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1.5.0 • Public • Published

TypeScript VFS

A Map based TypeScript Virtual File System.

Useful when you need to:

  • Run TypeScript in the browser
  • Run virtual TypeScript environments where files on disk aren't the source of truth


You start with creating a map which represents all the files in the virtual ts.System:

import { createSystem } from "@typescript/vfs"

const fsMap = new Map<string, string>()
fsMap.set("index.ts", 'const a = "Hello World"')

const system = createSystem(fsMap)

Then you can create a virtual TypeScript Environment:

import { createSystem, createVirtualTypeScriptEnvironment } from "@typescript/vfs"
import ts from "typescript"

const fsMap = new Map<string, string>()
const system = createSystem(fsMap)

const compilerOpts = {}
const env = createVirtualTypeScriptEnvironment(system, ["index.ts"], ts, compilerOpts)

// You can then interact with the languageService to introspect the code
env.languageService.getDocumentHighlights("index.ts", 0, ["index.ts"])

When working in tests, or in environments with file system access, you can switch your virtual system with ts.sys to use the real filesystem with the virtual environment.


You're most likely interested in the API available in env.languageService, here it is as of 3.7.4:

interface LanguageService {
    cleanupSemanticCache(): void;
    getSyntacticDiagnostics(fileName: string): DiagnosticWithLocation[];
    getSemanticDiagnostics(fileName: string): Diagnostic[];
    getSuggestionDiagnostics(fileName: string): DiagnosticWithLocation[];
    getCompilerOptionsDiagnostics(): Diagnostic[];
    getEncodedSyntacticClassifications(fileName: string, span: TextSpan): Classifications;
    getEncodedSemanticClassifications(fileName: string, span: TextSpan): Classifications;
    getCompletionsAtPosition(fileName: string, position: number, options: GetCompletionsAtPositionOptions | undefined): WithMetadata<CompletionInfo> | undefined;
    getCompletionEntryDetails(fileName: string, position: number, name: string, formatOptions: FormatCodeOptions | FormatCodeSettings | undefined, source: string | undefined, preferences: UserPreferences | undefined): CompletionEntryDetails | undefined;
    getCompletionEntrySymbol(fileName: string, position: number, name: string, source: string | undefined): Symbol | undefined;
    getQuickInfoAtPosition(fileName: string, position: number): QuickInfo | undefined;
    getNameOrDottedNameSpan(fileName: string, startPos: number, endPos: number): TextSpan | undefined;
    getBreakpointStatementAtPosition(fileName: string, position: number): TextSpan | undefined;
    getSignatureHelpItems(fileName: string, position: number, options: SignatureHelpItemsOptions | undefined): SignatureHelpItems | undefined;
    getRenameInfo(fileName: string, position: number, options?: RenameInfoOptions): RenameInfo;
    findRenameLocations(fileName: string, position: number, findInStrings: boolean, findInComments: boolean, providePrefixAndSuffixTextForRename?: boolean): readonly RenameLocation[] | undefined;
    getSmartSelectionRange(fileName: string, position: number): SelectionRange;
    getDefinitionAtPosition(fileName: string, position: number): readonly DefinitionInfo[] | undefined;
    getDefinitionAndBoundSpan(fileName: string, position: number): DefinitionInfoAndBoundSpan | undefined;
    getTypeDefinitionAtPosition(fileName: string, position: number): readonly DefinitionInfo[] | undefined;
    getImplementationAtPosition(fileName: string, position: number): readonly ImplementationLocation[] | undefined;
    getReferencesAtPosition(fileName: string, position: number): ReferenceEntry[] | undefined;
    findReferences(fileName: string, position: number): ReferencedSymbol[] | undefined;
    getDocumentHighlights(fileName: string, position: number, filesToSearch: string[]): DocumentHighlights[] | undefined;
    getNavigateToItems(searchValue: string, maxResultCount?: number, fileName?: string, excludeDtsFiles?: boolean): NavigateToItem[];
    getNavigationBarItems(fileName: string): NavigationBarItem[];
    getNavigationTree(fileName: string): NavigationTree;
    getOutliningSpans(fileName: string): OutliningSpan[];
    getTodoComments(fileName: string, descriptors: TodoCommentDescriptor[]): TodoComment[];
    getBraceMatchingAtPosition(fileName: string, position: number): TextSpan[];
    getIndentationAtPosition(fileName: string, position: number, options: EditorOptions | EditorSettings): number;
    getFormattingEditsForRange(fileName: string, start: number, end: number, options: FormatCodeOptions | FormatCodeSettings): TextChange[];
    getFormattingEditsForDocument(fileName: string, options: FormatCodeOptions | FormatCodeSettings): TextChange[];
    getFormattingEditsAfterKeystroke(fileName: string, position: number, key: string, options: FormatCodeOptions | FormatCodeSettings): TextChange[];
    getDocCommentTemplateAtPosition(fileName: string, position: number): TextInsertion | undefined;
    isValidBraceCompletionAtPosition(fileName: string, position: number, openingBrace: number): boolean;
    getJsxClosingTagAtPosition(fileName: string, position: number): JsxClosingTagInfo | undefined;
    getSpanOfEnclosingComment(fileName: string, position: number, onlyMultiLine: boolean): TextSpan | undefined;
    toLineColumnOffset(fileName: string, position: number): LineAndCharacter;
    getCodeFixesAtPosition(fileName: string, start: number, end: number, errorCodes: readonly number[], formatOptions: FormatCodeSettings, preferences: UserPreferences): readonly CodeFixAction[];
    getCombinedCodeFix(scope: CombinedCodeFixScope, fixId: {}, formatOptions: FormatCodeSettings, preferences: UserPreferences): CombinedCodeActions;
    applyCodeActionCommand(action: CodeActionCommand, formatSettings?: FormatCodeSettings): Promise<ApplyCodeActionCommandResult>;
    applyCodeActionCommand(action: CodeActionCommand[], formatSettings?: FormatCodeSettings): Promise<ApplyCodeActionCommandResult[]>;
    applyCodeActionCommand(action: CodeActionCommand | CodeActionCommand[], formatSettings?: FormatCodeSettings): Promise<ApplyCodeActionCommandResult | ApplyCodeActionCommandResult[]>;
    getApplicableRefactors(fileName: string, positionOrRange: number | TextRange, preferences: UserPreferences | undefined): ApplicableRefactorInfo[];
    getEditsForRefactor(fileName: string, formatOptions: FormatCodeSettings, positionOrRange: number | TextRange, refactorName: string, actionName: string, preferences: UserPreferences | undefined): RefactorEditInfo | undefined;
    organizeImports(scope: OrganizeImportsScope, formatOptions: FormatCodeSettings, preferences: UserPreferences | undefined): readonly FileTextChanges[];
    getEditsForFileRename(oldFilePath: string, newFilePath: string, formatOptions: FormatCodeSettings, preferences: UserPreferences | undefined): readonly FileTextChanges[];
    getEmitOutput(fileName: string, emitOnlyDtsFiles?: boolean, forceDtsEmit?: boolean): EmitOutput;
    getProgram(): Program | undefined;


When working with Web

It's very likely that you will need to set up your lib *.d.ts files to use this.

If you are running in an environment where you have access to the node_modules folder, then you can can write some code like this:

const getLib = (name: string) => {
  const lib = dirname(require.resolve("typescript"))
  return readFileSync(join(lib, name), "utf8")

const addLib = (name: string, map: Map<string, string>) => {
  map.set("/" + name, getLib(name))

const createDefaultMap2015 = () => {
  const fsMap = new Map<string, string>()
  addLib("lib.es2015.d.ts", fsMap)
  addLib("lib.es2015.collection.d.ts", fsMap)
  addLib("lib.es2015.core.d.ts", fsMap)
  addLib("lib.es2015.generator.d.ts", fsMap)
  addLib("lib.es2015.iterable.d.ts", fsMap)
  addLib("lib.es2015.promise.d.ts", fsMap)
  addLib("lib.es2015.proxy.d.ts", fsMap)
  addLib("lib.es2015.reflect.d.ts", fsMap)
  addLib("lib.es2015.symbol.d.ts", fsMap)
  addLib("lib.es2015.symbol.wellknown.d.ts", fsMap)
  addLib("lib.es5.d.ts", fsMap)
  return fsMap

This list is the default set of definitions (it's important to note that different options for target or lib will affect what this list looks like) and you are grabbing the library's content from the local dependency of TypeScript.

Keeping on top of this list is quite a lot of work, so this library ships functions for generating a map with with these pre-filled from a version of TypeScript available on disk.

Note: it's possible for this list to get out of sync with TypeScript over time. It was last synced with TypeScript 3.8.0-rc.

import { createDefaultMapFromNodeModules } from "@typescript/vfs"
import ts from "typescript"

const fsMap = createDefaultMapFromNodeModules({ target: ts.ScriptTarget.ES2015 })
fsMap.set("index.ts", "const hello = 'hi'")
// ...

If you don't have access to node_modules, then you can use the TypeScript CDN or unpkg to fetch the lib files. This could be up to about 1.5MB, and you should probably store the values in localStorage so that users only have to grab it once.

This is handled for you via createDefaultMapFromCDN.

import { createDefaultMapFromCDN } from "@typescript/vfs"
import ts from "typescript"
import lzstring from "lz-string"

const start = async () => {
  const shouldCache = true
  // This caches the lib files in the site's localStorage
  const fsMap = await createDefaultMapFromCDN({ target: ts.ScriptTarget.ES2015 }, "3.7.3", shouldCache, ts)

  // This stores the lib files as a zipped string to save space in the cache
  const otherMap = await createDefaultMapFromCDN({ target: ts.ScriptTarget.ES2015 }, "3.7.3", shouldCache, ts, lzstring)

  fsMap.set("index.ts", "const hello = 'hi'")
  // ...


The CDN cache:

  • Automatically purges items which use a different version of TypeScript to save space
  • Can use a copy of the lz-string module for compressing/decompressing the lib files

When working with node

If you can reliably access the file-system, then you can have a simpler time:

const compilerOpts: ts.CompilerOptions = { target: ts.ScriptTarget.ES2016, esModuleInterop: true }
const fsMap = new Map<string, string>()

// If using imports where the types don't directly match up to their FS representation (like the
// imports for node) then use triple-slash directives to make sure globals are set up first.
const content = `/// <reference types="node" />\nimport * as path from 'path';\npath.`
fsMap.set("index.ts", content)

// By providing a project root, then the system knows how to resolve node_modules correctly
const projectRoot = path.join(__dirname, "..")
const system = createFSBackedSystem(fsMap, projectRoot)
const env = createVirtualTypeScriptEnvironment(system, ["index.ts"], ts, compilerOpts)

// Requests auto-completions at `path.|`
const completions = env.languageService.getCompletionsAtPosition("index.ts", content.length, {})

createFSBackedSystem will let you hover a virtual environment on top of the file system at a particular path.

A full example

What does a full example look like? This comes basically verbatim from the TypeScript Sandbox codebase:

import ts from "typescript"
import tsvfs from "@typescript/vfs"
import lzstring from "lz-string"

const fsMap = await tsvfs.createDefaultMapFromCDN(compilerOptions, ts.version, true, ts, lzstring)
fsMap.set("index.ts", "// main TypeScript file content")

const system = tsvfs.createSystem(fsMap)
const host = tsvfs.createVirtualCompilerHost(system, compilerOptions, ts)

const program = ts.createProgram({
  rootNames: [...fsMap.keys()],
  options: compilerOptions,
  host: host.compilerHost,

// This will update the fsMap with new files
// for the .d.ts and .js files

// Now I can look at the AST for the .ts file too
const index = program.getSourceFile("index.ts")

Using this Dependency

This package can be used as a commonjs import, an esmodule and directly via a script tag which edits the global namespace. All of these files are embedded inside the released packages.




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