To bring static typing to JavaScript, Flow specifies a number of syntax extensions, which are used to describe types, annotate programs, and share types between modules.

Flow’s syntax extensions are only additions which can be easily stripped away and don’t change the runtime behavior of JavaScript in any way.

This page gives a high-level overview of the various syntax elements introduced by Flow.

Remember that Flow has sophisticated type inference and it is often unnecessary to explicitly annotate your program with types.

Stripping Flow syntax#

Before type-annotated code can run in a browser or Node, the Flow syntax must be stripped away.

Babel 5.x#

Babel 5.x supports Flow’s syntax extensions out of the box.

Babel 6.x#

Babel 6.x introduced a plugin architecture, which makes it easier to customize to your particular flavor of JavaScript.

To use Flow syntax with Babel 6.x, you need to include the transform-flow-strip-types plugin.

Another option is to use the React preset which has Flow support (including transform-flow-strip-types) already built-in in addition to React-specific plugins.

Comment syntax#

Flow also supports a comment-based syntax, which makes it possible to use Flow without requiring any compilation.

See Comment Syntax for more information and examples.

Variable declarations#

To specify a type for a binding introduced in a variable declaration statement, add a type annotation after the binding.

var foo: string = "Hello, World!";

When declaring multiple variables at once, type annotations can be provided for each variable.

var bar: number = 0,
    baz: boolean = true;

Function declarations#

Type annotations for function parameters and the return type are supplied inline.

function numVowels(word: string): number {
  const vowels = new Set("aeiou");
  let count = 0;
  for (let char of word)
    if (vowels.has(char))
  return count;

Type parameters for generic functions are also provided inline.

function reversed<T>(array: T[]): T[] {
  let ret = [];
  let i = array.length;
  while (i--)
  return ret;

Rest parameters should be annotated with an array type.

function sum(...xs: number[]): number {
  return xs.reduce((a,b) => a + b);

Arrow functions also support inline annotation syntax.

const flip = <A,B>([a,b]: [A,B]): [B,A] => [b,a];

See Functions for more information and examples.

Class declarations#

Type annotations for class fields are specified inside the class body. This syntax works well with the class properties proposal currently being reviewed by TC39 for inclusion in a future JavaScript standard.

Methods can also specify types for their parameter and return types.

class Point {
  x: number;
  y: number;

  constructor(x: number, y: number) {
    this.x = x;
    this.y = y;

  move(x: number, y: number) {
    this.x += x;
    this.y += y;

  copy(): Point {
    return new Point(this.x, this.y);

Type parameters for generic classes can also be specified inline.

class Box<T> {
  _value: T;

  constructor(value: T) {
    this._value = value;

  get(): T {
    return this._value;

See Classes for more information and examples.

Type aliases#

Type aliases make it easy to refer to a potentially complex type by a simple name. Type aliases are completely removed during the compile step.

See Type Aliases for more information and examples.

Object types#

The type of an object specifies the type of each of its properties.

type Person = {
  name: string,
  age: number,

Objects types can include a callable property, which allows values of that type be called like a function.

type Callable = {
  (x: string): number,
  foo: string,

Object types can include an indexer property, which allows values of that type to be used like a dictionary.

type Dictionary = {
  [x: number]: string,
  foo: string,

See Objects for more information and examples.

Function types#

In addition to the syntax for annotating a function inline, it’s possible to express the type of a function in isolation.

type TimesTwo = (value: number) => number;

Type parameters for generic function types are specified before the parameter list.

type Identity = <T>(x: T) => T;

See Functions for more information and examples.

Array types#

Array types can be specified by adding [] to the end of a type, or using Array<T> syntax. Note that maybe types of the short-hand syntax apply to the entire array, not the contained value type.

var array_of_num: number[] = [];
var array_of_num_alt: Array<number> = [];
var optional_array_of_num: ?number[] = null;
var array_of_optional_num: Array<?number> = [null, 0];

Tuple types describe arrays of known length holding heterogeneous values.

var tuple_of_str_and_num: [string, number] = ["Hi", 42];

See Arrays for more information and examples.

Interface declarations#

Declaring an interface creates a type that multiple classes can satisfy without being part of the same inheritance hierarchy.

Like type aliases, interfaces are entirely erased at compile time and have no runtime presence.

interface Comparable<T> {
  compare(a: T, b: T): number;

Importing and exporting types#

It’s possible to export types defined in one file for use in another. import type and export type are entirely erased at compile time and have no runtime presence.

# foo.js
export type Foo = string;
import type { Foo } from "./foo";
var foo: Foo = "Hello";

See Modules for more information and examples.

Destructured bindings#

JavaScript’s destructuring syntax can be used to introduce many bindings at once. Currently, type annotations must be applied to the entire pattern, not the individual bindings.

var {a, b: {c}}: {a: string, b: {c: number}} = {a: "", b: {c: 0}};

See Destructuring for more information and examples.


A typecast expression is a simple way to annotate any JavaScript expression.

(1 + 1 : number);

var obj = {
  name: (null: ?string)

([1, "a", true]: Array<mixed>).map(x => x);

See Typecasts for more information and examples.

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