Express.js – wrap every middleware/route in "decorator"

Issue

I have Express.js instance and couple of routes which I want to wrap in some function. Example:

const wrapper = (route) => {
  return (req, res, next) => {
    let result = route(req, res, next);

    // do some independent processing
  }
};

app.get('/', wrapper((req, res, next) => {
  // respond to request somehow
}));

While this works fine, I don’t like the idea to explicitly call wrapper on every route or middleware which requires such processing.

Is there any way to be able to wrap every required route/middleware in certain wrapper (given that wrapper function can check that this route/middleware needs to be wrapped) implicitly (via Express.js extension, monkey-patching or some special middleware)?

UPDATE:

More solid example. Let’s assume I want to make an async router functions. But I don’t want to catch errors in each and every route function. So I wrap them up:

const wrapper = func => (req, res, next) => {
  const promise = func(req, res, next);

  if (promise.catch) {
    promise.catch(err => next(err));
  }

  next();
};

app.get('/one', wrapper(async (req, res, next) => {
  // respond to request somehow
}));

app.get('/two', wrapper(async (req, res, next) => {
  // respond to request somehow
}));

app.get('/three', wrapper(async (req, res, next) => {
  // respond to request somehow
}));

// and so on...

app.use((err, req, res, next) => {
  // do something with intercepted error
});

This explicit wrapper around all routes is actually the thing I want to get rid of.

Solution

It turned out to be a bit of a PITA because, ultimately, Express doesn’t propagate the return value of a route handler function.

This is what I came up with (a monkey-patch):

const Layer          = require('express/lib/router/layer');
const handle_request = Layer.prototype.handle_request;

Layer.prototype.handle_request = function(req, res, next) {
  if (! this.isWrapped && this.method) {
    let handle  = this.handle;
    this.handle = function(req, res, next) { // this is basically your wrapper
      let result = handle.apply(this, arguments);
      // do some independent processing
      return result;
    };
    this.isWrapped = true;
  }
  return handle_request.apply(this, arguments);
};

I would probably suggest using a similar approach as express-promise-router though, which implements a drop-in replacement for Express’ Router. However, it’s not implicit.

Answered By – robertklep

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