How do I combine Connect middleware into one middleware? - node.js

Question

I have a few middlewares that I want to combine into one middleware. How do I do that?
For example...
// I want to shorten this...
app.use(connect.urlencoded())
app.use(connect.json())
// ...into this:
app.use(combineMiddleware([connect.urlencoded, connect.json]))
// ...without doing this:
app.use(connect.urlencoded()).use(connect.json())
I want it to work dynamically -- I don't want to depend on which middleware I use.
I feel like there's an elegant solution other than a confusing for loop.

Answer 1

Express accepts arrays for app.use if you have a path:
var middleware = [connect.urlencoded(), connect.json()];
app.use('/', middleware)
However, if you want a generic combineMiddleware function, you can build a helper easily without any additional libraries. This basically takes advantage of the fact that next is simply a function which takes an optional error:
/**
* Combine multiple middleware together.
*
* #param {Function[]} mids functions of form:
* function(req, res, next) { ... }
* #return {Function} single combined middleware
*/
function combineMiddleware(mids) {
return mids.reduce(function(a, b) {
return function(req, res, next) {
a(req, res, function(err) {
if (err) {
return next(err);
}
b(req, res, next);
});
};
});
}

Answer 2

If you like fancy stuff, here is one of possible solutions:
var connect = require('connect')
var app = connect()
function compose(middleware) {
return function (req, res, next) {
connect.apply(null, middleware.concat(next.bind(null, null))).call(null, req, res)
}
}
function a (req, res, next) {
console.log('a')
next()
}
function b (req, res, next) {
console.log('b')
next()
}
app.use(compose([a,b]))
app.use(function (req, res) {
res.end('Hello!')
})
app.listen(3000)
Here is what it does: compose function takes array of middleware and return composed middleware. connect itself is basically a middleware composer, so you can create another connect app with middlewares you want: connect.apply(null, middleware). Connect app is itself a middleware, the only problem is that it doesn't have a next() call in the end, so subsequent middleware will be unreachable. To solve that, we need another last middleware, which will call next : connect.apply(null, middleware.concat(last)). As last only calls next we can use next.bind(null, null) instead. Finally, we call resulting function with req and res.

Answer 3

Old question, but the need is still frequent for all the things using middlewares, like connect, express or custom made req/res/next patterns.
This is a very elegant and purely functional solution:
File ./utils/compose-middleware.js:
function compose(middleware) {
if (!middleware.length) {
return function(_req, _res, next) { next(); };
}
var head = middleware[0];
var tail = middleware.slice(1);
return function(req, res, next) {
head(req, res, function(err) {
if (err) return next(err);
compose(tail)(req, res, next);
});
};
}
module.exports = compose;
The final result of the compose(middlewareList) is a single middleware that encapsulates the whole chain of middleware initially provided.
Then simply import it and use like this:
File app.js:
var connect = require('connect');
var compose = require('./utils/compose-middleware');
var middleware = compose([
connect.urlencoded(),
connect.json()
]);
var app = connect();
app.use(middleware);

Answer 4

A simple and native way, and you don't need to install anything.
const {Router} = require('express')
const combinedMiddleware = Router().use([middleware1, middleware2, middleware3])
Then you can use the combinedMiddleware where you want. For example, you may want to run different set of middlewares/handlers for the same route depending on some conditions (a request attributes, for example):
app.get('/some-route', (req, res, next) => {
req.query.someParam === 'someValue'
? combinedMiddleware1(req, res, next)
: combinedMiddleware2(req, res, next)
})

Answer 5

If you're willing to use a library:
https://www.npmjs.org/package/middleware-flow
var series = require('middleware-flow').series;
var app = require('express')();
app.use(series(mw1, mw2, mw2)); // equivalent to app.use(mw1, mw2, mw3);

Answer 6

Make a list and use a loop.
const connect = require('connect')
const { urlencoded, json } = require('body-parser')
const app = connect()
[ urlencoded(), json() ].forEach(app.use, app)
The second argument of .forEach is used for this, but if you like you can also do the same with:
[ urlencoded(), json() ].forEach(app.use.bind(app))