I should also add that I attempted to replicate the error on Android 7.0 and API 24 using a Nexus_5x emulator, and I got NO error. So this seems to be a platform specific error.

I'm getting the same error. Seems like this exception is thrown when window.postMessage has been overwritten. But since webViewDidFinishLoad is called multiple times, it tries to overwrite window.postMessage over and over. Not sure though. Can we have you expertise @jacobp100?

It’s because the page you’re visiting is overriding postMessage, which may be due to a shim. Showing an error was chosen over invoking the shim’s value of postMessage when you call the value of postMessage that we set.

I thought I’d made it easier to workaround this problem, but it looks like that won’t work. The best you can do at the moment, which is an awful hack, is,

injectedJavaScript="window.postMessage = String(Object.hasOwnProperty).replace('hasOwnProperty', 'postMessage');"

Or just fork the project and remove the code you pointed to. Whatever’s easiest for you!

Ping @satya164 for input on how to resolve.

Well, when I try String(window.postMessage) === String(Object.hasOwnProperty).replace('hasOwnProperty', 'postMessage') on the page I visit, it returns true, so I am guessing postMessage has not been overridden.

Oh I see. The JS is injected after postMessage is overwritten. Your best bet is to fork it then, and I’ll see what I can do about getting this fixed! Sorry. :(

I have same error, without injecting any javascript to WebView - just setting WebView-properties:

source={require('../lib/html/somefile.html')}
onMessage={() => console.log('Hello World')}

and in my ../lib/html/somefile.html' I have javascript code doing both window.postMessage() and document.addEventListener("message",.......

Everything worked fine when I had the HTML in a variable "someHtml", like:
source={{html: someHtml}}

So difference seem to be when changing source to HTML-file instead of HTML-string.

Still for me I had other reasons, not related to this, why I really need to read HTML from file instead of string so it would be great to get this issue fixed.

+1, still has this problem

There is an ongoing discussion in #10941 for the best way to go forward with this.

It won't be in the next release or probably the release after. If you need this fixed now, fork the project and use the fix in the PR.

+1, I'm getting the same error. When there is a iframe on web, it occurs the same error

+1，encounter this problem

Again, there is a pr for this. We're aware this problem is affecting a lot of people.

+1 getting the same error.

injectedJavascript should instead be the following

(function() {
  var originalPostMessage = window.postMessage;

  var patchedPostMessage = function(message, targetOrigin, transfer) { 
    originalPostMessage(message, targetOrigin, transfer);
  };

  patchedPostMessage.toString = function() { 
    return String(Object.hasOwnProperty).replace('hasOwnProperty', 'postMessage'); 
  };

  window.postMessage = patchedPostMessage;
})();

And then it works! 😄

And if any one is interested, in my code instead of writing a giant string I did the following

const patchPostMessageFunction = function() {
  var originalPostMessage = window.postMessage;

  var patchedPostMessage = function(message, targetOrigin, transfer) { 
    originalPostMessage(message, targetOrigin, transfer);
  };

  patchedPostMessage.toString = function() { 
    return String(Object.hasOwnProperty).replace('hasOwnProperty', 'postMessage');
  };

  window.postMessage = patchedPostMessage;
};

const patchPostMessageJsCode = '(' + String(patchPostMessageFunction) + ')();';

...

<WebView injectedJavaScript={patchPostMessageJsCode} />

@kyle-ilantzis tried this with RN 0.38.1, but still get the Red Screen. What version of RN are you on?