Idiomatic way of parsing multiple types of request bodies (express)

Question

I'm trying to find the best way of parsing the request body of multiple (some non-json) mime types.

Let's say I have a simple app like this:

const app = express()
app.use(express.json())

app.post('/test', (req, res) => {
  res.status(200).json(req.body)
})

How would I handle both of these requests and get the proper request body?

fetch('/test', {
  method: 'POST',
  body: YAML.stringify({ foo: 'bar' }),
  headers: {
    'content-type': 'application/yaml',
  },
})

fetch('/test', {
  method: 'POST',
  body: JSON.stringify({ foo: 'bar' }),
  headers: {
    'content-type': 'application/json',
  },
})

Do I need to write body parsers from scratch? Can I even do this as express seems to not expose the raw body after the json bodyparser has ran?

Answer 1

I have not searched for body parsers that support YAML.

express.json() consumes the incoming request stream only if the request has Content-Type: application/json. So you can add middleware that parses YAML into JSON after that:

const yaml = require("js-yaml");
const {Writable} = require("stream");
app.use(express.json(), function(req, res, next) {
  if (req.get("content-type").startsWith("text/yaml")) {
    var buffers = [];
    req.pipe(new Writable({
      write(chunk, encoding, callback) {
        buffers.push(chunk);
        callback();
      },
      final(callback) {
        try {
          req.body = yaml.load(Buffer.concat(buffers).toString());
          next();
        } catch(e) {
          next(e);
        }
      }
    }))
  } else
    next();
});

Note that this does not deal with compressed request payloads.