How to restrict app to portrait mode only in ionic for all platforms?

If you want to restrict to portrait mode only on all devices you should add this line to the config.xml in your projects root folder.

<preference name="orientation" value="portrait" />

After that, please rebuild the platform by typing below text in command line:

ionic build

Ionic v2+ Update

For Ionic versions 2 and later, you will need to also install the corresponding Ionic Native package for any plugin you use, including cordova-plugin- and phonegap-plugin- plugins.

1. Install Ionic Native Plugin

   Install Ionic Native's TypeScript module for the plugin from the CLI.

   $ ionic cordova plugin add --save cordova-plugin-screen-orientation
   $ npm install --save @ionic-native/screen-orientation
   

   * Note that the --save command is optional and can be omitted if you do not wish to add the plugin to your package.json file

2. Import ScreenOrientation Plugin

   Import plugin into your controller, more details available in the documentation.

   import { ScreenOrientation } from '@ionic-native/screen-orientation';
   
   @Component({
       templateUrl: 'app.html',
       providers: [
           ScreenOrientation
       ]
   })
   
   constructor(private screenOrientation: ScreenOrientation) {
     // Your code here...
   }
   

3. Do Your Thing

   Lock and unlock the screen orientation programatically.

   // Set orientation to portrait
   this.screenOrientation.lock(this.screenOrientation.ORIENTATIONS.PORTRAIT);
   
   // Disable orientation lock
   this.screenOrientation.unlock();
   

4. Bonus Points

   You can also get the current orientation.

   // Get current orientation
   console.log(this.screenOrientation.type);  // Will return landscape" or "portrait"
   

If you want to do something on your orientation means you want to perform any action when change your app orientation then you have to use ionic framework plugin... https://ionicframework.com/docs/native/screen-orientation/

If you just want to restrict your app on portrait or landscape mode then you have to just add these following line in your config.xml

<preference name="orientation" value="portrait" />
                 OR
<preference name="orientation" value="landscape" />

According to https://cordova.apache.org/docs/en/4.0.0/config_ref/#global-preferences,

Orientation allows you to lock orientation and prevent the interface from rotating in response to changes in orientation. Possible values are default, landscape, or portrait. Example:

<preference name="Orientation" value="landscape" />

Be aware that these values are case sensitive, it is "Orientation", not "orientation".