How do you execute Javascript in React-Native WebView?

I'm trying to execute javascript in a WebView that is loaded on an iOS advice. I'm trying to get a painfully simple example to work but it is consistently throwing an undefined or TypeError.

Here is the code:

import React, { Component } from 'react';
import { StyleSheet, Text, View, Button } from 'react-native';
import { WebView } from 'react-native-webview';

export default class App extends Component {

  constructor(props){
    super(props)
    this.onPressButton = this.onPressButton.bind(this)
  }

  onPressButton(){
    this.webview.injectJavascript(`alert('hello')`)
  }

  render() {
    return (
      <View style={{ height: '100%' }}>
        <WebView
          ref={ref => (this.webview = ref)}
          javaScriptEnabled={true}
          source={{ uri: 'https://google.com' }}
        />
        <Button onPress={this.onPressButton} style={{ height: '10%' }} title="Test Javascript" />
      </View>

    );
  }
}

Please don't recommend to use injectedJavascript because that is not my desired functionality.

Would appreciate any other approaches as well.