TestClass.tsx

import * as React from 'react';
import { Component } from 'react';
import { Text } from 'react-native';

interface IProps {
  count?: number;
}
interface IState {
  count: number;
}

class TestClass extends Component<IProps, IState>  {

  state: IState = {
    count: this.props.count || 0,
  };

  render() {
    return(<Text testID='counter'>{this.state.count}</Text>);
  }
}

export default TestClass;