Error message in expo-service-worker.js on project start

[alaindresse]

Description

expo-service-worker.js generates an error in the log when listening to message events because event.data is not necessarily of type string (I have 4 messages in which event.data is a json object, not a string).

Expected Behavior

I just launched a website created from an expo project, and the errors appeared. I don't know what sends these messages (but I suspect it is firebase related).

Observed Behavior

I get the following message in my console (4 displays of this error)

Uncaught SyntaxError: Unexpected token o in JSON at position 1
    at JSON.parse (<anonymous>)
    at expo-service-worker.js:8
(anonymous) @ expo-service-worker.js:8

Environment

Expo CLI 3.4.1 environment info:
    System:
      OS: macOS 10.14.6
      Shell: 3.2.57 - /bin/bash
    Binaries:
      Node: 10.16.0 - ~/.nvm/versions/node/v10.16.0/bin/node
      npm: 6.11.3 - ~/.nvm/versions/node/v10.16.0/bin/npm
      Watchman: 4.9.0 - /usr/local/bin/watchman
    IDEs:
      Android Studio: 3.5 AI-191.8026.42.35.5791312
      Xcode: 11.1/11A1027 - /usr/bin/xcodebuild
    npmPackages:
      @types/react: ^16.9.5 => 16.9.5 
      @types/react-native: ^0.57.65 => 0.57.65 
      expo: ^35.0.0 => 35.0.0 
      react: 16.8.3 => 16.8.3 
      react-native: https://github.com/expo/react-native/archive/sdk-35.0.0.tar.gz => 0.59.8 
    npmGlobalPackages:
      expo-cli: 3.4.1

Reproducible Demo

I'll try to create a project that illustrates the issue.

I believe the following should do it, but I'm a bit short on time (will try it tomorrow)

import * as React from 'react';
import { View,Button } from 'react-native';

export default class App extends React.Component {
  handlePress = ()=>{
     navigator.serviceWorker.controller.postMessage({content:"non text message"});
  }
  render() {
    return (
      <View>
          <Button onPress={this.handlePress} title="Send message"/>
      </View>
    );
  }
}

[alaindresse]

A github public project illustrating the issue is available here: https://github.com/alaindresse/post-message-error.
To reproduce,

• build the web project

  expo build:web
  

• then serve the build folder

  serve web-build
  

• click on the send message button
• open the console to see the error message.

[EvanBacon]

fixed with #1154. Also thanks so much for the example, it was extremely helpful for debugging quickly.