req object :

req object is passed to any route handler function or in middlewares. It is the object which contains the details sent by the client.

It contains the informations like the headers, url details including pathname, query, port, href, protocol , cookies, any type of form data in string or json format and so on.

res object :

res object is passed by the server to the client. It has various fields like headers, the status code, the page data which the user has asked for, details regarding http protocol etc.

next() :

Another parameter which is generally passed to a middleware function is “next”. next is a reference to the next middleware or route handler function in event loop. If we don’t call next at the end of middleware execution the request will be kept hanging and the user will just see the page loading.

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Session starts when you request the server for the first time and it remains till you close that site. The amount of time you have spent on the site is called a session. As sessions get deleted when the connection to the client and server gets closed, so we have to store the session data somewhere to reuse them again in future.

Cookies

Cookies are the information stored on the client’s browser by the server. It helps in tracking the user and for various personalized recommendations.

Difference between Sessions and Cookies

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How to implement sessions in Express Js

First of all, install the necessary packages using the command given below :

Everytime the page you will refresh the views increase unless the server is restarted.

Note:  The above code is just for understanding the sessions, in production level applications you need to be careful while using sessions and cookies.

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Advantages of using Express Js

1. Lightweight : It is a lightweight framework of Node Js for creating web servers. As it is based on Node Js, it offers all the features of Node Js with less coding.
2. Time saving : It is developed to reduce the development time which generally takes, when we create applications in Node Js. Express Js provides many features of Node Js in less lines of code. There are many tasks, which in Node Js we have to do manually, but here in Express Js, we have various inbuilt functions available for that.
3. Huge Library : Another feature of Express Js is the availability of libraries. As the community of Express Js is large, everyday new packages are getting developed.
4. Realtime applications : Express Js is used heavily in realtime applications. It is best suited for realtime user interaction systems, due its non-blocking asynchronous event-driven model.
5. Learning curve : As it is based on JS, it becomes easier for developers knowing JS to learn Express Js. Another benefit is that we can develop the whole application (frontend and backend) using a single language.

Disadvantages of Express Js

1. Callback hell: For new developers, it becomes hard to grasp the asynchronous nature of JS. They find it difficult to manage the callbacks. Callback hell is a condition, where we have multiple callbacks nested inside each other and every callback waits for another callback. Eventually resulting a non readable and difficult to maintain code.
2. Less features: As it is a lightweight framework, many of the features are not available out of the box and we have to install multiple packages to get those features.
3. Lack of strong typing: Express Js doesn’t have strong type checking while compilation, which results in complex bugs.
4. Error handling: Although Express Js has excellent error handling capabilities, but sometimes it becomes hard for new developers to handle errors with the code containing multiple callbacks and promises.

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MongoDB database can perform all types of CRUD (Create, Read, Update, Delete) operations. We use a module called mongoose to connect to the MongoDB database.

In the above code, we have connected our project to the MongoDB database. After that we have created a schema, which defines the structure of the data going to be stored. Then we have created a model and exported that to use in the other parts of the application.

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Scaffolder is the tool used to automate this task. A popular scaffolder for Express Js is an express-generator.

To use the express-generator, run the following command :

Folder Structure

1. Routes: This folder contains various routes of the application, divided in different files.
2. Public: This folder contains the files we want to be public like images and CSS files.
3. Views: Views folder contains different templating files that we want to serve.
4. app.js: This is the place where all the code is managed. It handles all the other files.

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There are various templating engines available in Express Js like ejs, pug, etc. All the templating engines have their own syntax. In this article we will discuss about ejs.

Templating in Express Js using EJS

EJS stands for Embedded Javasript Templating. EJS is used to produce the final HTML document after adding the dynamic content from database or after some calculations. We can also write javascript inside HTML pages using react. We can use almost all the necessary things like conditional statements, loops, etc.

Setting up the EJS and using it in the Express Js project

First we have to install it in our project, using the command :

npm i –save ejs

For using EJS inside our HTML pages, we have to save the HTML files using the .ejs extension.

Using loops in EJS

Using conditional statements in EJS

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Error handling is useful in handling runtime error. If we don’t handle runtime errors, it will result in the incomplete program termination. Using error handling we can display a message or handle the error, so that the program can complete its execution properly.

Using try catch blocks

As we know try-catch blocks are used everywhere to handle errors. But this approach may not handle all types of errors in JS, as JS is based asynchronous methodology.

Using Callbacks

Callbacks are also a function supplied to a function as an argument. After the completion of that function, the callback function is called. In simple terms, callback functions are executed after the completion of asynchronous operations.

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Cookies helps us to store the necessary information on the client side. Later we can use this information for session management, for personalized recommendations.

Cookies also helps the server to recognize the user back, if he accesses the website after some time. In this way the user doesn’t have to provide the details each time.

Confidential informations must not be stored in cookies, as it can be easily accessed by anyone, through internet or the web browser itself.

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Why should we use middlewares?

1. We can handle the authentication and authorization part in the middlewares.
2. We can also use third party middleware like cookie-parser to handle cookies.
3. We can divide the tasks between multiple middlewares.
4. We can also manipulate req and res object inside the middlewares, like setting the headers of the response object.

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