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Language Server Overview

The Deno Language Server provides a server implementation of the 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.

Most users will never interact with the server directly, but instead will via vscode_deno or another editor extension. This documentation is for those implementing a editor client.

Structure Jump to heading

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.

Settings Jump to heading

There are several settings that the language server supports for a workspace:

  • deno.enable
  • deno.enablePaths
  • deno.cache
  • deno.certificateStores
  • deno.config
  • deno.importMap
  • deno.internalDebug
  • deno.codeLens.implementations
  • deno.codeLens.references
  • deno.codeLens.referencesAllFunctions
  • deno.codeLens.test
  • deno.suggest.completeFunctionCalls
  • deno.suggest.names
  • deno.suggest.paths
  • deno.suggest.autoImports
  • deno.suggest.imports.autoDiscover
  • deno.suggest.imports.hosts
  • deno.lint
  • deno.tlsCertificate
  • deno.unsafelyIgnoreCertificateErrors
  • deno.unstable

There are settings that are supported on a per resource basis by the language server:

  • deno.enable
  • deno.enablePaths
  • deno.codeLens.test

There are several points in the process where Deno analyzes these settings. First, when the initialize request from the client, the initializationOptions will be assumed to be an object that represents the deno namespace of options. For example, the following value:

  "enable": true,
  "unstable": true

Would enable Deno with the unstable APIs for this instance of the language server.

When the language server receives a workspace/didChangeConfiguration notification, it will assess if the client has indicated if it has a workspaceConfiguration capability. If it does, it will send a workspace/configuration request which will include a request for the workspace configuration as well as the configuration of all URIs that the language server is currently tracking.

If the client has the workspaceConfiguration capability, the language server will send a configuration request for the URI when it received the textDocument/didOpen notification in order to get the resources specific settings.

If the client does not have the workspaceConfiguration capability, the language server will assume the workspace setting applies to all resources.

Commands Jump to heading

There are several commands that might be issued by the language server to the client, which the client is expected to implement:

  • deno.cache - This is sent as a resolution code action when there is an un-cached module specifier that is being imported into a module. It will be sent with and argument that contains the resolved specifier as a string to be cached.
  • deno.showReferences - This is sent as the command on some code lenses to show locations of references. The arguments contain the specifier that is the subject of the command, the start position of the target and the locations of the references to show.
  • deno.test - This is sent as part of a test code lens to, of which the client is expected to run a test based on the arguments, which are the specifier the test is contained in and the name of the test to filter the tests on.

Requests Jump to heading

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:

    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/reloadImportRegistries - Reloads any cached responses from import registries.

    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:

    interface VirtualTextDocumentParams {
      textDocument: TextDocumentIdentifier;
  • deno/task - Requests the return of available deno tasks, see task_runner.

    It does not expect any parameters.

Notifications Jump to heading

There is currently one custom notification that is sent from the server to the client:

  • deno/registryState - when deno.suggest.imports.autoDiscover is true and an origin for an import being added to a document is not explicitly set in deno.suggest.imports.hosts, the origin will be checked and the notification will be sent to the client of the status.

    When receiving the notification, if the param suggestion is true, the client should offer the user the choice to enable the origin and add it to the configuration for deno.suggest.imports.hosts. If suggestion is false the client should add it to the configuration of as false to stop the language server from attempting to detect if suggestions are supported.

    The params for the notification are:

    interface RegistryStatusNotificationParams {
      origin: string;
      suggestions: boolean;

Language IDs Jump to heading

The language server supports diagnostics and formatting for the following text document language IDs:

  • "javascript"
  • "javascriptreact"
  • "jsx" non standard, same as javascriptreact
  • "typescript"
  • "typescriptreact"
  • "tsx" non standard, same as typescriptreact

The language server supports only formatting for the following language IDs:

  • "json"
  • "jsonc"
  • "markdown"