"expo": {
 "extra": {
   "fonts": [
     {
       "name": "Roboto",
       "asset": "assets/fonts/Roboto-Regular.ttf"
     }
   ]
 }
}

import * as Font from 'expo-font';
import { Text } from 'react-native';

export default class App extends React.Component {
  state = {
    fontLoaded: false,
  };

  async componentDidMount() {
    await Font.loadAsync({
      'Roboto': require('./assets/fonts/Roboto-Regular.ttf'),
    });
    this.setState({ fontLoaded: true });
  }

  render() {
    return this.state.fontLoaded ? (
      Hello, world!
    ) : null;
  }
}

Hello, world!