.info('createVideoRecorder success');
     } else {
@@ -2105,7 +2105,7 @@ getInputSurface(callback: AsyncCallback\<string>): void;
 // asyncallback
 let surfaceID = null;                                               // 传递给外界的surfaceID
 videoRecorder.getInputSurface((err, surfaceId) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('getInputSurface success');
         surfaceID = surfaceId;
     } else {
@@ -2166,7 +2166,7 @@ start(callback: AsyncCallback\<void>): void;
 ```js
 // asyncallback
 videoRecorder.start((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('start videorecorder success');
     } else {
         console.info('start videorecorder failed and error is ' + err.message);
@@ -2222,7 +2222,7 @@ pause(callback: AsyncCallback\<void>): void;
 ```js
 // asyncallback
 videoRecorder.pause((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('pause videorecorder success');
     } else {
         console.info('pause videorecorder failed and error is ' + err.message);
@@ -2276,7 +2276,7 @@ resume(callback: AsyncCallback\<void>): void;
 ```js
 // asyncallback
 videoRecorder.resume((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('resume videorecorder success');
     } else {
         console.info('resume videorecorder failed and error is ' + err.message);
@@ -2330,7 +2330,7 @@ stop(callback: AsyncCallback\<void>): void;
 ```js
 // asyncallback
 videoRecorder.stop((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('stop videorecorder success');
     } else {
         console.info('stop videorecorder failed and error is ' + err.message);
@@ -2384,7 +2384,7 @@ release(callback: AsyncCallback\<void>): void;
 ```js
 // asyncallback
 videoRecorder.release((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('release videorecorder success');
     } else {
         console.info('release videorecorder failed and error is ' + err.message);
@@ -2438,7 +2438,7 @@ reset(callback: AsyncCallback\<void>): void;
 ```js
 // asyncallback
 videoRecorder.reset((err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('reset videorecorder success');
     } else {
         console.info('reset videorecorder failed and error is ' + err.message);