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Editors

What you will be able to do: set up your editor, know exactly what the server answers and what it does not, and make sure it formats and lints to your project's configuration rather than to uf's defaults.

What you need first: uf installed, and a project with a uf.config.js — Your first project makes one.

What the server answers

A session with one file, app/flags.js, which declares export type Flag = bool;:

initialize
{"serverInfo":{"name":"uf-lsp","version":"…"},"capabilities":{"textDocumentSync":1,"documentFormattingProvider":true,"hoverProvider":true,"codeActionProvider":{"codeActionKinds":["quickfix","source.fixAll.uf"]}}}

textDocument/publishDiagnostics
flow/deprecated-type  severity 1  4:20  the `bool` type alias is deprecated; write `boolean`

textDocument/codeAction
quickfix          Replace `bool` with `boolean`  (preferred)
source.fixAll.uf  Fix all uf lint problems in this file

textDocument/hover on `bool`
**`flow/deprecated-type`** · error

the `bool` type alias is deprecated; write `boolean`

---

Flow's own lint set · default `error` · decided from the source text

What it does not answer: completion, go-to-definition, rename, find-references, document symbols, and the type at a position. It advertises none of them, and a request for one is refused rather than left waiting:

textDocument/completion
{"code":-32601,"message":"uf lsp does not serve `textDocument/completion`"}

Completion for uf.config.js is being written under #941. Until the server answers types, the terminal is where they are: uf check.

Which configuration it reads

The server reads the project's configuration once, when it starts: from the directory --cwd names, or its working directory, walking up to the nearest uf.config.js, package.json or .git. Three consequences:

• Editing uf.config.js needs a restart. The server has no way to be told. The VS Code extension watches the file and restarts by itself; every other integration says below how to restart it.
• A configuration it cannot read is not reported. The server falls back to uf's defaults, and the editor formats and lints to those without a word. The check is to format a file in the editor and run uf fmt --check in a terminal: if they disagree, the server is reading something else.
• The walk stops at the first package.json. In a workspace, a member with its own package.json and no uf.config.js gets uf's defaults, not the root's configuration.

Every integration starts the server in the project folder, which is correct however the directory is chosen. The READMEs under editors/ say that uf lsp --cwd is ignored; it is honoured, and #998 is correcting them.

The project's ignore list does not reach the server either: a file you have opened is linted, even one uf lint would skip.

VS Code and Cursor

An extension in editors/vscode, built from source. It is not published to the Marketplace or Open VSX yet (#977):

cd editors/vscode
npm install
npx @vscode/vsce package        # writes uf-0.0.0.vsix
code --install-extension uf-0.0.0.vsix

Cursor installs the same file with cursor --install-extension uf-0.0.0.vsix.

It activates in a workspace containing uf.config.js and starts one server per workspace folder that has a uf.config.js at its root, in that folder. A folder without one gets no server, and a uf project nested below the folder you opened is not picked up — open that folder, or add it to the workspace.

A uf.server.path that points nowhere is an error naming the path, not a quiet fall back to another uf. To apply every safe fix on save:

"editor.codeActionsOnSave": { "source.fixAll.uf": "explicit" }

uf: Restart Language Server re-resolves the binary and restarts every server; uf: Show Language Server Log lists every path it searched. The extension's logic — finding the binary, what counts as a project, the command it starts — is tested in tests/library/vscode-extension.test.js; the wiring inside VS Code itself is not, because no editor host runs in CI.

Neovim

editors/neovim/lua/uf.lua, on core vim.lsp.start, with no plugin manager and no nvim-lspconfig. Copy it to ~/.config/nvim/lua/uf.lua, then:

require("uf").setup({
  cmd = { "./node_modules/.bin/uf", "lsp" }, -- optional; "uf" on PATH by default
  format_on_save = true,                     -- optional; off by default
})

A buffer gets a client when its filetype is javascript or javascriptreact and a uf.config.js is found in or above its directory; the server starts in that directory. Fix all is vim.lsp.buf.code_action({ context = { only = { "source.fixAll" } }, apply = true }). The README has the vim.lsp.config form for a setup that already uses it.

Vim

editors/vim/uf.vim, a registration for vim-lsp:

Plug 'prabirshrestha/vim-lsp'
source /path/to/uf/editors/vim/uf.vim

vim-lsp starts a server in Vim's working directory, so start Vim from the project root, or set let g:uf_cd_to_root = 1 before the source line, which starts the server through /bin/sh -c 'cd <root> && exec uf lsp'. g:uf_executable names a different binary. Format on save is a commented-out autocommand at the bottom of the file; restart with :LspStopServer uf and reopen the file.

Helix

editors/helix/languages.toml, copied into the project or merged into ~/.config/helix/languages.toml:

mkdir -p .helix && cp path/to/uf/editors/helix/languages.toml .helix/languages.toml

It replaces Helix's default servers for JavaScript and JSX with uf, roots them at uf.config.js, and formats on write. Restart with :lsp-restart.

Emacs

editors/emacs/uf.el, for Eglot, built into Emacs 29, or lsp-mode:

(add-to-list 'load-path "/path/to/uf/editors/emacs")
(require 'uf)

Then M-x uf-start in a file inside a uf project. uf.el makes a directory with a uf.config.js a project.el project, which is what starts Eglot's server in the right directory. uf-executable names a different binary; restart with M-x eglot-reconnect.

Zed

Not working. editors/zed holds the extension manifest, and a Zed extension that provides a language server also needs a Rust half compiled to WebAssembly, which is not written (#977). Meanwhile uf fmt, uf lint and uf check run as tasks from Zed's terminal.

Where to go next

Build a reading list is next: one application end to end, written with the server's findings in front of you. Formatting and linting is where those findings and the formatting come from, and Type checking is the half an editor does not show yet.

Next: Build a reading list →