After modifying express.js res.send function it won't emit any data

Question

Basically what I'm trying to do is unify every response of my application just so they look like this

{
  success: true,
  error: "Error description",
  errorCode: "SomeError",
  data: {
    ...
  }
}

Copying and pasting this structure everywhere as well as wrapper function looks quite ugly to me, so I've tried to modify send function so that it accepts 4 params instead of only 1 and then makes given structure.

I've found various examples on how to modify send function and came up with this middleware

app.use(function (req, res, next) {
  const oldSend = res.send;

  res.send = function (data = {}, success = true, error = '', errorCode = '') {
    let response = {
      data,
      success,
      error,
      errorCode
    };

    oldSend.apply(res, response);
  };

  next();
});

so the res.send call looks like this

res.json(req.session.key != null, true, null, null);

Which works fine except it doesn't return any response to the client. Could you help me solve this? Is it possible to achieve expected behaviour?

Answer 1

You are using function.apply which expects a this and then an array of the function arguments (see MDN for function.prototype.apply ). What you need to use instead is function.call (again see MDN for function.prototype.call ).

However, a word of caution to be careful of overwriting a low level part of express (or any library) - you don't know what else might be calling this so you may get some unexpected results. You might like to check (for example) that the supplied parameter is of a type which you expect (eg an object) and pass anything unexpected (eg a Buffer ) straight through to the original send.