Express.js Interview Questions & Answers Part- 2

To enable debugging, you can follow these steps:

• Install the debug module (if not already installed): you can do so using npm or yarn.
• Import and Configure Debugging: In your Express.js application code, import the debug module and create a debug function with a specific namespace. Namespaces help you control which parts of your application you want to enable debugging for.
• Run the Application with Debugging: To enable debugging, you’ll need to set the DEBUG environment variable to the namespace(s) you’ve defined. You can do this in the command line before starting your Express.js application.
• View Debugging Output: With debugging enabled, you’ll see output in the console related to the namespace(s) you’ve specified. This output can be helpful for diagnosing issues, tracking the flow of your application, and understanding how different parts of your code are interacting.

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In Express.js, the next() function is a callback function used to pass control to the next middleware function in the middleware stack. The next() function is used within a middleware function to signal that the current middleware has completed its task and that Express should move on to the next middleware in the chain. It is useful when you have multiple middleware functions that need to be executed in a specific order.

Answer:

In Express JS, you can configure properties using two different methods:

1. Utilizing env: Create a file named “.env” within your project folder. Place all the desired properties within this “.env” file. You can then access any of these properties within your “server.js” file.
2. Using require: Create a file named “config.json” within the “config” folder in your project directory. Define the configuration properties within the “config.json” file. To access the “config.json” file, you can make use of the require.

Answer:

Express.js is primarily used for backend development. It helps developers build robust and efficient web applications by providing a set of tools and utilities for handling HTTP requests, routing, middleware, and more.

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The main difference between Express.js and Node.js is their focus and role:

• js: It’s a runtime environment that allows you to run JavaScript on the server. It provides the foundation for building server-side applications, including web applications.
• js: It’s a web application framework built specifically for Node.js. It provides a higher-level abstraction to simplify the process of building web applications by providing tools and features for handling common web-related tasks.

So, you use Node.js to write server-side JavaScript code, while you use Express.js to structure and manage your web application built with Node.js.

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Here are a few MVC frameworks, other than Express.js, that support scaffolding:

• Ruby on Rails
• NET MVC
• Django
• Grails
• Laravel

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Yes, Express.js does support template engines. A template engine is a tool that allows you to generate HTML dynamically by inserting data into templates. Some of the commonly used template engines with Express.js are:

• EJS (Embedded JavaScript)
• Pug (formerly known as Jade)
• Handlebars

Answer:
Application-level middleware in the context of Express.js refers to the middleware functions that are used to handle specific tasks or actions in an Express application. Middleware functions in Express are essentially functions that have access to the request (req) and response (res) objects, as well as the next function in the application’s request-response cycle.
Application-level middleware is configured to be executed for specific routes or for all routes in an Express application. It’s used to perform various tasks such as authentication, logging, data validation, and more, before the actual route handler is invoked. This allows for modular and reusable code that can be applied to multiple routes.

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Router-level middleware refers to a concept commonly used in web development, particularly in the context of server-side frameworks and libraries. Middleware, in general, is a software component that sits between the client and the main application logic, intercepting and processing requests and responses. It’s often used for tasks like authentication, logging, input validation, and more.

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Error handling middleware is a component used to manage errors that occur during the processing of incoming requests in an Express.js application. When an error occurs in an Express.js application, it’s important to handle it gracefully to prevent crashes and provide meaningful responses to clients. Error-handling middleware serves as a central point for catching and processing errors that might happen at various stages of request processing.

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An Express application generator is a tool used to quickly set up the basic structure of a web application using the Express.js framework. The generator automates the process of creating the initial files, folders, and code structure needed to start building a web application.

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Dynamic routing in Express.js refers to the practice of defining routes in a way that allows for dynamic parameters to be included in the URL paths. These dynamic parameters are used to capture values from the URL and are often utilized to make the routes more flexible and versatile.

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Some of the common database integrations that Express.js supports include:

• Neo4j
• Oracle
• CouchDB
• LevelDB
• MySQL
• MongoDB
• Cassandra
• Couchbase
• PostgreSQL

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Express.js supports several HTTP methods, each serving a different purpose. Here are some of the commonly used HTTP methods in Express.js:

• GET
• POST
• PUT
• PATCH
• DELETE

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In Express.js, there is no specific file extension required to save programs. However, the common practice is to save Express.js programs in files with a .js extension, which indicates that they are JavaScript files. This is because Express.js applications are typically written in JavaScript, and using the .js extension helps maintain consistency and clarity in your project’s file structure.

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Enabling CORS (Cross-Origin Resource Sharing) in an Express.js application allows you to control how your server responds to requests from different domains. Here’s how you can allow CORS in an Express.js application:

1. Install the necessary package: First, ensure you have the cors package installed. If not, you can install it using npm or yarn.
2. Import and Use the cors middleware: In your Express.js application code, you need to import the cors middleware and use it before your route handlers.
3. Handling CORS Preflight Requests (Optional): When making certain types of requests, the browser sends a preflight request using the HTTP OPTIONS method. You can handle these requests by explicitly allowing them in your route handlers.

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Rendering plain HTML in Express.js can be done using the res.send() method.

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In Express.js, the app.use() function is used to mount middleware in the application’s request processing pipeline. Middleware functions are functions that have access to the request (req) and response (res) objects, and they can perform various tasks like authentication, logging, data parsing, and more.

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Structuring an Express.js application is crucial for maintaining a clean and organized codebase. Here’s a typical way to structure an Express.js application:

• Project Root
• Folders
• Inside routes
• Inside controllers
• Inside models
• Middleware
• Configuration
• Testing
• Error Handling
• Logging
• Validation
• Security

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In the context of Express.js, the term “sanitizing input process” refers to the practice of cleaning and validating user input before it is processed by the application. This is done to prevent various security vulnerabilities, such as cross-site scripting (XSS) attacks and SQL injection attacks, that can occur if malicious or malformed input is directly used in application logic or database queries.