Create function in iframe from parent so that it has Function prototype

I have an application where an iframe loads a Blazor application that needs to use a function (invoked from Blazor) available only in the main page. I can add the parent function into the iframe's window successfully, but the Blazor library performs an instanceof Function check, which for some reason is not true when loading a function to an iframe's contentWindow this way, even though the function itself is executable.

Is there a way to construct this so instanceof Function returns true? findJsFunction is provided by Blazor and cannot be modified.

I've tried:

manually setting the prototype to Function
using parent.test
defining the function test also in the iframe and then overwriting it

main

<iframe id="myFrame" src="url"></iframe>
<script>
    function test() {
        console.info('test');
    }
    document.querySelector('#myFrame').contentWindow.test = test;
</script>

iframe

<!-- blazor library does something like this -->
<script>
    function findJsFunction(name) {
        // the below is what fails because window[name] is never instanceof Function right now
        // is there a way I can get this to return true?
        if (window[name] instanceof Function) {
            window[name]();
        }
    }
</script>
<button onclick="findJsFunction('test')">Click me</button>

Solution

console.log(Function == document.querySelector('#myFrame').contentWindow.Function)

returns false, so that is likely the cause of your issue.

Using Object.setPrototypeOf to set the prototype of the test function in the iframe, to the Function of the iframe, appears to work:

<iframe id="myFrame" srcdoc="&lt;button onclick=&quot;console.log(window.test instanceof Function)&quot;&gt;Click me&lt;/button&gt;"></iframe>

<script>
    function test() {
        console.info('test');
    }
    document.querySelector('#myFrame').contentWindow.test = test;
    Object.setPrototypeOf(document.querySelector('#myFrame').contentWindow.test,
        document.querySelector('#myFrame').contentWindow.Function)
</script>

That throws security errors here in the snippet, but under https://jsfiddle.net/2j8oLgmt/ I am getting true in the console, when clicking the button in the iframe. (You need to check the real browser console, not the one jsfiddle itself provides.)