@lucas-bur/effect-memfs

Platform-agnostic in-memory file system for Effect v4. Everything is kept in RAM – no disk, no native bindings. Works in Node, browsers, edge runtimes, anywhere.

Ideal for testing, mocking, and scenarios where you need a filesystem but don't want to touch the real one. Exposes the standard FileSystem service so your Effect code doesn't know the difference.

Install

npm install @lucas-bur/effect-memfs effect

Usage

import { Effect, FileSystem } from "effect"
import * as MemoryFileSystem from "@lucas-bur/effect-memfs"

const program = Effect.gen(function* () {
  const fs = yield* FileSystem.FileSystem
  yield* fs.writeFileString("hello.txt", "world")
  return yield* fs.readFileString("hello.txt")
})

Effect.runPromise(program.pipe(Effect.provide(MemoryFileSystem.layer())))
// => "world"

Pre-populate files:

import { Effect, FileSystem } from "effect"
import { layer } from "@lucas-bur/effect-memfs"

const program = Effect.gen(function* () {
  const fs = yield* FileSystem.FileSystem
  return yield* fs.readFileString("/src/index.ts")
})

Effect.runPromise(
  program.pipe(
    Effect.provide(
      layer({
        src: {
          "index.ts": "export const x = 1",
          utils: { "math.ts": "export const add = (a, b) => a + b" },
        },
        test: {
          "foo.test.ts": "import { x } from '../src'",
        },
        assets: null, // empty directory
      }),
    ),
  ),
)

Relative paths use / as their working directory by default, so src/index.ts and /src/index.ts address the same file. Pass a custom cwd to mirror a specific Node working directory instead; the initial file tree is mounted at that same location:

const testLayer = layer(
  {
    src: { "index.ts": "export const x = 1" },
  },
  { cwd: process.cwd() },
)

Snapshot a real directory tree into a fresh in-memory layer. Files, empty directories, symlinks, file mode, owner, and access/modification times are all replicated. The caller provides the FileSystem.FileSystem service used for the walk, so the source can live on Node, Bun, Deno, or any other Effect-TS FileSystem backend.

import { NodeFileSystem } from "@effect/platform-node-shared"
import { Effect, FileSystem } from "effect"
import { layerFromPath } from "@lucas-bur/effect-memfs"

const program = Effect.gen(function* () {
  const fs = yield* FileSystem.FileSystem
  return yield* fs.readFileString("/data/fixtures/test.json")
}).pipe(Effect.provide(layerFromPath("/data/fixtures")), Effect.provide(NodeFileSystem.layer))

The source path is mounted at the same location in the in-memory filesystem by default; pass { mountAt: "/some/other/path" } to remap.

Roadmap

• [ ] 1:1 parity with Node's filesystem — @lucas-bur/effect-memfs must be a drop-in replacement for NodeFileSystem: identical behavior, error tags, and edge cases. Enforced by test/file-system-suite.test.ts, which runs the same suite against both backends. Any divergence is a bug to fix, not a quirk to document. Pinned via the TODO(1:1-parity) marker in src/index.ts.

License

MIT