In-Memory Filesystem (memory-fs)

Common errors

• TypeError: MemoryFileSystem is not a constructor

  cause Attempting to call `new` on the result of `require('memory-fs')` when the variable was incorrectly reassigned or handled.
  fix Ensure `const MemoryFileSystem = require('memory-fs');` directly assigns the constructor to `MemoryFileSystem` and then `const fs = new MemoryFileSystem();` is called.

• ReferenceError: require is not defined in ES module scope

  cause Trying to use `require` in a file or project configured for ES modules (`"type": "module"` in package.json).
  fix This package is CommonJS-only. For projects using ES modules, you might need to use a bundler (like Webpack) that handles CommonJS dependencies, or restructure your code to load this module in a CommonJS context.

• TypeError: fs.someMethod is not a function

  cause Attempting to call an `fs` module method that is not implemented by `memory-fs` (e.g., `fs.watch`, `fs.createReadStream`).
  fix Verify that the specific `fs` method you need is supported by `memory-fs`. If not, you may need to reconsider its use for that particular operation or find an alternative library.

Warnings

• breaking The `engines` field in package.json restricts compatible Node.js versions. Specifically, it requires Node.js `>=4.3.0 <5.0.0 || >=5.10`. Using unsupported Node.js versions may lead to unexpected behavior or installation failures.
  Fix: Ensure your Node.js environment matches the specified engine compatibility range or use a tool like `nvm` to manage Node.js versions.
• gotcha memory-fs provides only a subset of the standard Node.js `fs` module API. While it implements common methods like `readFile`, `writeFile`, `readdir`, and `stat`, many advanced or less frequently used `fs` methods are not available.
  Fix: Always check the `memory-fs` documentation or source code for method availability. Do not assume full `fs` compatibility.
• gotcha This package is CommonJS-only and does not natively support ES module `import` syntax. Attempting to use `import` will result in a runtime error.
  Fix: Use CommonJS `require()` syntax: `const MemoryFileSystem = require('memory-fs');`.
• gotcha Unlike the native `fs` module, `memory-fs` requires explicit instantiation via `new MemoryFileSystem()`. Importing the package directly does not yield a functional filesystem object.
  Fix: After `require('memory-fs')`, always call `new MemoryFileSystem()` to get an instance, e.g., `const fs = new MemoryFileSystem();`.

Install

• npm install memory-fs npm
• yarn add memory-fs yarn
• pnpm add memory-fs pnpm

Imports

• MemoryFileSystem
  wrong:

  import MemoryFileSystem from 'memory-fs';

  correct:

  const MemoryFileSystem = require('memory-fs');

  This package is CommonJS-only. Attempting to use ES module import syntax will result in errors.
• fsInstance
  wrong:

  const fs = require('memory-fs'); // Does not return an instance

  correct:

  const fs = new MemoryFileSystem();

  The `require` call returns the constructor function, which must be instantiated to get a functional filesystem object.

Quickstart

Demonstrates basic synchronous and asynchronous file operations (create, write, read, delete, list, stat) using the `memory-fs` API.

const MemoryFileSystem = require('memory-fs');
const fs = new MemoryFileSystem(); // Optionally pass an initial JavaScript object, e.g., { 'src': { 'main.js': 'console.log("Hello");' } }

// Create directories
fs.mkdirpSync("/a/test/dir");
console.log('Created /a/test/dir');

// Write and read a file
const filePath = "/a/test/dir/file.txt";
fs.writeFileSync(filePath, "Hello World from memory-fs");
console.log(`Content of ${filePath}: ${fs.readFileSync(filePath, 'utf8')}`);

// Async file unlink
fs.unlink(filePath, function(err) {
  if (err) {
    console.error(`Error unlinking file: ${err.message}`);
    return;
  }
  console.log(`${filePath} unlinked successfully.`);
});

// List directory contents
console.log(`Contents of /a/test: ${fs.readdirSync("/a/test")}`);

// Get file/directory stats
const stats = fs.statSync("/a/test/dir");
console.log(`/a/test/dir is a directory: ${stats.isDirectory()}`);

// Clean up directory
fs.rmdirSync("/a/test/dir");
console.log('Removed /a/test/dir');