Language Features

Language Features provide the actual smarts in the language server protocol. They are usually executed on a [text document, position] tuple. The main language feature categories are: code comprehension features like Hover or Goto Definition. coding features like diagnostics, code complete or code actions.

Go to Definition

textDocument/definition: TextDocumentPositionParams → Location | Location[] | LocationLink[] | null

Go to Declaration

textDocument/declaration: TextDocumentPositionParams → Location | Location[] | LocationLink[] | null

Go to Type Definition

textDocument/typeDefinition: TextDocumentPositionParams → Location | Location[] | LocationLink[] | null

Go to Implementation

textDocument/implementation: TextDocumentPositionParams → Location | Location[] | LocationLink[] | null

Find References

textDocument/references: ReferenceParams → Location[] | null

interface ReferenceParams extends TextDocumentPositionParams {
  context: { includeDeclaration: boolean; }
}

Information Features

Hover

textDocument/hover: TextDocumentPositionParams → Hover | null

interface Hover {
  contents: MarkedString | MarkedString[] | MarkupContent;
  range?: Range;
}

Signature Help

textDocument/signatureHelp: SignatureHelpParams → SignatureHelp | null

interface SignatureHelp {
  signatures: SignatureInformation[];
  activeSignature?: uinteger;
  activeParameter?: uinteger;
}

Document Symbols

textDocument/documentSymbol: DocumentSymbolParams → DocumentSymbol[] | SymbolInformation[] | null

Workspace Symbols

workspace/symbol: WorkspaceSymbolParams → SymbolInformation[] | WorkspaceSymbol[] | null

Code Intelligence Features

Code Completion

textDocument/completion: CompletionParams → CompletionItem[] | CompletionList | null

interface CompletionList {
  isIncomplete: boolean;
  items: CompletionItem[];
}

interface CompletionItem {
  label: string;
  kind?: CompletionItemKind;
  detail?: string;
  documentation?: string | MarkupContent;
  sortText?: string;
  filterText?: string;
  insertText?: string;
  textEdit?: TextEdit;
  additionalTextEdits?: TextEdit[];
}

Completion Triggers:

User invoked (Ctrl+Space)
Trigger characters (., ->, etc.)
Incomplete completion re-trigger

Code Actions

textDocument/codeAction: CodeActionParams → (Command | CodeAction)[] | null

interface CodeAction {
  title: string;
  kind?: CodeActionKind;
  diagnostics?: Diagnostic[];
  isPreferred?: boolean;
  disabled?: { reason: string; };
  edit?: WorkspaceEdit;
  command?: Command;
}

Code Action Kinds:

quickfix - Fix problems
refactor - Refactoring operations
source - Source code actions (organize imports, etc.)

Code Lens

textDocument/codeLens: CodeLensParams → CodeLens[] | null

interface CodeLens {
  range: Range;
  command?: Command;
  data?: any; // For resolve support
}

Formatting Features

Document Formatting

textDocument/formatting: DocumentFormattingParams → TextEdit[] | null

Range Formatting

textDocument/rangeFormatting: DocumentRangeFormattingParams → TextEdit[] | null

On-Type Formatting

textDocument/onTypeFormatting: DocumentOnTypeFormattingParams → TextEdit[] | null

Semantic Features

Semantic Tokens

Since version 3.16.0. The request is sent from the client to the server to resolve semantic tokens for a given file. Semantic tokens are used to add additional color information to a file that depends on language specific symbol information.

textDocument/semanticTokens/full: SemanticTokensParams → SemanticTokens | null
textDocument/semanticTokens/range: SemanticTokensRangeParams → SemanticTokens | null
textDocument/semanticTokens/full/delta: SemanticTokensDeltaParams → SemanticTokens | SemanticTokensDelta | null

Token Encoding:

5 integers per token: [deltaLine, deltaStart, length, tokenType, tokenModifiers]
Relative positioning for efficiency
Bit flags for modifiers

Inlay Hints

textDocument/inlayHint: InlayHintParams → InlayHint[] | null

interface InlayHint {
  position: Position;
  label: string | InlayHintLabelPart[];
  kind?: InlayHintKind; // Type | Parameter
  tooltip?: string | MarkupContent;
  paddingLeft?: boolean;
  paddingRight?: boolean;
}

Diagnostics

Push Model (Traditional)

textDocument/publishDiagnostics: PublishDiagnosticsParams

interface PublishDiagnosticsParams {
  uri: DocumentUri;
  version?: integer;
  diagnostics: Diagnostic[];
}

Pull Model (Since 3.17)

textDocument/diagnostic: DocumentDiagnosticParams → DocumentDiagnosticReport
workspace/diagnostic: WorkspaceDiagnosticParams → WorkspaceDiagnosticReport

Diagnostic Structure:

interface Diagnostic {
  range: Range;
  severity?: DiagnosticSeverity; // Error | Warning | Information | Hint
  code?: integer | string;
  source?: string; // e.g., "typescript"
  message: string;
  tags?: DiagnosticTag[]; // Unnecessary | Deprecated
  relatedInformation?: DiagnosticRelatedInformation[];
}

Dialectic