node.js - How Does Middleware Actually Work? -

i'm curious nodejs , express. how middleware work? how go creating own middleware?

does rely on type of dependency injection? understand add middleware in order want executed, , http request/response passed middleware, , onto next, , next, until 1 of them returns/renders request/response.

are request , response objects passed through each middleware reference?

sorry if sounds confusing. i'm trying learn how create own middleware better understanding of in general.

edit: post written express 3. minor details have changed since then, it's conceptually same.

a note before start: express built atop connect, handles middleware. when write express in these examples, i'd able write connect.

at bottom level, have node's http server module. looks kind of this:

var http = require("http"); http.createserver(function(request, response) {   response.end("hello world!\n"); }).listen(1337, "localhost"); 

basically, make single function handles all http requests. try running above code , visiting localhost:1337/hello or localhost:1337/wow-anime. technically, need!

but let's want many functions run every time. maybe want add command-line logger, , maybe want make every request plain text.

var http = require("http"); http.createserver(function(request, response) {    // logger   console.log("in comes " + request.method + " " + request.url);    // plain text   response.writehead(200, { "content-type": "text/plain" });    // send response         response.end("hello world!\n");  }).listen(1337, "localhost"); 

in express/connect, might write instead:

var express = require("express"); var app = express(); app.use(express.logger()); app.use(function(request, response, next) {   response.writehead(200, { "content-type": "text/plain" });   next(); }); app.use(function(request, response) {   response.end("hello world!\n"); }); app.listen(1337); 

i think of middlewares list of functions. when http request comes in, start @ top , go through each middleware top bottom, , stop when response.end called (or response.send in express).

i wrote a blog post explains express , middleware in more detail, if you're interested.