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 {
@@ -307,7 +307,7 @@ 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;
     }
@@ -380,7 +380,7 @@ 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]);
         }
@@ -416,7 +416,7 @@ function printfDescription(obj) {
 }
 
 audioPlayer.getTrackDescription().then((arrlist) => {
-    if (typeof (arrlist) != 'undefined') {
+    if (arrlist != null) {
         arrayDescription = arrlist;
     } else {
         console.log('audio getTrackDescription fail');
@@ -491,7 +491,7 @@ 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;
     }
@@ -546,7 +546,7 @@ 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;
     }
@@ -634,7 +634,7 @@ 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!');
@@ -690,7 +690,7 @@ 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!');
@@ -740,7 +740,7 @@ 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!');
@@ -790,7 +790,7 @@ 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!');
@@ -840,7 +840,7 @@ 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!');
@@ -890,7 +890,7 @@ 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!');
@@ -941,7 +941,7 @@ seek(timeMs: number, callback: AsyncCallback\<number>): void
 
 ```js
 videoPlayer.seek((seekTime, err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('seek success!');
     } else {
         console.info('seek fail!');
@@ -969,7 +969,7 @@ seek(timeMs: number, mode:SeekMode, callback: AsyncCallback\<number>): void
 
 ```js
 videoPlayer.seek((seekTime, seekMode, err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('seek success!');
     } else {
         console.info('seek fail!');
@@ -1033,7 +1033,7 @@ setVolume(vol: number, callback: AsyncCallback\<void>): void
 
 ```js
 videoPlayer.setVolume((vol, err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('setVolume success!');
     } else {
         console.info('setVolume fail!');
@@ -1089,7 +1089,7 @@ 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!');
@@ -1147,7 +1147,7 @@ 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]);
         }
@@ -1184,7 +1184,7 @@ function printfDescription(obj) {
 
 let arrayDescription;
 videoPlayer.getTrackDescription().then((arrlist) => {
-    if (typeof (arrlist) != 'undefined') {
+    if (arrlist != null) {
         arrayDescription = arrlist;
     } else {
         console.log('video getTrackDescription fail');
@@ -1216,7 +1216,7 @@ setSpeed(speed:number, callback: AsyncCallback\<number>): void
 
 ```js
 videoPlayer.setSpeed((speed:number, err) => {
-    if (typeof (err) == 'undefined') {
+    if (err == null) {
         console.info('setSpeed success!');
     } else {
         console.info('setSpeed fail!');
@@ -1437,7 +1437,7 @@ 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
         }