Webview sendMessage function

sendMessage.js

 <Map
  style={{
    width: '100%',
    backgroundColor: 'lightblue'
  }}
  ...
  // handling a react-leaflet map click event
  onClick={(event) => {
      this.onMapEvent('onMapClicked', {
        coords: [event.latlng.lat, event.latlng.lng]
      });
    }}
  ...
/>

// An event has occured in the react-leaflet map rendered by the webpage
  onMapEvent = (event, payload) => {
    // build a default payload consisting of map information if one is not provided
    if (!payload) {
      payload = {
        center: this.mapRef.current.leafletElement.getCenter(),
        bounds: this.mapRef.current.leafletElement.getBounds(),
        zoom: this.mapRef.current.leafletElement.getZoom()
      };
    }

    this.sendMessage({
      event,
      payload
    });
  }; 

// data to send is an object containing key value pairs that will be
  // spread into the destination's state or seen as an event
  sendMessage = (payload) => {
    const message = JSON.stringify({
      prefix: MESSAGE_PREFIX,
      payload: payload
    });

    if (document.hasOwnProperty('postMessage')) {
      document.postMessage(message, '*');
    } else if (window.hasOwnProperty('postMessage')) {
      window.postMessage(message, '*');
    } else {
      console.log('unable to find postMessage');
    }
  };