File System Operations (fs Module)

1// File System Operations — Comprehensive Guide
2
3const fs = require("fs");
4const fsp = require("fs").promises; // Promise-based API
5const path = require("path");
6
7// 1. Reading files
8async function readFileExamples() {
9  // Promise-based (recommended)
10  const content = await fsp.readFile("data.txt", "utf-8");
11  console.log("Content:", content);
12
13  // Read as Buffer (binary files)
14  const buffer = await fsp.readFile("image.png");
15  console.log("File size:", buffer.length, "bytes");
16
17  // Read JSON file
18  const jsonStr = await fsp.readFile("config.json", "utf-8");
19  const config = JSON.parse(jsonStr);
20
21  // Synchronous (only for startup/scripts)
22  const syncContent = fs.readFileSync("data.txt", "utf-8");
23}
24
25// 2. Writing files
26async function writeFileExamples() {
27  // Write (creates or overwrites)
28  await fsp.writeFile("output.txt", "Hello, World!\n", "utf-8");
29
30  // Write JSON
31  const data = { name: "Alice", age: 30 };
32  await fsp.writeFile("data.json", JSON.stringify(data, null, 2));
33
34  // Append to file
35  await fsp.appendFile("logs.txt", `[${new Date().toISOString()}] Event occurred\n`);
36
37  // Write with exclusive flag (error if exists)
38  try {
39    await fsp.writeFile("unique.txt", "data", { flag: "wx" });
40  } catch (err) {
41    if (err.code === "EEXIST") console.log("File already exists");
42  }
43}
44
45// 3. Directory operations
46async function directoryExamples() {
47  // Create directory (recursive = create parent dirs too)
48  await fsp.mkdir("output/reports/2024", { recursive: true });
49
50  // List directory contents
51  const files = await fsp.readdir("src");
52  console.log("Files:", files);
53
54  // List with file type info
55  const entries = await fsp.readdir("src", { withFileTypes: true });
56  entries.forEach((entry) => {
57    console.log(`${entry.isDirectory() ? "📁" : "📄"} ${entry.name}`);
58  });
59
60  // Remove directory (recursive)
61  await fsp.rm("temp", { recursive: true, force: true });
62}
63
64// 4. File metadata
65async function statExamples() {
66  const stats = await fsp.stat("data.txt");
67  console.log("Is file:", stats.isFile());
68  console.log("Is directory:", stats.isDirectory());
69  console.log("Size:", stats.size, "bytes");
70  console.log("Created:", stats.birthtime);
71  console.log("Modified:", stats.mtime);
72  console.log("Permissions:", stats.mode.toString(8));
73}
74
75// 5. Path module — cross-platform path handling
76function pathExamples() {
77  // ✅ GOOD: Use path.join for cross-platform paths
78  const filePath = path.join(__dirname, "data", "users.json");
79  // Linux: /app/data/users.json
80  // Windows: C:\app\data\users.json
81
82  // path.resolve — Always returns absolute path
83  const absolute = path.resolve("data", "users.json");
84  console.log("Absolute:", absolute);
85
86  // Parse a path into components
87  const parsed = path.parse("/home/user/docs/report.pdf");
88  console.log(parsed);
89  // { root: '/', dir: '/home/user/docs', base: 'report.pdf',
90  //   ext: '.pdf', name: 'report' }
91
92  console.log("Extension:", path.extname("app.test.js"));  // .js
93  console.log("Basename:", path.basename("/foo/bar.txt"));  // bar.txt
94  console.log("Directory:", path.dirname("/foo/bar.txt"));  // /foo
95  console.log("Relative:", path.relative("/a/b", "/a/c")); // ../c
96}
97
98// 6. Safe file existence check
99async function fileExists(filePath) {
100  try {
101    await fsp.access(filePath, fs.constants.F_OK);
102    return true;
103  } catch {
104    return false;
105  }
106}
107
108// 7. Recursive directory walker
109async function walkDir(dir) {
110  const entries = await fsp.readdir(dir, { withFileTypes: true });
111  const files = [];
112
113  for (const entry of entries) {
114    const fullPath = path.join(dir, entry.name);
115    if (entry.isDirectory()) {
116      files.push(...(await walkDir(fullPath)));
117    } else {
118      files.push(fullPath);
119    }
120  }
121  return files;
122}
123
124// Usage
125// const allFiles = await walkDir("./src");
126// console.log("All files:", allFiles);
127
128// 8. File watching
129function watchExample() {
130  const watcher = fs.watch("./src", { recursive: true }, (eventType, filename) => {
131    console.log(`[${eventType}] ${filename}`);
132  });
133
134  // Clean up on exit
135  process.on("SIGINT", () => {
136    watcher.close();
137    process.exit(0);
138  });
139}
140
141// 9. Temp files and atomic writes
142async function atomicWrite(filePath, data) {
143  const tempPath = `${filePath}.${process.pid}.tmp`;
144  try {
145    await fsp.writeFile(tempPath, data);
146    await fsp.rename(tempPath, filePath); // Atomic on same filesystem
147  } catch (err) {
148    // Clean up temp file on failure
149    await fsp.unlink(tempPath).catch(() => {});
150    throw err;
151  }
152}
153
154pathExamples();