3.9.0 • Public • Published

    Babel Typecheck

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

    Build Status

    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 = {
        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


    npm i babel-plugin-typecheck

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