Express.js Flashcards

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Express.js is a fast, unopinionated, and minimalist web framework for Node.js. It provides a robust set of features for building web applications and APIs, including routing, middleware support, template engines, and HTTP utility methods. Express.js has become the de facto standard for Node.js web development due to its simplicity, flexibility, and extensive middleware ecosystem. It allows developers to create everything from simple REST APIs to complex web applications with ease. Express.js is designed to be minimal and flexible, making it an excellent choice for developers who want control over their application architecture while benefiting from a solid foundation.

Our flashcard app contains carefully selected Express.js interview questions, complete with comprehensive answers, to effectively prepare you for any interview requiring Express.js knowledge. IT Flashcards is not only a valuable tool for job seekers but also a great way to strengthen and test your understanding of Express.js framework. Regular practice with the app will keep you updated with the latest trends in Node.js backend development and enhance your expertise in building scalable server-side applications.

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Express.js

What is middleware in Express.js?

Middleware in Express.js are functions that have access to the request object (req), the response object (res), and the next middleware function (next) in the request lifecycle. Middleware are key components of the application and API structure in Express.js.

Middleware functions can perform the following tasks:
- Execute any code.
- Make changes to the request and response objects.
- End the request-response cycle.
- Call the next middleware function in the stack.

If the current middleware function does not end the request-response cycle, it must call `next()` to pass control to the next middleware function. Otherwise, the request will be left hanging.

Express.js

What is the significance of the next parameter in Express.js middleware?

The **next** parameter in middleware in Express.js is a function that, when called, moves the call to the next middleware in the stack.

If the middleware function does not end the request-response cycle, it must call `next()` to pass control to the next middleware function. Otherwise, the request will remain pending.

The operation principle of `next()` is quite simple: When `next()` is called, execution goes to the next middleware at the current level.

Example code using the next function:

app.use((req, res, next) => {
    console.log('First middleware');
    next();
});

app.use((req, res, next) => {
    console.log('Second middleware');
    res.end();
});

In the above example, after receiving a request, the first middleware calls `console.log`, then `next()`, which passes control to the second middleware. In the second middleware, we call `res.end()`, concluding the request-response cycle.

The `next()` function can also be called with an error, which will pass control to the next error-handling middleware:

app.use((req, res, next) => {
    console.log('First middleware');
    next(new Error('Error'));
});

app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).send('Server error!');
});

In the above example, after receiving a request, the first middleware creates a new error and passes it to `next()`, which moves to the next error-handling middleware. The error-handling middleware logs the error and returns a response to the user.

Express.js

How do you define static files in Express.js?

Express.js is a highly configurable Node.js framework designed to facilitate the creation of web servers. It allows easy definition of static files to extend server functionality.

Defining static files in Express.js is straightforward. We'll use the built-in middleware function **express.static()** for this purpose.

To define static files, we first need to import express, and then apply the express.static() function with the path to the directory we want to serve. Typically, this is the 'public' directory in the main application directory.

const express = require('express');
    const app = express();

    app.use(express.static('public'));

In the above code, express.static('public') instructs the Express server to serve the contents of the 'public' directory as static files. Once these files are defined, they will be directly accessible from the main URL.

In practice, this means that if we have an `index.html` file in the `public` directory, it will be available at `http://localhost:3000/index.html` (assuming our server is running locally on port 3000). If we have a `styles.css` file in the `css` subdirectory within `public`, it will be available at `http://localhost:3000/css/styles.css` and so on.

Express.js

What is res.json() in Express.js and what is it used for?

The **res.json()** function in Express.js is used to send a JSON response to the client. It is a special method available on the response object that Express passes to the request-handling function.

The name "json" comes from JavaScript Object Notation, which is a standard format for data exchange over the network. The **res.json()** method automatically converts a JavaScript object or value to JSON and then sends it as an HTTP response.

Additionally, this method sets the appropriate Content-Type header for the response, informing the client that the response is JSON.

Here is a sample code snippet using **res.json()**:

app.get('/api/data', (req, res) => {
  const data = {
    id: 1,
    name: 'Test',
  };
  
  res.json(data);
});

In the above example, when a client makes a GET request to '/api/data', Express.js will return a JSON response containing the `data` object.

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