Skip to content


Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?

Latest commit


Git stats


Failed to load latest commit information.
Latest commit message
Commit time

jstyper Build Status npm version

Turns your JavaScript to TypeScript / Flow (adds types, converts requires to imports, etc); ALPHA QUALITY

JsTyper adds {TypeScript, Flow, Closure} types to JavaScript programs using iterative type propagation and the TypeScript Language Services.

Run it

  • Interactive online demo
  • npm i -g jstyper then jstyper input.js
    • Noteworthy flags:
      • -o <outputDir> (or --outputDir=<outputDir>): where files should be generated
      • --declarations: only generate interface files (*.d.ts)


(take a look at our tests)

  • Assumes your JS code is correct, and propagates inferred types globally (across different files):
    • From call sites: if you call f(1), it will assume function f can take a number argument. If you call f() somewhere else, it will assume that argument is optional.
    • From function bodies: in function f(x) { return x * 2 } it's obvious x must be a number. Similar constraints appear when you test arguments for nullity or for equality with other values, etc.
    • From call shapes: from x.y() * x.n; it infers x: {y(): void, readonly n: number}
  • Rewrites require & module.exports to ES2015 (ES6) imports & exports
  • Supports writing declarations (*.d.ts) files only with the --declarations flag.
  • Writes definitions for imported / required module interfaces (e.g. if you var foo = require('foo'); it will infer declare module "foo" { export function bar(): void })
  • Rewrites var to let



function f(x) {
  return x * 2;

function g(x, o) {
  if (o.addValue) {
    return f(x) + o.value;
  return == 'default' ? x : 'y';

function gg(x, y) {
  var v = g(x, y);
  return v;


function f(x: number): number {
  return x * 2;

function g(x: number, o: {addValue: boolean, value: any, name: string}) {
  if (o.addValue) {
    return f(x) + o.value;
  return == 'default' ? x : 'y';

function gg(x: number, y: {addValue: boolean, value: any, name: string}) {
  let v = g(x, y);
  return v;


  • Bundle & support DOM & ES2015+ libs
  • Support var foo = require('./bar').foo pattern
  • Support var Foo = function() {}; = ... (create companion interface Foo { bar... })
  • Parse and output flow comment / flow types (+ compare with Flow)
  • Lint mode
  • Split large tests into smaller specs
  • Better propagate contextual types in expressions (e.g. in (a && b && c, a ? b : c)
  • Use type constraints instead of types to allow local inference passes
  • Support merging of symbols added after structural members {x: number} | Foo
  • Handle index operators
  • Infer x: Promise<T> from (await x): T
  • WebDriver test for demo
  • Support literal types
  • Use const when appropriate in var rewrite.
  • Proper cli args parsing + --help
  • Better new F() class creation inference

Hack it

  • Clone this repo
  • Run npm i
  • Debug the demo npm start (will auto-reload and auto-run tests after any changes made to the TypeScript sources)
  • More options: take a look at package.json's script entries


Adds Flow / TypeScript types to JavaScript files... because that hassle can be automated.






No packages published