Language-service features

The user-facing IDE capabilities the language service provides. This page is the per-feature index; for the architecture of the service itself see systems/language-service, and for refactors and code-fixes specifically see features/refactors-and-codefixes.

Feature catalogue

Feature	File	Notes
Completions	src/services/completions.ts (6,173 lines)	Symbol-based, keyword, JSX-attribute, string-literal, paths, JSDoc-tag completions; auto-import support
String completions	src/services/stringCompletions.ts	Specifically string-literal contexts: imports, process.env, etc.
Hover (quickInfo)	src/services/services.ts → symbolDisplay.ts	Type + JSDoc panel
Signature help	src/services/signatureHelp.ts	Per-parameter info while typing a call
Go to definition	src/services/goToDefinition.ts	Definition / type-definition / implementation
Find all references	src/services/findAllReferences.ts (2,803 lines)	Powers find-references and rename
Rename	services/rename.ts + reuses findAllReferences	Single-symbol cross-file rename
Document highlights	src/services/documentHighlights.ts	Same-file occurrence highlighting
Inlay hints	src/services/inlayHints.ts	Inferred-type / parameter-name hints
Outlining (folding)	src/services/outliningElementsCollector.ts	Foldable regions
Smart selection	src/services/smartSelection.ts	"Expand selection"
Navigate to	src/services/navigateTo.ts	Fuzzy symbol search
Navigation bar	src/services/navigationBar.ts	Top-of-document outline
Call hierarchy	src/services/callHierarchy.ts	Incoming/outgoing calls
Breakpoints	src/services/breakpoints.ts	Validate/compute breakable spans
Classifier	src/services/classifier.ts + classifier2020.ts	Syntactic + semantic colourisation
Suggestion diagnostics	src/services/suggestionDiagnostics.ts	"Did you mean async?", unused-symbol style suggestions
Format	src/services/formatting/	On-type, range, document formatting
Map code	src/services/mapCode.ts	Suggest-diff-style mapping for AI assistants
Paste edits	src/services/pasteEdits.ts	Update imports when pasting code from another file
Rename file	src/services/getEditsForFileRename.ts	Update import paths when a file is renamed
Organize imports	src/services/organizeImports.ts	Sort + remove unused imports
Code fixes	src/services/codefixes/	See features/refactors-and-codefixes
Refactors	src/services/refactors/	See features/refactors-and-codefixes
Transpile	src/services/transpile.ts	transpileModule / transpileDeclaration (used by tools like Babel and Vite)

Cross-feature primitives

Several files are heavily reused:

• src/services/utilities.ts (170k+ effective lines) — finding the right node/symbol for a position, walking AST contexts, etc.
• src/services/textChanges.ts — the ChangeTracker used by every feature that returns edits.
• src/services/exportInfoMap.ts — auto-import index of exports across the program.
• src/services/importTracker.ts — find-all-references support across import boundaries.
• src/services/patternMatcher.ts — fuzzy matcher for navigateTo and completions.

Feature lifecycle

sequenceDiagram
    participant Editor
    participant tsserver as src/server/session.ts
    participant Service as createLanguageService
    participant Provider as e.g. completions.ts
    participant Checker as checker.ts

    Editor->>tsserver: { command: "completionInfo", ... }
    tsserver->>Service: getCompletionsAtPosition(file, pos, options)
    Service->>Provider: getCompletionsAtPosition
    Provider->>Service: program.getTypeChecker
    Service->>Checker: getSymbolsInScope, getTypeAtLocation
    Checker-->>Provider: types/symbols
    Provider->>Service: CompletionInfo
    Service->>tsserver: CompletionInfo (translated to protocol shape)
    tsserver->>Editor: response message

Tests

Almost all language-service features are tested with fourslash. The pattern:

/// <reference path="fourslash.ts" />

////import { /*1*/ } from "./util";

verify.completions({ marker: '1', includes: ['foo', 'bar'] });

tests/cases/fourslash/ contains roughly 3,500 of these. See how-to-contribute/testing.

Entry points for modification

• New feature: extend LanguageService in src/services/types.ts, implement under src/services/, expose through tsserver via src/server/protocol.ts and a Session handler in src/server/session.ts.
• New language-service mode behaviour: see LanguageServiceMode in src/services/types.ts and the dispatch in services.ts.

See systems/language-service for the surrounding architecture.