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.

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.

• javascript
• node.js
• express
• middleware

• Is your ultimate goal to run some code when the response was sent? – robertklep Commented Jun 2, 2017 at 11:27
• No, my goal is decorate the router function so I can get direct response result (for example, return Promise) and do something with it. – Damaged Organic Commented Jun 2, 2017 at 12:40

3 Answers 3

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.

Why not just use next()?

You can add stuff on req like

app.get('/', (req, res, next) => {
    req.somestupidfieldthatidontevenknowwhyinamedthisway = 42;
    next();
});

app.get('/', (req, res, next) => {
    //req.somestupidfieldthatidontevenknowwhyinamedthisway is now accessible as 42
    var valueFromPreviousMiddleware = req.somestupidfieldthatidontevenknowwhyinamedthisway;
    .....
});

You could wrap middleware and router as below

function wrapper(func) {
    return function inner(req, res, next) {
        const start = Date.now();
        func(req, res, function () {
            let elapsedMS = Date.now() - start
            console.log('time elapsed for function ' + func.prototype.constructor.name + ' is ' + elapsedMS)

            next.apply(this, arguments);
        });
    };
}


var originalAppUse = app.use;
app.use = function () {
    lastArg = arguments.length - 1;

    if (typeof arguments[lastArg] === 'function') {
        arguments[lastArg] = wrapper(arguments[lastArg])
    }

    originalAppUse.apply(this, arguments)
}