Update doc (16796)

Signed-off-by: Nester.zhou <ester.zhou@huawei.com>

en/application-dev/reference/arkui-js-lite/js-framework-js-file.md

 # app.js
 
+## Application Lifecycle<sup>4+</sup>
 
 You can implement lifecycle logic specific to your application in the **app.js** file. Available application lifecycle functions are as follows:
 
@@ -11,14 +12,76 @@ You can implement lifecycle logic specific to your application in the **app.js**
 
 In the following example, logs are printed only in the lifecycle functions.
 
+
+
+```js
+// app.js
+export default {
+  onCreate() {
+    console.info('Application onCreate');
+  },
+  onDestroy() {
+    console.info('Application onDestroy');
+  },
+}
 ```
+
+## Application Object<sup>10+</sup>
+
+| Attribute    | Type      | Description                                      |
+| ------ | -------- | ---------------------------------------- |
+| getApp | Function | Obtains the data object exposed in **app.js** from the page JS file. This API works globally.|
+
+> **NOTE**
+>
+> The application object is global data and occupies JS memory before the application exits. Although it facilitates data sharing between different pages, exercise caution when using it on small-system devices, whose memory is small. If they are overused, exceptions may occur due to insufficient memory when the application displays complex pages.
+
+The following is an example:
+
+Declare the application object in **app.js**.
+
+```javascript
 // app.js
 export default {
+    data: {
+        test: "by getAPP"
+    },
     onCreate() {
         console.info('Application onCreate');
     },
     onDestroy() {
         console.info('Application onDestroy');
     },
+};
+```
+
+Access the application object on a specific page.
+
+```javascript
+// index.js
+export default {
+    data: {
+        title: ""
+    },
+    onInit() {
+        if (typeof getApp !== 'undefined') {
+            var appData = getApp().data;
+            if (typeof appData !== 'undefined') {
+                this.title = appData.name; // read from app data
+            }
+        }
+    },
+    clickHandler() {
+        if (typeof getApp !== 'undefined') {
+            var appData = getApp().data;
+            if (typeof appData !== 'undefined') {
+                appData.name = this.title; // write to app data
+            }
+        }
+    }
 }
 ```
+
+> **NOTE**
+>
+> To ensure that the application can run properly on an earlier version that does not support **getApp**, compatibility processing must be performed in the code. That is, before using **getApp**, check whether it is available.