How can I generate an apk that can run without server with react-native?

You will have to create a key to sign the apk. Use below to create your key:

 keytool -genkey -v -keystore my-app-key.keystore -alias my-app-alias -keyalg RSA -keysize 2048 -validity 10000

Use a password when prompted

Once the key is generated, use it to generate the installable build:

 react-native bundle --platform android --dev false --entry-file index.android.js --bundle-output android/app/src/main/assets/index.android.bundle --assets-dest android/app/src/main/res/

Generate the build using gradle

 cd android && ./gradlew assembleRelease

Upload the APK to your phone. The -r flag will replace the existing app (if it exists)

adb install -r ./app/build/outputs/apk/app-release-unsigned.apk

A more detailed description is mentioned here: https://facebook.github.io/react-native/docs/signed-apk-android.html

UPDATE: Based on comments by @shashuec and @Fallen

if you get error

ENOENT: no such file or directory, open 'android/app/src/main/assets/index.android.bundle'

run mkdir android/app/src/main/assets

Following Aditya Singh's answer the generated (unsigned) apk would not install on my phone. I had to generate a signed apk using the instructions here.

The following worked for me:

$ keytool -genkey -v -keystore my-release-key.keystore -alias my-key-alias -keyalg RSA -keysize 2048 -validity 10000

Place the my-release-key.keystore file under the android/app directory in your project folder. Then edit the file ~/.gradle/gradle.properties and add the following (replace **** with the correct keystore password, alias and key password)

MYAPP_RELEASE_STORE_FILE=my-release-key.keystore
MYAPP_RELEASE_KEY_ALIAS=my-key-alias
MYAPP_RELEASE_STORE_PASSWORD=****
MYAPP_RELEASE_KEY_PASSWORD=****

If you're using MacOS, you can store your password in the keychain using the instructions here instead of storing it in plaintext.

Then edit app/build.gradle and ensure the following are there (the sections with signingConfigs signingConfig may need to be added) :

...
android {
    ...
    defaultConfig { ... }
    signingConfigs {
        release {
            if (project.hasProperty('MYAPP_RELEASE_STORE_FILE')) {
                storeFile file(MYAPP_RELEASE_STORE_FILE)
                storePassword MYAPP_RELEASE_STORE_PASSWORD
                keyAlias MYAPP_RELEASE_KEY_ALIAS
                keyPassword MYAPP_RELEASE_KEY_PASSWORD
            }
        }
    }
    buildTypes {
        release {
            ...
            signingConfig signingConfigs.release
        }
    }
}
...

Then run the command cd android && ./gradlew assembleRelease ,

For Windows 'cd android' and then run gradlew assembleRelease command , and find your signed apk under android/app/build/outputs/apk/app-release.apk, or android/app/build/outputs/apk/release/app-release.apk