I am building an API with express and JSON middleware. I want to have a generic middleware that wraps each JSON response. The wrapping is an asynchronous process - there is a backend process that asynchronously generates a signature for the given JSON object. Example:

A route:

app.use("/", (req, res) => res.json({status: "ok"}))

The middleware would ideally intercept the JSON response, asynchronously generate a signature and ultimately send the user the following JSON:

{
    "signature": "...",
    "content": { "status": "ok" }
}

What's the best approach to building such a middleware?

I have tried replacing res.json with a custom function. The problem with this solution is that the signature generation is asynchronous, but the res.json is a synchronous function. This was my approach:

export function signatureMiddleware(req, res, next) {
    var json = res.json;
    res.json = async function (obj) {
      json.call(this, await wrapAndSign(obj));
    };
    next();
}