In JavaScript development, which statement correctly describes how you can read from and write to files?

Introduction / Context:
Reading and writing files is a common requirement in real world applications, but how you do it in JavaScript depends heavily on the environment. Interviewers use this topic to test whether candidates understand the difference between browser side JavaScript and server side JavaScript in platforms such as Node.js. Knowing the correct way to handle files is important for security, architecture, and user experience.

Given Data / Assumptions:

• We are working with JavaScript in both browser and server environments.
• We want to read and write data from or to files on a file system.
• We recognise that browsers and Node.js have different security and API models.
• The question asks for the most accurate general statement about file access in JavaScript.

Concept / Approach:
In the browser, JavaScript runs in a sandbox for security reasons. It cannot freely read or write arbitrary files on the user disk. Instead, it works with user selected files via the File API, drag and drop, or uploads, and it sends or receives data from servers using HTTP. On the server side with Node.js, JavaScript has access to the host file system through the fs module, which provides synchronous and asynchronous functions to read and write files. Understanding this separation is vital when deciding where to implement file handling logic in a web application.

Step-by-Step Solution:
Step 1: In a browser, JavaScript does not have unrestricted access to the user file system. Users must choose files explicitly, for example by using an input type="file" element. Step 2: Once a user selects a file, the browser provides a File or Blob object that JavaScript can read using APIs such as FileReader, but it still cannot silently modify arbitrary files on disk. Step 3: To save data persistently from the browser, JavaScript typically sends the data to a server, uses downloads, or stores it in browser storage APIs rather than writing directly to the operating system file system. Step 4: In Node.js, by contrast, JavaScript uses the built in fs module to work with the file system. Functions like fs.readFile, fs.writeFile, and streams allow server side code to read and write files on the server machine. Step 5: Therefore, the correct high level description is that Node.js handles direct file operations on the server, while browser JavaScript interacts with files indirectly and in a controlled way for security.

Verification / Alternative check:
You can verify this model by attempting to run fs.readFile in browser JavaScript, which fails because fs is not defined. In Node.js, the same code works after requiring the fs module. Conversely, browser specific APIs like FileReader are not available in plain Node.js scripts. This clear separation confirms that the correct summary involves Node.js file system access on the server and limited, user mediated access in browsers.

Why Other Options Are Wrong:
Option A is wrong because there is no built in FileSystem object in standard browser JavaScript that allows direct access to arbitrary user files; this would be a serious security risk. Option C is incorrect because JavaScript clearly can access files in some environments, such as Node.js on the server, even though browser access is restricted. Option D is completely incorrect because folders are not automatically mapped to arrays, and using array methods like push() has nothing to do with writing files to disk.

Common Pitfalls:
New developers often assume that JavaScript in the browser should be able to browse and modify any file on the user machine, not realising that the security model strictly blocks this. Another pitfall is writing code that only works in Node.js and expecting it to run unmodified in the browser. A good practice is to keep file system code on the server and communicate between browser and server using HTTP APIs, while using browser specific APIs only for user selected files or local storage needs.

Final Answer:
In summary, you use the Node.js fs module to read and write files on the server, while browser JavaScript relies on the File API and server communication rather than direct disk access.