# Deno Language Server
The Deno Language Server provides a server implementation of the
[Language Server Protocol](https://microsoft.github.io/language-server-protocol/)
which is specifically tailored to provide a _Deno_ view of code. It is
integrated into the command line and can be started via the `lsp` sub-command.
> :warning: The Language Server is highly experimental and far from feature
> complete. This document gives an overview of the structure of the language
> server.
## Structure
When the language server is started, a `LanguageServer` instance is created
which holds all of the state of the language server. It also defines all of the
methods that the client calls via the Language Server RPC protocol.
## Custom requests
The LSP currently supports the following custom requests. A client should
implement these in order to have a fully functioning client that integrates well
with Deno:
- `deno/cache` - This command will instruct Deno to attempt to cache a module
and all of its dependencies. If a `referrer` only is passed, then all
dependencies for the module specifier will be loaded. If there are values in
the `uris`, then only those `uris` will be cached.
It expects parameters of:
```ts
interface CacheParams {
referrer: TextDocumentIdentifier;
uris: TextDocumentIdentifier[];
}
```
- `deno/performance` - Requests the return of the timing averages for the
internal instrumentation of Deno.
It does not expect any parameters.
- `deno/virtualTextDocument` - Requests a virtual text document from the LSP,
which is a read only document that can be displayed in the client. This allows
clients to access documents in the Deno cache, like remote modules and
TypeScript library files built into Deno. The Deno language server will encode
all internal files under the custom schema `deno:`, so clients should route
all requests for the `deno:` schema back to the `deno/virtualTextDocument`
API.
It also supports a special URL of `deno:/status.md` which provides a markdown
formatted text document that contains details about the status of the LSP for
display to a user.
It expects parameters of:
```ts
interface VirtualTextDocumentParams {
textDocument: TextDocumentIdentifier;
}
```