App.js

import React, { Component } from 'react';
import backgroundTask from 'react-native-background-task';

backgroundTask.define(() => {
  console.log('Background Task Activated!');
  backgroundTask.finish();
});

export default class App extends Component {
  componentDidMount() {
    backgroundTask.schedule();
    backgroundTask.statusAsync().then(status => console.log(status));
    // { available: true }
  }
  render() { return null }
}