What worked for us was this:

injectJavaScript(
                "window.callSomeFunction(); void(0);",
              )

That didn't work 😢

I've noticed that if I pause between one call to webView.injectJavaScript and the next one with the debugger it does not throw an error

Throttling the calls to injectJavaScript seems to have fixed the problem.

this.injectJavaScript = throttle(this.injectJavaScript.bind(this), 500);

@EgidioCaprino's solution worked for me. I had to stop using the injectedJavaScript prop and used the method instead to inject the same string.

This problem still occurs for us 😢

See your JS code carefully, this error is due to the fact that WebView can not execute JS code!

The same code was working with WebView imported from react-native 😕

Android or iOS ?

just upgraded from 2.10 to the current latest and started running into this. The injection seems a wee bit more unreliable in general after the upgrade. (iOS)

This seems to happen when you inject code too early before webview is loaded, but I'm not sure

Facing it with some form submission .
Working -
const jsCodeHello = 'alert("hello")';
Not Working -

const jsCode = `
     document.getElementById("name").value = "${name}"; 
     document.f1.submit(); 
`; 

Fixed using a timer.

const jsCode = `
    setTimeout(() => {
        document.getElementById("name").value = "${name}"; 
        document.f1.submit(); 
    }, 100);
`;

When I inject stuff inside the webview i'll always wrap it inside a function and instantly execute it. It allows me to avoid wrong return types:

(function(lol){
	//do stuff
})()

@legion-zver is right. Try running that same code in a Web browser console and verify that it can execute. There is likely a minor syntax error.

i have the same issue , The same code was working with WebView imported from react-native,Who has a good solution?

This works for me!

this.webviewRef.injectJavaScript(this.onLoadGetStore);

This is really late, but for anyone else stuck on this issue and having problems finding a solution, for me the issue was that in the original react-native WebView I could just call postMessage(), but this library changed it to window.ReactNativeWebView.postMessage(). Since postMessage is now undefined, your existing code will throw an error, and the new library responds with the generic (and very unhelpful) error message:

Error evaluating injectedJavaScript: This is possibly due to an unsupported return type. Try adding true to the end of your injectedJavaScript string.

You will see this message for any JS error thrown by the webview. By updating your postMessage() calls you should be able to get things working again. Good luck!

TL;DR => Replace postMessage() calls with window.ReactNativeWebView.postMessage() to fix this error.

In my case I replaced injectedJavaScript prop with injectJavaScript() method and it fixed the error.

@sillysachin's solution worked for me, and even with a large injection on a very slow simulator, using 0 for the amount of time works just as well as 100.

This bug is present in iOS <= 9.2. For recent releases this bug is not present.

using setTimeout solved my problem as described here.
#341 (comment)

try and catch works fine, if you are adding an event to an element that does not exist you are gonna get (Error evaluating injectedJavaScript: This is possibly due to an unsupported return type. Try adding true to the end of your injectedJavaScript string.
#)
so if u use try and catch if the element does not exist u are gonna get whatever you put in catch, but if element exist you are gonna get what u put on try, don't forget to add a true at the end of the injection.

for example I used this:

const INJECTED_JS = `
      try{
            document.querySelector('form[data-refresh-athlete=true]').addEventListener('submit', _ => {
                window.ReactNativeWebView.postMessage(
                    JSON.stringify({
                        requireUserFecth: true,
                    }),
                )
            })
        } catch(e) {
            window.ReactNativeWebView.postMessage(
                JSON.stringify({
                    injected: false,
                }),
            )
        }
        true;
`

react-native-webview outputs the message "Please add true to the end of the inserted JavaScript string" regardless of the error that occurred in injectedJavaScript.
The latest version of react-native-webview adds a dump of the actual error that occurred at the end of the message.
This will help identify the actual cause.
https://github.com/react-native-community/react-native-webview/blob/5e8b4d5c2b08b982b1f80ecd34160597ec482a4f/ios/RNCWebView.m#L999

In most cases, you will see the error "WKWebView was invalidated."

This seems to happen when you inject code too early before webview is loaded, but I'm not sure

It is not because inject code too early before webview is loaded, the reason is inject code too early before window is loaded
this solution is worked。

In our error, we were trying to inject during componentWillUnmount to try and clean up some resources the webview was using. Removing this fixed it.

This seems to happen when you inject code too early before webview is loaded, but I'm not sure

issue still （some time）
this works for me

    `
      try {
        window.onload = function() {
          callFunc()
        }
        if(document.readyState === 'complete') {
         callFunc()
        }
      } catch (err) {
        console.error(err)
      }
      true;
      `
      // it's may be call function for twice

Had this problem, realized it was a function in my code that was looping indefinitely within the webview. So don't forget to check if that's happening.

Amazing still issue after 3 years. Crazy.

Yeah I'm having this issue as well.

I've noticed it only happens when I try to call window.ReactNativeWebView.postMessage

Fixed using a timer.

const jsCode = `
    setTimeout(() => {
        document.getElementById("name").value = "${name}"; 
        document.f1.submit(); 
    }, 100);
`;

this could be an alternative

<WebView 
javaScriptEnabledAndroid
javaScriptEnabled
injectedJavaScriptBeforeContentLoaded={'document.getElementById("name").value = "${name}";document.f1.submit();'}
/>

Fixed using a timer.

const jsCode = `
    setTimeout(() => {
        document.getElementById("name").value = "${name}"; 
        document.f1.submit(); 
    }, 100);
`;

this could be an alternative

<WebView 
javaScriptEnabledAndroid
javaScriptEnabled
injectedJavaScriptBeforeContentLoaded={'document.getElementById("name").value = "${name}";document.f1.submit();'}
/>

Except it doesn't because you didn't interpolate the string in the injected JS. Replace ' with ` and you're golden.