dotnet/maui · info · Error

This is an async test error thrown from JavaScript code!

Error message

This is an async test error thrown from JavaScript code!

What it means

A deliberate asynchronous JavaScript Error thrown by the sample's ThrowJavaScriptErrorAsync() function after a 100ms setTimeout. Like its sync counterpart it exists only to verify that C# can catch exceptions from async JS invoked through the HybridWebView bridge. It is sample/test code.

Source

Thrown at src/Controls/samples/Controls.Sample/Resources/Raw/HybridSamplePage/index.html:131

            try {
                const result = await window.HybridWebView.InvokeDotNet('ThrowExceptionAsync');
                LogMessage("ThrowExceptionAsync unexpectedly succeeded with result: " + result);
            } catch (ex) {
                LogMessage("Caught async exception from ThrowExceptionAsync: " + ex.message);
                LogMessage("Exception type: " + (ex.dotNetErrorType || "Unknown"));
            }
        }

        // JavaScript functions that throw exceptions (for testing C# exception handling of JS calls)
        function ThrowJavaScriptError() {
            LogMessage("ThrowJavaScriptError called - about to throw");
            throw new Error("This is a test error thrown from JavaScript code!");
        }

        async function ThrowJavaScriptErrorAsync() {
            LogMessage("ThrowJavaScriptErrorAsync called - about to throw");
            await new Promise(resolve => setTimeout(resolve, 100));
            throw new Error("This is an async test error thrown from JavaScript code!");
        }

    </script>
</head>
<body>
    <div>
        Hybrid sample!
    </div>
    <div>
        <button onclick="window.HybridWebView.SendRawMessage('Message from JS! ' + (count++))">Send message to C#</button>
    </div>
    <div>
        <button onclick="InvokeDoSyncWork()">Call C# sync method (no params)</button>
        <button onclick="InvokeDoSyncWorkParams()">Call C# sync method (params)</button>
        <button onclick="InvokeDoSyncWorkReturn()">Call C# method (no params) and get simple return value</button>
        <button onclick="InvokeDoSyncWorkParamsReturn()">Call C# method (params) and get complex return value</button>
    </div>
    <div>

View on GitHub (pinned to f377ff1c5e)

Solutions

If you reused the sample HTML, remove ThrowJavaScriptErrorAsync or unwire it from production UI.
If testing, await the bridge call inside try/catch on the C# side to observe the rejection.

Example fix

// before
async function ThrowJavaScriptErrorAsync() {
  await new Promise(resolve => setTimeout(resolve, 100));
  throw new Error("This is an async test error thrown from JavaScript code!");
}

// after
async function DoRealAsyncWork() {
  await new Promise(resolve => setTimeout(resolve, 100));
  return "ok";
}

Defensive patterns

Strategy: try-catch

Try / catch

try
{
    await hybridWebView.EvaluateJavaScriptAsync("ThrowJavaScriptErrorAsync()");
}
catch (Exception ex)
{
    Debug.WriteLine($"Async JS rejected: {ex.Message}");
}

Prevention

Strip demo async-throw functions from production HTML.
Await all JS-evaluation calls so promise rejections propagate to C#.
Keep test/demo JS behind a clear naming or build flag.

When it happens

Trigger: C# invokes the async function through the JS bridge, or the sample's 'Test JS Async Exception' button is clicked, causing the awaited promise to reject after the timeout.

Common situations: Running the sample JS-exception test; copying the sample HTML and leaving the demo async throw reachable; automated tests that walk sample buttons.

Related errors

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/eb04d4d51841e923. Report an issue: GitHub.