How to handle malformed response from WP REST API?

Question

Happy day scenario: I implement WP REST API that sends JSONs to a JavaScript client, which parses it and displays the data to the user. All is fine.

What actually happens, inevitably: some WordPress code or plugin generates a PHP notice or warning, and the server is configured so that this garbage finds its way to the JSON response. (WP REST API tries to avoid that but for example if ini_set() is disallowed by server configuration, it can still happen.)

One way to approach is to simply blame it on the users – if their servers are configured so that they send PHP notices and warnings in production it's certainly bad. However, even the core REST infrastructure tries to support users who have badly configured servers and it's my goal as well in this case.

Is there a technique established for dealing with this? I can see two ways:

1. Detect in JavaScript that the response is malformed, show some error and refuse to work.
2. Modify the actual JSON by adding some unique key to it on the server and let the JavaScript client work with that. For example, instead of the plain value 123, it would be wrapped into an object like { "__VALID__": true, "data": 123 } and the JS code would use response.body.data instead of response.body.

Or is there some other approach?

Answer 1

If you are creating classes, the first thing in the construct would be this. Or you can drop it in with an action as early as possible. Maybe plugins_loaded.

/*
     * If WP DEBUG is not on do NOT return any php warning, notices, and/or fatal errors.
     * Well If it is a fatal error then this return is FUBAR anyway...
     * We do this because some badly configured servers will return notices and warnings switch get prepended or appended to the rest response.
     */
    if( defined('WP_DEBUG') ){
        if( false === WP_DEBUG ){
            error_reporting(0);
        }
    }

Answer 2

You can not fix in your own plugin general problems caused by other parts of the software as there are just too many possibilities. It is the site owner's responsibility to run a clean operation if he expects to be able to use additional plugin/themes.

The only reasonable approach that you may have is to log this kind of responses in the browser console so it will be easier for you to track down why things are not working when a plugin user complains.

Answer 3

You need to use a combination of exception catching and a custom error handler.

If you use this to suppress errors to the end user, you might want to add in some method that flags the error to the administrator/developer/yourself, so you are at least aware of the issue.

Alternatively, instead of suppressing the errors, you could return an object with both the return value, and an array of any errors, to the client. It would then be up to the client whether it did anything with that information. Whether you want to do this depends on who the clients are, and what sensitive information might be exposed by these errors. Of course you have complete control over what gets output, you don't need to just output the notices word for word.