Node Path Fs Patterns

Node.js Path and FS Patterns

Perform file system operations correctly using fs.promises, path utilities, and file watching

When to Use

• Reading, writing, or manipulating files and directories
• Building file paths safely across operating systems
• Watching files for changes (hot reload, build tools)
• Handling file operations asynchronously without blocking the event loop

Instructions

1. Always use fs/promises for async operations:

import { readFile, writeFile, mkdir, readdir, stat, rm } from 'node:fs/promises';
import { join } from 'node:path';

const content = await readFile(join(dir, 'data.json'), 'utf-8');
const data = JSON.parse(content);

2. Build paths with path.join and path.resolve:

import { join, resolve, basename, dirname, extname } from 'node:path';

join('src', 'utils', 'helper.ts'); // 'src/utils/helper.ts'
resolve('src', 'utils'); // '/absolute/path/to/src/utils'
basename('/path/to/file.ts'); // 'file.ts'
basename('/path/to/file.ts', '.ts'); // 'file'
dirname('/path/to/file.ts'); // '/path/to'
extname('file.test.ts'); // '.ts'

Never concatenate paths with string templates — use join() for OS-safe separators.

3. Create directories recursively:

await mkdir(join(outputDir, 'images', 'thumbnails'), { recursive: true });

4. Write files safely (atomic write pattern):

import { writeFile, rename } from 'node:fs/promises';

async function writeFileAtomic(filePath: string, content: string) {
  const tempPath = `${filePath}.tmp`;
  await writeFile(tempPath, content, 'utf-8');
  await rename(tempPath, filePath); // Atomic on most file systems
}

5. Check if a file exists:

import { access, constants } from 'node:fs/promises';

async function fileExists(path: string): Promise<boolean> {
  try {
    await access(path, constants.F_OK);
    return true;
  } catch {
    return false;
  }
}

6. List directory contents recursively:

import { readdir } from 'node:fs/promises';

const files = await readdir(srcDir, { recursive: true, withFileTypes: true });
const tsFiles = files
  .filter((f) => f.isFile() && f.name.endsWith('.ts'))
  .map((f) => join(f.parentPath, f.name));

7. Watch files for changes:

import { watch } from 'node:fs/promises';

const watcher = watch(srcDir, { recursive: true });

for await (const event of watcher) {
  console.log(`${event.eventType}: ${event.filename}`);
}

8. Stream large files instead of loading into memory:

import { createReadStream, createWriteStream } from 'node:fs';
import { pipeline } from 'node:stream/promises';

await pipeline(
  createReadStream('large-input.csv'),
  transformStream,
  createWriteStream('output.csv')
);

9. Clean up with rm:

await rm(tempDir, { recursive: true, force: true });

Details

node:fs/promises provides Promise-based file system operations. Always prefer it over the callback-based fs and the synchronous fs.*Sync variants.

readFile vs createReadStream: Use readFile for files under ~50MB that you need in memory. Use createReadStream for larger files or when you can process data incrementally.

recursive: true: Available on mkdir (create nested directories), readdir (list all descendants), rm (delete directory trees), and watch (monitor subdirectories). Node.js 20+ supports readdir with recursive and withFileTypes together.

File watching: fs.watch is OS-dependent and can emit duplicate events. For production file watching, consider chokidar which normalizes behavior across platforms.

Trade-offs:

• fs/promises is non-blocking — but adds async overhead for small operations
• path.join handles OS differences — but you must remember to use it consistently
• Atomic writes prevent corruption — but require temporary file cleanup on failure
• recursive: true on readdir is convenient — but can be slow on large directory trees

Source

https://nodejs.org/api/fs.html

Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

Harness Integration

• Type: knowledge — this skill is a reference document, not a procedural workflow.
• No tools or state — consumed as context by other skills and agents.

Success Criteria

• The patterns described in this document are applied correctly in the implementation.
• Edge cases and anti-patterns listed in this document are avoided.