How to disable landscape mode in React Native Android dev mode?

As @morenoh149 stated above the property name and value to do this is android:screenOrientation="portrait". React Native generates a file called AndroidManifest.xml in your project directory under the Android folder. Within that xml file under the tag manifest/application/activity you would add the line android:screenOrientation="portrait"

An example is shown below

<uses-permission android:name="android.permission.INTERNET" />

<application
  android:allowBackup="true"
  android:label="@string/app_name"
  android:icon="@mipmap/ic_launcher"
  android:theme="@style/AppTheme">
  <activity
    android:name=".MainActivity"
    android:label="@string/app_name"
    android:screenOrientation="portrait" 
    android:configChanges="keyboard|keyboardHidden|orientation|screenSize">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
  </activity>
  <activity android:name="com.facebook.react.devsupport.DevSettingsActivity" />
</application>

Harry Moreno's comment is correct. Add it to android/app/src/main/AndroidManifest.xml in the 'activity' section. Also change

android:configChanges="keyboard|keyboardHidden|orientation|screenSize"

to android:configChanges="keyboard|keyboardHidden|screenSize"

removing orientation so it doesn't conflict with the new line added, android:screenOrientation="portrait"

http://developer.android.com/guide/topics/manifest/activity-element.html

add android:screenOrientation="portrait" to your activity xml

Add android:screenOrientation="portrait" to the activity section in android/app/src/main/AndroidManifest.xml file, so that it end up looking like this:

<activity
    android:name=".Activity"
    android:label="Activity"
    android:screenOrientation="portrait"
    android:configChanges="keyboardHidden|orientation|screenSize">
</activity>

There are several different values for the android:screenOrientation property; for a comprehensive list take a look at the following: https://developer.android.com/guide/topics/manifest/activity-element.html