Builtin MiddleWar in ExpressJS

Introduction to UsiMiddlewareare

Express is a web framework for routing and middleware with only the following basic functionality: In essence, an Express program is a collection of middleware function calls. In the request-response cycle of an application, express middleware functions are functions that have access to the request object (req), the answer object (res), and the subsequent express middleware function. A variable with the name next is frequently used to identify the next middleware procedure.

The following activities can be carried out by express middleware functions:

• Put any code to use.
• Modify the objects representing the request and the response.
• Stop the loop of requests and responses* Invoke the middleware procedure that is next on the stack.
• The current express middleware function must execute next() to transfer control to the following middleware function if it does not stop the request-response cycle. If not, the proposal will be ignored.

An Express application can use the following types of middleware:

• Application-level middleware
• Router-level middleware
• Error-handling middleware
• Built-in middleware
• Third-party middleware

Application Level Middleware

To attach express middleware that operates at the application level to a specific app object instance, you can utilize the app.use() and app.METHOD() functions. The METHOD parameter should be in lowercase and correspond to the HTTP method (such as GET, PUT, or POST) that the middleware function handles.

Example 1

The express middleware method in this example doesn't have a mount path. Every time the app gets a request, the function is run.

Here, the app object represents a server, app.use() method is used to set up the middleware function

Using this line the middleware function logs in the current time to the console

What makes this middleware function intriguing is that it lacks a mount path, meaning that it's triggered by every HTTP request the server receives. This can be beneficial when implementing features that need to apply to all requests, like authentication or logging. However, it's essential to keep in mind that if needed, middleware functions can also be confined to a specific set of paths using a mount path.

Example 2

This example shows a middleware function mounted on the /user/:id path. which means that the express js middleware function will only be called for HTTP requests that match this route pattern

The middleware function logs the HTTP request method to the console using

This kind of scoped middleware function can help add features that are unique to particular routes, like input validation or authentication. You can make sure that the expressjs middleware function is only used when it is required and not for each HTTP request to the server by using a mount path to scope the function to particular routes.

Furthermore, you can alter code like this

This would show a route and its handler function (middleware system).

Example 3

Here is an illustration of how to use a mount path to install several express middleware functions at a mount point. It displays an example of a middleware sub-stack that outputs request information for any kind of HTTP request to the /user/:id path

Example 4

Route handlers allow you to define multiple routes for a path. The example below defines two routes for GET requests to the /user/:id path. The request-response loop is ended by the first route, so the second route won't cause any problems but will never be used.

It shows an express middleware sub-stack that handles GET requests to the /user/:id path

Call next('route') to transfer control to the next route in a router middleware stack to skip the remaining middleware operations.

Built-In Middleware Functions

For common tasks in web applications, Express.js offers a set of integrated middleware functions. The following are some of the most popular built-in middleware features:

1. express.static: This middleware feature is used to deliver static files from a server directory, including images, CSS, and JavaScript files. It accepts as an argument the root directory from which to serve files.

2. express.json: JSON payloads in HTTP requests are parsed using this middleware function. It adds the parsed info to the req.body object.

3. express.urlencoded: The HTTP requests' inbound URL-encoded payloads are decoded using this middleware method. It adds the parsed info to the req.body object.

4. express.Router: Create modular routes that can be mounted to various paths in your program using this middleware function.

5. express.cookieParser: To process cookies in HTTP requests, middleware is used. The decoded cookies are added to the req.cookies object.

6. express.session: In your program, user sessions are created and managed using this middleware function.

Let's discuss some of them in detail

express.static

The express.static() function is a built-in middleware function in Express. It serves static files and is based on serve-static. Syntax:

Parameters: The root parameter describes the root directory from which to serve static assets.

Return Value: It returns an Object.

Explanation Files that clients receive directly from the server are known as static files. Make a new subfolder called public. By default, Express forbids serving inactive files. Use the following built-in middleware to activate it.

express.json

express.json() is a built-in middleware function in Express. This method is used to parse the incoming requests with JSON payloads and is based on the body parser.

This technique provides middleware that only parses JSON and examines requests where the content-type header corresponds to the type option.

Syntax:

Parameters: The options parameter have various property like inflate, limit, type, etc.

Return Value: It returns an Object.

Now let's take a look at an example

Example 1:

Create a file named index.js

On port 3000 an express js server is created to parse the JSON payloads from HTTP POST requests using the express.json middleware method

The callback code is executed whenever a POST request is sent to the root endpoint ('/'). This function uses the console.log command to record the value of the name attribute of the parsed JSON data from the request body (req.body.name). The parsed JSON data from the request body is contained in the req.body object, which is filled out by the express.json middleware method.

Then you will see the following output on your console:

express.urlencoded

Express.js comes with a built-in wrapper called express.urlencoded(). This approach, which is built on the body parser, aims to parse incoming requests with urlencoded payloads.

The middleware that decodes all of the urlencoded bodies is returned by this function.

Syntax:

Parameter: The options parameter contains various properties like extended, inflate, limit, veri,fy etc.

Return Value: It returns an Object.

Let's see an example to understand better

Example 1:

Filename: index.js

On port 3000 an express js server is created to decode inbound URL-encoded form data from HTTP POST requests

The output from the console.log statement in your code would look like this:

{ name: 'ScalarTopics' }