Skip to content

The Svelte Language Server, and official extensions which use it


Notifications You must be signed in to change notification settings


Repository files navigation

Cybernetically enhanced web apps: Svelte npm version license

IDE docs and troubleshooting

What is Svelte Language Tools?

Svelte Language Tools contains a library implementing the Language Server Protocol (LSP). LSP powers the VSCode extension, which is also hosted in this repository. Additionally, LSP is capable of powering plugins for numerous other IDEs.

A .svelte file would look something like this:

    let count = 1;

    // the `$:` means 're-run whenever these values change'
    $: doubled = count * 2;
    $: quadrupled = doubled * 2;

    function handleClick() {
        count += 1;

<button on:click="{handleClick}">Count: {count}</button>

<p>{count} * 2 = {doubled}</p>
<p>{doubled} * 2 = {quadrupled}</p>

Which is a mix of HTMLx and vanilla JavaScript (but with additional runtime behavior coming from the svelte compiler).

This repo contains the tools which provide editor integrations for Svelte files like this.


This repo uses pnpm workspaces, which TLDR means if you want to run a commands in each project then you can either cd to that directory and run the command, or use pnpm -r [command].

For example pnpm -r test.

The language server for Svelte. Built from UnwrittenFun/svelte-language-server and heavily inspired by Vetur to become the official language server for the language.

A command line tool to check your svelte files for type errors, unused css, and more. Built from Vetur's VTI.

The official vscode extension for Svelte. Built from UnwrittenFun/svelte-vscode to become the official vscode extension for the language.

Converts a .svelte file into a legal TypeScript file. Built from halfnelson/svelte2tsx to provide the auto-complete and import mapping inside the language server.

Want to see how it's transformed? Check out this REPL


High Level Overview

flowchart LR
    %% IDEs
    VSC[IDE: VSCode + Svelte for VS Code extension]
    click VSC "" "Svelte for VSCode extension"
    %% Tools
    CLI[CLI: svelte-check]
    click CLI "" "A command line tool to get diagnostics for Svelte code"
    %% Svelte - Extensions
    click VSC_TSSP "" "A TypeScript plugin for Svelte intellisense"
    %% Svelte - Packages
    TS_SERVICE["TS/JS intellisense using TypeScript language service"]
    SVELTE_SERVICE["Svelte intellisense using Svelte compiler"]
    click SVELTE_LANGUAGE_SERVER "" "A language server adhering to the LSP"
    click SVELTE_COMPILER_SERVICE "" "Transforms Svelte code into JSX/TSX code"
    click TS_SERVICE ""
    click SVELTE_SERVICE ""
    %% External Packages
    HTML_SERVICE[HTML intellisense using vscode-html-languageservice]
    CSS_SERVICE[CSS intellisense using vscode-css-languageservice]
    click HTML_SERVICE ""
    click CSS_SERVICE ""
    click VSC_TS ""
    subgraph EMBEDDED_SERVICES[Embedded Language Services]
      direction LR
    VSC -- Language Server Protocol --> SVELTE_LANGUAGE_SERVER
    CLI -- Only using diagnostics feature --> SVELTE_LANGUAGE_SERVER
    VSC -- includes --> VSC_TS
    VSC_TS -- loads --> VSC_TSSP

More information about the internals can be found HERE.


Pull requests are encouraged and always welcome. Pick an issue and help us out!

To install and work on these tools locally:

Make sure to uninstall the extension from the marketplace to not have it clash with the local one.

git clone svelte-language-tools
cd svelte-language-tools
pnpm install
pnpm bootstrap

Do not use npm to install the dependencies, as the specific package versions in pnpm-lock.yaml are used to build and test Svelte.

To build all of the tools, run:

pnpm build

The tools are written in TypeScript, but don't let that put you off — it's basically just JavaScript with type annotations. You'll pick it up in no time. If you're using an editor other than Visual Studio Code you may need to install a plugin in order to get syntax highlighting and code hints etc.

Making Changes

There are two ways to work on this project: either by working against an existing project or entirely through tests.

Running the Dev Language Server Against Your App

To run the developer version of both the language server and the VSCode extension:

  • open the root of this repo in VSCode
  • Go to the debugging panel
  • Make sure "Run VSCode Extension" is selected, and hit run

This launches a new VSCode window and a watcher for your changes. In this dev window you can choose an existing Svelte project to work against. If you don't use pure Javascript and CSS, but languages like Typescript or SCSS, your project will need a Svelte preprocessor setup. When you make changes to the extension or language server you can use the command "Reload Window" in the VSCode command palette to see your changes. When you make changes to svelte2tsx, you first need to run pnpm build within its folder.

Running Tests

You might think that as a language server, you'd need to handle a lot of back and forth between APIs, but actually it's mostly high-level JavaScript objects which are passed to the npm module vscode-languageserver.

This means it's easy to write tests for your changes:

pnpm test

For tricker issues, you can run the tests with a debugger in VSCode by setting a breakpoint (or adding debugger in the code) and launching the task: "Run tests with debugger".

Supporting Svelte

Svelte is an MIT-licensed open source project with its ongoing development made possible entirely by the support of awesome volunteers. If you'd like to support their efforts, please consider:

Funds donated via Open Collective will be used for compensating expenses related to Svelte's development such as hosting costs. If sufficient donations are received, funds may also be used to support Svelte's development more directly.




  • James Birtles for creating the foundation which this language server, and the extensions are built on
  • Vue's Vetur language server which heavily inspires this project
  • halfnelson for creating svelte2tsx
  • jasonlyu123 for his ongoing work in all areas of the language-tools