Feature request: ability to set settings per response

[bradvogel]

I'd like to implement middleware that returns formatted json in the response when pretty_ouput is present on the query. I attempted this with the following code:

  // This code does not work as expected.
  app.use(function(req, res, next) {
    // Querystring `pretty_output` should prettify json output.
    if (req.query['pretty_ouput']) {
      app.set('json spaces', 2); // I'd like to set this only for this one request.
    }
    next();
  });

However, subsequent requests without the pretty_ouput querystring are also formatted.

Is there a way to set the setting json spaces for only one request? Ideally I could call res.set('json spaces', 2) that would set this option for only this one response. I'd think this would be generically useful for any Express option.

[graphicbeacon]

@bradvogel What if you disable the setting when the request does not contain the query string?

if (req.query['pretty_output']) {
    app.set('json spaces', 2);
} else {
    app.disable('json spaces'); // or app.set('json spaces', false)
}

[wesleytodd]

The above will most likely not work as you want it. This would result in a race condition between incoming requests. At the moment I think your best bet is just returning your own json response logic. Here is a reference for what the internal res.json is doing, maybe that would help:

https://github.com/expressjs/express/blob/b7817ab1b0bbadabe79fc88819c8338838ead707/lib/response.js#L239-L268

[wesleytodd]

You could do something simple like:

function sendPrettyJson (req, res, value) {
  this.set('Content-Type', 'application/json')
  var body = req.query['pretty_output'] ? JSON.stringify(value, null, 2) : JSON.stringify(value)
  res.send(body)
}  

[bradvogel]

Yes - thanks. I know I can take over res.json and format it myself - but I'd prefer to keep using the built-in res.json and benefit from its future improvements.

Isn't there still a generic need for setting options at the response level, e.g. to temporarily disable something like x-powered-by for a single response? Or any future options that are added?

[wesleytodd]

There is a generic need for this, and I believe there are some issues open for this. I am at work right now and so cant dig for them, but if you search the issues and pr's you can probably find them. I do not remember if anyone has created a strong enough proposal for it to get marked for inclusion into 5.x, but feel free to do so :)

[dougwilson]

@wesleytodd I also thought this was discussed before, but I wasn't able to locate. I think this is useful, though even just looking at the way it's used now it likely wouldn't be compatible with Express 4.x due to a few places where the settings are looked up without any references to req or res to key off of.

[wesleytodd]

Same, I could not find it either. I think we should add this to the 5.x road map. It should be simple enough to pass req/res to those places, and the value add is worth it IMO.

[sanji-programmer]

Hi, I have investigated some open issues involving races (as a research project) and took a look at the solution of @graphicbeacon. I created a small app (https://github.com/sanji-programmer/express-issue-3536-race) and as correctly pointed out by @wesleytodd this solution is subjected to races (https://github.com/sanji-programmer/express-issue-3536-race/blob/master/express01/server-test.js). Using a tool we have developed, I could explore different callback interleavings that reveal the race.

I also verified the solution proposed by @wesleytodd (https://github.com/sanji-programmer/express-issue-3536-race/blob/master/express01/server-test-fixed.js) and this seems alright and no subjected to races.

I also noticed that modifying global settings of express may also causes races in different scenarios.