When the WooCommerce checkout is processed it requires that gateways return an array (which is then converted into JSON) telling the checkout whether or not it was successful. It’s been this way since v1. Here is a basic example from the PayPal gateway:

return array(
    'result'   => 'success',
    'redirect' => $paypal_request->get_request_url( $order, $this->testmode )
);

The above for example returns a successful response and tells the checkout to redirect the user to the URL provided.

In 2.4+ this was made more strict in that the response must be valid JSON. Anything else in the response (such as HTML, notices or whitespace) will invalidate this JSON and you may see an error along the lines of:

SyntaxError: Unexpected token

WooThemes own gateways have all been tested for 2.4, however, we’ve seen some less-supported gateways break due to returning invalid content.

I’ve seen a few vocal users complaining about this recently on the forums (some even feeling the need to attack me personally, thanks guys) and thought it best to post a few steps on how you can narrow down the issue.

First off its always best to rule out your plugins and theme by disabling them, even though a lot of users seem to ignore this step. It is only temporary and helps narrow down the issue. Much better than shouting at me on WordPress.org.

Lets assume this does not narrow down the issue for a moment. How would you know if content is being output? The answer is in your browser console.

Lets simulate an error. In the paypal gateway, just before the array is returned I’ll add:

trigger_error( 'Uh oh' );

This triggers the error as expected:

2015-11-12 at 16.29

Now lets try to debug this like someone who does not know where the error is coming from.

In Chrome, go to View > Developer > Javascript Console whilst viewing the checkout.

Now trigger the error again.

After the error is shown, in the console go to the Network tab and click XHR:

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In the list you’re see one which looks like this: ?wc-ajax=checkout. Click on that and go to the response tab:

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Notice in my above example, there is the notice. This invalidates the JSON.

A valid JSON response {starts and ends with a bracket} – if there is an error, white space or content before or after those brackets, that is your issue.

In some cases the error will be obvious and allow you to patch or disable the cause.

Hopefully that helps.