eslint_d

Makes eslint the fastest linter on the planet.

"But eslint is pretty fast already, right?"

Yes, it's really fast. But the node.js startup time and loading all the required modules slows down linting times for a single file to ~700 milliseconds. eslint_d reduces this overhead by running a server in the background. It brings the linting time down to ~160 milliseconds. If you want to lint from within your editor whenever you save a file, eslint_d is for you.

Install

This will install the eslint_d command globally:

$ npm install -g eslint_d

Usage

To start the server and lint a file, just run:

$ eslint_d file.js

On the initial call, the eslint_d server is launched and then the given file is linted. Subsequent invocations are super fast.

How does this work?

The first time you use eslint_d, a little server is started in the background and bound to a random port. You can then run eslint_d commands the same way you would use eslint and it will delegate to the background server. It will load a separate instance of eslint for each working directory to make sure settings are kept local. If eslint is found in the current working directories node_modules folder, then this version of eslint is going to be used. Otherwise, the version of eslint that ships with eslint_d is used as a fallback.

It's possible to force eslint_d to only resolve local eslint by setting the ESLINT_D_LOCAL_ESLINT_ONLY environment variable to a truthy value (ie. true or 1).

To keep the memory footprint low, eslint_d keeps only the last 10 used instances in the internal nanolru cache.

What if eslint or a plugin is updated?

The cached version of eslint and the Node require cache for the current working directory are cleared whenever a change to one of these files is detected: package.json, package-lock.json, npm-shrinkwrap.json, yarn.lock and pnpm-lock.yaml. If changes are not automatically detected, remember to run eslint_d restart to bounce the background server.

Note: Change detection was switched from mtime to content hash with v12.

Which versions of eslint are supported?

You can use eslint_d with multiple projects depending on different versions of eslint. If no local eslint is found, eslint_d falls back to the eslint version that ships with eslint_d.

Experimental support for eslint flat config is available since v13.1.0 if the ESLINT_USE_FLAT_CONFIG environment variable is defined.

Commands

Control the server like this:

$ eslint_d <command>

Available commands:

• start: start the server
• stop: stop the server
• status: print out whether the server is currently running
• restart: restart the server
• [options] file.js [file.js] [dir]: invoke eslint with the given options. The eslint engine will be created in the current directory. If the server is not yet running, it is started.

Type eslint_d --help to see the supported eslint options.

When the server starts, eslint_d selects a free port automatically and decides on a random access token. Both the port and token are written to an .eslint_d file so that future usages of eslint_d can connect to the already running server. The .eslint_d file is stored under the XDG_RUNTIME_DIR directory if this environment variable is defined. If the variable is not defined then the file is stored in the user's home directory.

Editor integration

Linting

• Vim:

  • With syntastic:

    let g:syntastic_javascript_checkers = ['eslint']
    let g:syntastic_javascript_eslint_exec = 'eslint_d'

  • With ale:

    let g:ale_javascript_eslint_executable = 'eslint_d'
    let g:ale_javascript_eslint_use_global = 1

• WebStorm: Configure your IDE to point to the eslint_d package instead of eslint. In the ESLint configuration dialog, under 'ESLint package', select your eslint_d package.

• Emacs: Use flycheck with the javascript-eslint checker:

  (setq flycheck-javascript-eslint-executable "eslint_d")

• Sublime: The official SublimeLinter-eslint plugin automatically prefers eslint_d if it finds one.

• Atom, VSCode: You will not gain any performance from this module as these editors already cache eslint instances for you.

If you're using eslint_d in any other editor, please let us know!

Automatic Fixing

eslint_d has an additional flag that eslint does not have, --fix-to-stdout which prints the fixed file to stdout. This allows editors to add before save hooks to automatically fix a file prior to saving. It must be used with --stdin.

• Vim: Add this to your .vimrc to lint the current buffer or visual selection on <leader>f:

  " Autofix entire buffer with eslint_d:
  nnoremap <leader>f mF:%!eslint_d --stdin --fix-to-stdout<CR>`F
  " Autofix visual selection with eslint_d:
  vnoremap <leader>f :!eslint_d --stdin --fix-to-stdout<CR>gv

• Emacs: See eslintd-fix

• If the above doesn't autofix: This can happen with .vue files
  Change eslint_d --stdin --fix-to-stdout to eslint_d --stdin --fix-to-stdout --stdin-filename % (% = path to file you want to autofix)
  In Vim, the above mapping should be replaced with:

  nnoremap <leader>f mF:%!eslint_d --stdin --fix-to-stdout --stdin-filename %<CR>`F

Moar speed

If you're really into performance and want the lowest possible latency, talk to the eslint_d server with netcat. This will also eliminate the node.js startup time.

You first need to extract the port and access token from the .eslint_d file. The location of this file may change depending on your system (see above). For example, if XDG_RUNTIME_DIR is specified, you can do this:

$ PORT=`cat $XDG_RUNTIME_DIR/.eslint_d | cut -d" " -f1`
$ TOKEN=`cat $XDG_RUNTIME_DIR/.eslint_d | cut -d" " -f2`

Then, you can do the following to run eslint on file.js:

$ echo "$TOKEN $PWD file.js" | nc localhost $PORT

Or if you want to work with stdin:

$ echo "$TOKEN $PWD --stdin" | cat - file.js | nc localhost $PORT

This runs eslint in under 50ms!

Tip For additional speed, did you know that you can lint only files that have changed? This is a feature of normal eslint, but it also works from eslint_d. Run:

$ eslint_d . --cache

References

If you're interested in building something similar to this: Most of the logic was extracted to core_d, a library that manages the background server.

Compatibility

• 13.0.0: eslint 4 - 8, node 12 - 20 (ships with eslint 8)
• 12.0.0: eslint 4 - 8, node 12 - 16 (ships with eslint 8)
• 11.0.0: eslint 4 - 8, node 12 - 16 (ships with eslint 7)
• 10.0.0: eslint 4 - 7, node 10 - 14 (using new ESLint API if available)
• 9.0.0: eslint 4 - 7, node 10 - 14 (using CLIEngine API)
• 8.0.0: eslint 4 - 6, node 8 - 12
• 7.2.0: eslint 4 - 5, node 6 - 10
• 7.0.0: eslint 5.4+, node 6, 8 and 10
• 6.0.0: eslint 5.0+, node 6+ (eslint dropped node 4)
• 5.0.0: eslint 4.0+
• 4.0.0: eslint 3.0+, node 4+ (eslint dropped node 0.10 and 0.12)
• 3.0.0: eslint 2.2+
• 1.0.0, 2.0.0: eslint 1.4+, node 4 (and probably older)

License

MIT