Recipe for mocking the filesystem

[jamestalmage]

mock-fs causes problems for AVA, but there are not great alternatives.

mountfs seems like a good solution. It could be used with mockFs.fs(..)(which does not directly modify the native fs module).

import mockfs from 'mock-fs';
import mountfs from 'mountfs';
import fs from 'fs';

mountfs.patchInPlace();

fs.mount('/some/test/directory/', mockfs.fs({
  '/some/test/directory': {
    'some-file.txt': 'mock contents'
  }
});

I haven't actually tested the above code, I just wanted to get the idea out there.

[novemberborn]

Couldn't we reference the required fs methods before loading any user code? Like we do for the various date and timer methods?

[jamestalmage]

We could, but we need to propagate that down to all the dependencies that might want to do that as well, including graceful-fs. I also think doing so would make testing most of those modules a pain (it is now harder to intercept what they are doing and mock it).

It would just be nice if mock-fs didn't clobber the entire file system the way it does. I just think it's going to be hard to stay on top of every place it might cause problems.

[novemberborn]

We could, but we need to propagate that down to all the dependencies that might want to do that as well, including graceful-fs.

Hmm yea, fair enough.

[thangngoc89]

I tried to test using this recipe. But I can't figure out what am I missing.

Here is my test (with debug info)

// compiler-template.js
import template from "lodash.template"
import { readFile } from "fs-promise"

export default function(filePath, templateVars) {
  return readFile(filePath, { encode: "utf-8" })
    .then((content) => template(content)(templateVars))
}

// test
import test from "ava"
import mockFs from "mock-fs"
import mountfs from "mountfs"
import fs from "fs"
import compiler from "../compile-template"

mountfs.patchInPlace()

test("compile template with lodash.template", async (t) => {
  fs.mount("/some/path", mockFs.fs({
    "/template.js": "foo <%= bar %>",
  }))

  console.log(fs.readFileSync("/some/path/template.js").toString())

  const result = await compiler("/some/path/template.js", { bar: "bar" })

  t.is(result, "foo bar")

  fs.umount("/some/path")
})

Log:

$ ava src/_utils/service-worker/__tests__/compile-template.js

foo <%= bar %>
  ✖ compile template with lodash.template failed with "ENOENT: no such file or directory, open '/some/path/template.js'"

  1 test failed

  1. compile template with lodash.template
  Error: ENOENT: no such file or directory, open '/some/path/template.js'

As you can see, I can read /some/path/template.js content using readFileSync. Not quite sure why the compiler can't read it.

[jamestalmage]

It looks like fs-promise stores a reference to readFile before you mock out the function.
Your only choice here may be to use require statements instead of imports 😢.

const mountfs = require('mount-fs');
mountfs.patchInPlace();
const compiler = require('../compile-template');

[thangngoc89]

Well. After changing the test like you suggested. Test is passed. But when running AVA with nyc. The old issue is still there.

ENOENT: no such file or directory, open '/some/path/template.js'

Maybe mocking fs with nyc is not a good idea.

[jamestalmage]

Maybe mocking fs with nyc is not a good idea.

@thangngoc89 - can you upload a minimal reproduction to GitHub? It should contain the minimal amount of steps required to reproduce the problem.

@novemberborn and I are maintainers on both the AVA and nyc projects. Understanding the problems you are having would help us to write up good documentation to help future users as well.

[thangngoc89]

There you go @jamestalmage https://github.com/thangngoc89/nyc-mock-fs

[jamestalmage]

OK, I am stumped. It is only happening with nyc, so let's continue discussing here.

[thangngoc89]

1 downside of this recipe is it doesn't work with Windows.

[sibelius]

should I mock the filesystem to make this work:

dotenv-safe do not work with ava
https://stackoverflow.com/questions/39065075/env-do-not-work-with-ava

[thangngoc89]

@sibelius could you post your AVA configuration/ test code ? Mocking filesystem is a different problem here

[sibelius]

ava configuration

"ava": {
    "files": [
      "./**/__tests__/**/*.blue.js"
    ],
    "babel": "inherit",
    "failFast": true,
    "require": [
      "babel-register",
      "babel-polyfill"
    ]
  },

I'm testing a koajs server using https://github.com/rolodato/dotenv-safe to require .env files

it is failing in this line of dotenv-safe

node_modules/dotenv-safe/index.js:26:42) (https://github.com/rolodato/dotenv-safe/blob/master/index.js#L26)

[sibelius]

@thangngoc89 this is a repo https://github.com/sibelius/koa-env-ava

that reproduces this error

[sibelius]

fixed here: sibelius/koa-env-ava#1 tks

[bitjson]

Looks like this issue hasn't moved in a while. Does anyone have a working example of mocking the filesystem in AVA tests?

[lo1tuma]

Instead of monkey patching globals, node core modules or any other third party modules I usually do dependency injection.

[theKashey]

Dependency injection to the rescue (rewiremock with babel plugin)

import test from 'ava';
// ^ needs a real FS

// nothing before would be mocked
import rewiremock from 'rewiremock/node';

import mockfs from 'mock-fs'; // mock-fs
import fs from 'fs'; // get-fs

rewiremock('fs').mockThrough(); // mock "fs" 
// ^ this command would be hoisted by babel plugin

// this is FS, mocked by rewiremock and "hacked" by mockfs
// "real" FS is kept safe
fs.mount('/some/test/directory/', mockfs.fs({
  '/some/test/directory': {
    'some-file.txt': 'mock contents'
  }
});

PS: With common js require it would be a bit easy, and you will be able to use mockery, for example, to do the same.