fs

The fs (File System) module is one of the most useful in Node.js. Let’s explore it step by step: basic read/write → async vs sync → streams.

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📂 1. The fs Module

• Built-in module for interacting with the file system.

• Gives access to both synchronous and asynchronous methods.

• Import it:

const fs = require("fs");

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📖 2. Reading Files

Asynchronous (non-blocking) → preferred

fs.readFile("data.txt", "utf8", (err, data) => {
  if (err) throw err;
  console.log("File content:", data);
});
console.log("Reading file...");

👉 Doesn’t block the event loop. The callback runs when reading is finished.

Synchronous (blocking)

const data = fs.readFileSync("data.txt", "utf8");
console.log("File content:", data);

👉 Blocks the thread until the file is fully read. Rarely recommended in servers.

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✍️ 3. Writing Files

Asynchronous

fs.writeFile("output.txt", "Hello World!", (err) => {
  if (err) throw err;
  console.log("File saved!");
});

Append to file

fs.appendFile("output.txt", "\nAnother line", (err) => {
  if (err) throw err;
  console.log("Line appended!");
});

Synchronous

fs.writeFileSync("output.txt", "Hello World!");

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🔄 4. Streams

For large files, reading/writing everything at once is inefficient.
Streams let you process data in chunks without loading the entire file into memory.

Read Stream

const readStream = fs.createReadStream("bigfile.txt", "utf8");
 
readStream.on("data", (chunk) => {
  console.log("Received chunk:", chunk.length);
});
 
readStream.on("end", () => {
  console.log("Finished reading file.");
});

👉 Useful for video/audio streaming, logs, big datasets.

Write Stream

const writeStream = fs.createWriteStream("output.txt");
 
writeStream.write("First line\n");
writeStream.write("Second line\n");
writeStream.end();
 
writeStream.on("finish", () => {
  console.log("Write complete.");
});

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🔗 5. Piping Streams

The real power: connecting streams together.

Example: Copy a file efficiently:

const readStream = fs.createReadStream("bigfile.txt");
const writeStream = fs.createWriteStream("copy.txt");
 
readStream.pipe(writeStream);

👉 Data flows in chunks from the read stream into the write stream.

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✅ In Short

• fs.readFile / fs.writeFile → simple file operations (async preferred).

• Sync methods block the event loop — avoid in servers.

• Streams (createReadStream, createWriteStream) let you handle large files efficiently.

• pipe() connects streams together for powerful data handling (file copy, compression, network).

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