Middlewares.md

Middlewares

A Middleware is a small piece of code that is used to alter web application request/response cycle, middleware function has access of request, response object and next middleware function in request/response cycle. Middlewares are the building block of routejs.

Middleware functions can be used to perform the following tasks:

• Execute code.
• Make changes to the request/response objects.
• End the request/response cycle.
• Call the next middleware function in request/response cycle.

If the current middleware function does not end the request/response cycle, it must call next() function to pass control to the next middleware function.

In routejs everything is a middleware, type of middlewares are the following:

• Global middleware
• Route middleware
• Error-handler middleware

Middleware defination:

app.use(CALLBACKS);

Where:

• CALLBACKS is the callback function or router instance or urls routes.

Example:

// Middleware with single callback
app.use(function (req, res, next) {
  next();
});

// Middleware with multiple callbacks
app.use(
  function (req, res, next) {
    next();
  },
  function (req, res, next) {
    next();
  },
);

If we pass skip in the next("skip") function it will skip all pending middlewares of current route.

If we pass any argument in the next("my error") function it will skip all the middlewares and routes and execute the next error-handler middleware, if there is no error-handler middleware is registered it will throw the exception.

Global Middleware

Global middleware is a function that is executed on every request.

app.use(function (req, res, next) {
  req.counter = 0;
  // Call next middleware
  next();
});

app.use(function (req, res, next) {
  req.counter++;
  next();
});

Route Middleware

Route middleware is a function that is executed when request route is matched.

app.get("/", function (req, res) {
  res.end("Ok");
});

// OR

app.get("/",
  function (req, res, next) {
    req.counter = 0;
    // Calls next middleware
    next();
  },
  function (req, res) {
    res.end(`Counter is ${req.counter}`);
  }
);

Error-handler Middleware

Error-handler middleware is used to handle errors in request/response cycle.

Error-handler middleware always takes four arguments, where the first argument is the error object. You must provide four arguments to identify a middleware as an error-handler middleware.

app.use(function (err, req, res, next) {
  res.writeHead(500).end("Internal Server Error");
});

Routejs catches error and automatically call the Error-handler middlewares. If we pass error as an argument in next() function, it will skip all middlewares and only calls the next error-handler middleware.

app.get("/", function (req, res) {
  next("error");
});

app.use(function (err, req, res, next) {
  res.writeHead(500).end("Internal Server Error");
});