openalpr.jsx

import React, { Component } from 'react';
import {
  StyleSheet,
  Text,
  View,
} from 'react-native';
import Camera from 'react-native-openalpr';

const styles = StyleSheet.create({
  container: {
    flex: 1,
  },
  textContainer: {
    position: 'absolute',
    top: 100,
    left: 50,
  },
  text: {
    textAlign: 'center',
    fontSize: 20,
  },
});

export default class PlateRecognizer extends React.Component {
  constructor(props) {
    super(props);
    this.camera = null;
    this.state = {
      camera: {
        aspect: Camera.constants.Aspect.fill,
      },
      plate: 'Scan a plate',
    };
  }

  onPlateRecognized = ({ plate, confidence }) => {
    if (confidence > 0.9) {
      this.setState({
        plate,
      })
    }
  }

  render() {
    return (
      <View style={styles.container}>
        <Camera
          ref={(cam) => { this.camera = cam }}
          style={styles.container}
          aspect={this.state.camera.aspect}
          captureQuality={Camera.constants.CaptureQuality.medium}
          onPlateRecognized={this.onPlateRecognized}
          showPlateOutline
          torchMode={Camera.constants.TorchMode.off}
          touchToFocus
        />
        <View style={styles.textContainer}>
          <Text style={styles.text}>{this.state.plate}</Text>
        </View>
      </View>

    );
  }
}

you're PlateRecognizer doesnt need to define a constructor function:

export default class PlateRecognizer extends React.Component {
  camera = null
  state = {
    camera: {
      aspect: Camera.constants.Aspect.fill,
    }
    plate: 'Scan a plate',
  }
  // ...
}