Implementation of the Language Server Protocol for JavaScript and TypeScript

Usage no npm install needed!

<script type="module">
  import elasticJavascriptTypescriptLangserver from 'https://cdn.skypack.dev/@elastic/javascript-typescript-langserver';



This is a fork of: https://github.com/sourcegraph/javascript-typescript-langserver

The goal of this fork is:

  1. Simplify file system model (only use local file system)
  2. Largely Reduce memory footprint
  3. Replace Observable with promise
  4. Add APIs that specific for Elastic Code

Non goal

  1. Make it a full featured langserver that do best for all LSP calls, specifically, it will optimize for readonly use cases

Q: Why not https://github.com/theia-ide/typescript-language-server? A: the fact it calls tsserver bring a lot of overhead, making the indexing speed very slow. However, we are going to brow code from them

JavaScript/TypeScript language server

npm downloads build appveyor build codecov dependencies OpenTracing: enabled semantic-release code style: prettier license chat: on gitter

This is a language server for JavaScript and TypeScript that adheres to the Language Server Protocol (LSP). It uses TypeScript's LanguageService to perform source code analysis.

Try it out


  • Hovers
  • Goto definition
  • Goto type definition
  • Find all references
  • Document symbols
  • Workspace symbol search
  • Rename
  • Completion
  • Signature help
  • Diagnostics
  • Quick fixes

Run it from source

# install dependencies
npm install

# compile
npm run build
# or compile on file changes
npm run watch

# run over STDIO
node lib/language-server-stdio
# or run over TCP
node lib/language-server

# run tests
npm test


  Usage: language-server [options]


    -h, --help            output usage information
    -V, --version         output the version number
    -s, --strict          enabled strict mode
    -p, --port [port]     specifies LSP port to use (2089)
    -c, --cluster [num]   number of concurrent cluster workers (defaults to number of CPUs, 8)
    -t, --trace           print all requests and responses
    -l, --logfile [file]  log to this file
    -j, --enable-jaeger   enable OpenTracing through Jaeger


This language server implements some LSP extensions, prefixed with an x.

  • Files extension Allows the server to request file contents without accessing the file system
  • SymbolDescriptor extension Get a SymbolDescriptor for a symbol, search the workspace for symbols or references to it
  • Streaming Supports streaming partial results for all endpoints through JSON Patches
  • Packages extension Methods to get information about dependencies
  • TCP / multiple client support When running over TCP, the exit notification will not kill the process, but close the TCP socket


This project follows semver for command line arguments and standard LSP methods. Any change to command line arguments, Node version or protocol breaking changes will result in a major version increase.

Debugging Performance with OpenTracing

The language server is fully traced through OpenTracing, which allows to debug what exact operations caused method calls to take long. You can pass a span context through an optional meta field on the JSON RPC message object.

For local development, there is built-in support for the open source OpenTracing implementation Jaeger, which can be set up to run on localhost with just one command (you need Docker installed):

docker run -d -p5775:5775/udp -p6831:6831/udp -p6832:6832/udp \
  -p5778:5778 -p16686:16686 -p14268:14268 jaegertracing/all-in-one:latest

After that, run the language server with the --enable-jaeger command line flag and do some requests from your client. Open http://localhost:16686 in your browser and you will see method calls broken down into spans.