Express.js Interview Questions & Answers

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Express.js is an open-source, lightweight, Node.js based web application framework. It was designed in 2010 by TJ Holowaychuk for building hybrid, single-page, and multi-page web applications and APIs.

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• Faster server-side development– Express.js offers many commonly used Node.js features that can be readily used anywhere in a program. It removes the need to code for several hours; thus, saves time.
• Middleware– It is the part of a program with access to client-request, database, and other middleware. It is mainly responsible for systematically organizing different Express.js functions.
• Routing-Express.js offers a highly advanced routing mechanism that helps preserve a webpage’s state with the help of its URLs.
• Templating– Express.js provides templating engines that enable developers to build dynamic content on the web pages by creating HTML templates on the server-side.
• Debugging– It is crucial for the successful development of web applications. Express.js can streamline debugging with a debugging mechanism that can pinpoint the exact part of a web application that has bugs.

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Network routing is the process of selecting a path across one or more networks. The principles of routing can apply to any network type, from telephone networks to public transportation. In packet-switching networks, like the Internet, routing selects the paths for Internet Protocol packets to travel from their origin to the destination. The Internet routing decisions are made by specialized network hardware pieces called routers.

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A template engine is used to merge HTML pages with data from a program. The engine is powerful and very simple. At runtime, it replaces variables in the template file with the actual values and transforms a template into the HTML file sent to a client.

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An HTTP cookie means a small piece of data stored on a user’s computer by the browser when browsing a website. Cookies were designed to be a reliable source for websites to remember stateful information or record the user’s browsing activities. They are also used to remember some information that a user previously entered in the form fields, such as addresses, names, payment card numbers, and passwords.

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Scaffolding implies a tool that sets up all necessary public directories, create separate route files, adds middleware, sets up the skeleton for web applications, and performs other important tasks so that one can directly get started with application development.

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Follow these steps to install Express.js in Node.js:

• First, ensure you’ve installed Node.js and create a directory to hold your app;
• Now, use the “npm init” command to build a package.json file for your app;
• This command will prompt you for various things, like the name and version of your app;
• You can enter app.js or any suitable name for the main file. You can also accept the suggested default file names;
• It’s time to install Express.js temporarily without adding it to the dependencies list using this command: $ npm install express –no-save.

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Express.js provides the following benefits:

• Express.js renders a faster I/O means it offers a rapid response for the user requests.
• Asynchronous callback and single thread communications feature of Express.js creates good request processing.
• It has a Model-View-Controller (MVC) structure that makes simplifies data manipulations.
• The routing prowess of the API makes it very promising.

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There is a wide range of template engines that work well with Express.js. The default template engine in Express.js is Jade, which is called Pug. However, the default Jade or Pug installed in Express.js still uses the old version.

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• Sails.js is powerful than Express.js because it has an ORM & can auto-generate REST APIs. It also has built-in WebSocket support along with a defined app structure. Express.js cannot execute the same without additional middlewares and libraries.
• Express.js is more flexible than Sails.js, as you can develop your app like a constructor. Sails.js limits the developers’ ability to change the application’s structure.
• Express.js is easier to learn and a great pick for beginner-level programmers.
• Sails have a potent CLI utility.

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A router means a device that connects two or more subnetworks or packet-switched networks. It primarily serves two functions:

• Handling or managing traffic between the networks by forwarding data packets to their intended IP addresses.
• Enabling multiple devices to utilize the same Internet connection.

There are different types of routers, but most routers pass data between Local Area Networks (LAN) and Wide Area Networks (WAN).  A LAN means a group of connected devices restricted to a particular geographic area. It often requires a single router.

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It is beneficial to divide all errors into two broad categories:

• Operational errors showcase the runtime problems experienced by the correctly-written programs. They’re not bugs in the program, but these problems are associated with something else like a system itself. For instance, too many open files or out of memory, the system’s configuration, network, or a remote service. Some examples of operation errors are:
  • Failed to connect to the server
  • Invalid user input
  • Request timeout
  • The server returns a 500 response
  • Failed to resolve hostname
  • Socket hang-up
  • The system runs out of memory
• Programmer errors are bugs in a program. These are things that can be avoided by changing the lines of code. Some examples of programmer errors are:
  • Try to read undefined property.
  • Call an asynchronous function without the callback.
  • Pass string where the object was expected.
  • Pass an object where the IP address string was expected.

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Middleware in Express.js are functions that execute during the lifecycle of the request to the Express.js server. Every middleware has access to an HTTP request and response for every route attached to it. Express.js is wholly compromised of middleware functions. Middleware can either terminate an HTTP request or pass it to another Middleware function using the next variable. The chaining of middleware enables you to compartmentalize a code and build reusable middleware.

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The Express Router allows you to create modular route handlers. It helps organize routes and middleware into separate files and directories, making the codebase more manageable and maintainable.

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Route parameters are placeholders in the route path that capture values from the URL. They are denoted by a colon (:) followed by the parameter name.

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Query parameters are part of the URL’s query string and can be accessed using the req.query object in Express.js.

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Express.js provides a built-in error handling mechanism using middleware. You can define an error-handling middleware with four parameters (err, req, res, next) and use it to catch and handle errors that occur during the request-response cycle.

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Template engines in Express.js are used to dynamically render HTML on the server-side. They allow you to embed variables, logic, and templates in your HTML views, making it easier to generate dynamic content.

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Database integration in Express.js refers to the process of connecting and interacting with a database within an Express.js application. Express.js is a popular web application framework for Node.js that simplifies the development of web applications and APIs. To store, retrieve, and manipulate data, web applications often need to interact with databases.

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In Express.js, you can redirect 404 errors to a specific page by creating a middleware function that catches requests that couldn’t be matched by any route handlers. Here’s how you can achieve this:

• Create a 404 Handler Middleware: You’ll need to create a middleware function that will catch any unmatched routes (404 errors). Place this middleware after all your other route handlers.
• Define the Error Page Route: Ensure that you have a route handler for the error page where users will be redirected.

Here, you can customize the content of the error page according to your preferences.

Remember to place the 404 handler middleware after all your other route handlers, so it catches requests that haven’t been matched by any other routes. This approach allows you to redirect users to a designated error page when they encounter a 404 error