Static and runtime type checking for JavaScript in the form of a Babel plugin.



Babel Typecheck

This is a Babel plugin for static and runtime type checking using flow type annotations.

Build Status


This project is now deprecated, we're now focussing on flow-runtime.

Note: Now requires babel 6.1, babel 5 users see the 2.x branch.


Turns code like this:

function sendMessage (to: User, message: string): boolean {
  return socket.send(to, message);

into code like this:

function sendMessage(to, message) {
  var _socket$send;

  if (!(to instanceof User)) throw new TypeError("Value of argument 'to' violates contract.");
  if (typeof message !== "string") throw new TypeError("Value of argument 'message' violates contract.");
  _socket$send = socket.send(to, message);
  if (typeof _socket$send !== "boolean") throw new TypeError("Function 'sendMessage' return value violates contract.");
  return _socket$send;

And guards against some silly mistakes, for example the following code will fail to compile with a SyntaxError, because the function can return the wrong type.

function foo (): boolean {
  if (Math.random() > 0.5) {
    return "yes"; // <-- SyntaxError - string is not boolean
  else {
    return false;

function bar (input: string = 123): string { // <-- SyntaxError: default value is not string
  return input + "456";


First, install via npm.

npm install --save-dev babel-plugin-typecheck

Then, in your babel configuration (usually in your .babelrc file), add "typecheck" to your list of plugins:

  "plugins": [
    ["typecheck", {
      "disable": {
        "production": true

The example configuration will disable typecheck when NODE_ENV=production which is usually preferable for performance reasons.

Important: This plugin has a dependency on babel-plugin-syntax-flow and babel-plugin-transform-flow-strip-types. Without syntax-flow, babel will be unable to parse the flow annotation syntax. Without transform-flow-strip-types, the type annotations will be included in the output which will make it unparsable by JS engines.

If you are not already using the babel-preset-react plugin, you must install those plugins and include them in your babel configuration (usually .babelrc). Put them after typecheck in the list, e.g.

  "plugins": ["typecheck", "syntax-flow", "transform-flow-strip-types"]

If you are using babel-preset-react you can ignore this warning.

Note Depending on your babel configuration you may encounter issues where typecheck interferes with other transformations. This can almost always be fixed by adjusting your preset order and setting "passPerPreset": true in your .babelrc.


The basic format is similar to Flow Type Annotations.

Here are a few examples of annotations this plugin supports:

function foo(
    aNum: number,
    anOptionalString: ?string, // will allow null/undefined
    anObject: Object,
    aDate: Date,
    anError: Error,
    aUnionType: Object|string,
    aClass: User,
    aShape: {foo: number, bar: ?string},
    anArray: Array,
    arrayOf: string[] | Array<string>,
    {x, y}: {x: string, y: number}, // destructuring works
    es6Defaults: number = 42
) : number {
  return aNum;

Importing and Exporting types.

You can reuse types across modules using an extension of the ES6 module syntax:


export type CsvDataType = Array<Array<String>>;
export type LocationType = {
    country: string,
    sourceNid: string,
    locationNid: string,
    name: string,
    url: string,
    alternativeUrl: ?string,
    street1: ?string


import type {
} from './places';

// You can now use CsvDataType and LocationType just like any other type.

Note that in contrast to flow, an imported type must be an actual type and cannot be a class or other concrete value.


In cases where typecheck can statically verify that the return value is of the correct type, no type checks will be inserted, for instance:

function bar (): string|Object {
  if (Math.random() > 0.5) {
    return "yes";
  else {
    return {
      message: "no"

will produce no type checks at all, because we can trivially tell that the function can only return one of the two permitted types. This is also true for simple cases like:

function createUser (): User {
  return new User(); // <-- no typecheck required

This is currently quite limited though, as the plugin can only statically infer the types of literals and very simple expressions, it can't (yet) statically verify e.g. the result of a function call. In those cases a runtime type check is required:

function createUser (): User {
  return User.create(); // <-- produces runtime typecheck

Changes in 3.5.0

Supports various number types:

  • int8
  • uint8
  • int16
  • uint16
  • int32
  • uint32
  • float32
  • float16


function demo (input: uint8): uint16 {
  return input * input;

demo(1); // ok
demo(128); // ok
demo(255); // ok
demo(-1); // TypeError
demo(12.34); // TypeError
demo(1024); // TypeError
demo('nope'); // TypeError

Changes in 3.0.0

Supports type aliases:

type Foo = string|number;

function demo (input: Foo): string {
  return input + '  world';

demo('hello'); // ok
demo(123); // ok
demo(["not", "a", "Foo"]); // fails

Better static type inference

function demo (input: string): string[] {
  return makeArray(input); // no return type check required, knows that makeArray is compatible

function makeArray (input: string): string[] {
  return [input];

Type propagation

function demo (input: string): User {
  const user = new User({name: input});
  return user; // No check required, knows that user is the correct type

Assignment tracking

let name: string = "bob";

name = "Bob"; // ok
name = makeString(); // ok
name = 123; // SyntaxError, expected string not number

function makeString (): string {
  return "Sally";

Type casting

let name: string = "bob";

name = "Bob";
((name: number) = 123);
name = 456;
name = "fish"; // SyntaxError, expected number;

Array type parameters

function demo (input: string[]): number {
  return input.length;

demo(["a", "b", "c"]); // ok
demo([1, 2, 3]); // TypeError

Shape tracking

type User = {
  name: string;
  email: string;

function demo (input: User): string {

demo({}); // TypeError
demo({name: 123, email: ""}); // TypeError
demo({name: "test", email: ""}); // ok


Sometimes you might need to disable type checking for a particular file or section of code. To ignore an entire file, add a comment at the top level scope of the file:

// typecheck: ignore file
export function wrong (input: string = 123): boolean {
  return input + ' nope';

To ignore a particular statement:

let foo: string = "hello world";
// typecheck: ignore statement
foo = 123;

Note: Because of how typecheck works, it's not possible to ignore individual lines, only entire statements or files. So if you ignore e.g. an if statement, the entire body of that statement will be ignored.

You can also control the disabling and enabling of type checking using the plugin options and the @typecheck pragma. Type checking will be enabled only for files where any of the configured only values are found in the @typecheck pragma. With babel configuration:

"plugins": [
  ["typecheck", { only: ["production", "test"] }],

This file would have typechecks enabled

// @typecheck: production, some

Whereas this file would not:

// @typecheck: any, some


Published by codemix under a permissive MIT License, see