To find a reCAPTCHA v2 success handler, check the widget’s data-callback attribute or the callback option passed to grecaptcha.render. Google’s documented API does not provide a supported way to inject an arbitrary token into the widget and trigger a genuine success callback. Calling your own handler with a test value can exercise your application code, but only server-side verification establishes whether a response is valid.
How do I find the reCAPTCHA v2 callback function?
The callback is an application function configured when the widget is created. Google calls it after a successful response and passes the response token as its argument. The exact function name depends on the site’s code; it cannot be determined from the widget alone.
Automatically rendered widget
Inspect the widget markup for data-callback. For example:
<div class="g-recaptcha" data-sitekey="..." data-callback="onCaptchaSuccess"></div>Here, onCaptchaSuccess is the configured handler. Search the page’s scripts or source maps for that name to find its implementation. Google documents this attribute in its reCAPTCHA v2 display documentation, last updated October 8, 2024.
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Explicitly rendered widget
If the page renders the widget in JavaScript, inspect the options passed to grecaptcha.render:
const widgetId = grecaptcha.render('captcha', {
sitekey: 'YOUR_SITE_KEY',
callback: onCaptchaSuccess
});The callback is configured through the callback option. Explicit rendering returns a widget ID, which you can retain when working with that widget, especially if the page has more than one.
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When no success callback is configured
After a user completes the challenge, you can retrieve the response with grecaptcha.getResponse(widgetId). If the ID is omitted, Google’s API uses the first widget. Use the correct ID when there are multiple widgets. To clear a widget, use grecaptcha.reset(widgetId). The display documentation also describes data-expired-callback / expired-callback for expiration and data-error-callback / error-callback for errors.
How should the callback be wired?
Define the API onload function before loading the reCAPTCHA script. Google warns that the onload function must exist before the API loads; use async and defer as shown in its explicit-render guidance to avoid a load race.
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<div id="captcha"></div>
<script>
function onCaptchaSuccess(responseToken) {
// Send the response to your application server for verification.
submitResponseForServerVerification(responseToken);
}
function onCaptchaExpired() {
// Ask the user to complete the challenge again.
}
function onCaptchaError() {
// Tell the user to retry when connectivity is restored.
}
function onRecaptchaApiLoaded() {
window.captchaWidgetId = grecaptcha.render('captcha', {
sitekey: 'YOUR_SITE_KEY',
callback: onCaptchaSuccess,
'expired-callback': onCaptchaExpired,
'error-callback': onCaptchaError
});
}
</script>
<script src="https://www.google.com/recaptcha/api.js?onload=onRecaptchaApiLoaded&render=explicit" async defer></script>The success handler receives the response token as a string. The sample’s submission function is application-specific; replace it with the code that sends the response to your own server.
Can I trigger the callback after injecting a token?
Not as a genuine reCAPTCHA success. Putting a string in an input, passing it directly to your application handler, or otherwise supplying it from page code does not make it a verified response. Google’s documented widget API does not support injecting an arbitrary token into a widget and manually triggering its success callback.
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You may call your own handler with a fixture to test what your application does next—for example, whether it updates the interface or sends a request. That tests your handler and downstream code only; it does not test Google’s token validation or turn the fixture into a valid challenge response.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do I programmatically run an invisible reCAPTCHA v2 challenge?
For an invisible v2 widget, configure it as invisible and call grecaptcha.execute(widgetId) when the application needs to start the challenge. After successful completion, the configured callback receives the response. Starting the challenge with execute() is different from retrieving the response with getResponse(), and neither replaces server verification. See Google’s invisible reCAPTCHA documentation.
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How must the server verify the response?
Send the response token and your secret from the server to Google’s siteverify endpoint, then base your application’s decision on Google’s verification response. Keep the secret on the server, not in client-side code. Google states that each response token is valid for two minutes and can be verified only once, to prevent replay attacks. An expired or already-used token cannot serve as a fresh successful verification; see Google’s response verification documentation.
Testing without confusing a callback with verification
- Unit test: Call your application’s success handler with a fixture to check downstream UI or request handling. Treat it as a handler test, not a real challenge or verification.
- Integration test: Use an authorized test configuration and exercise the documented widget and server-verification path. Do not represent a fabricated token as a successful challenge.
The cited API documentation establishes the widget and verification requirements, but not a framework-specific testing procedure. How to structure a test around your application depends on its code and framework.
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